Electrical Thumb Rules
Title: Illuminating Insights: Electrical Basics and General Thumb Rules
Introduction:
Understanding the fundamentals of electricity is crucial for anyone working with electrical systems, whether it’s in a professional capacity or simply for household maintenance. In this guide, we’ll explore some essential electrical basics and general thumb rules to help you navigate the world of electricity safely and effectively.
- Voltage, Current, and Resistance:
- Voltage (V): Voltage is the force that pushes electrical charges through a conductor. It’s measured in volts (V) and represents the potential energy difference between two points in an electrical circuit.
- Current (I): Current is the flow of electrical charges through a conductor. It’s measured in amperes (A) and represents the rate of flow of electric charge.
- Resistance (R): Resistance is the opposition to the flow of electrical current in a circuit. It’s measured in ohms (Ω) and is determined by the material, length, and cross-sectional area of the conductor.
- Ohm’s Law:
- Ohm’s Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) between them. Mathematically, it’s expressed as: V = I * R or I = V / R or R = V / I.
- Power:
- Power (P) in an electrical circuit is the rate at which work is done or energy is transferred. It’s measured in watts (W) and is calculated as the product of voltage (V) and current (I): P = V * I.
- Basic Safety Rules:
- Always turn off the power before working on any electrical circuit or device.
- Use insulated tools and wear appropriate personal protective equipment (PPE) when working with electricity.
- Never overload electrical circuits or outlets.
- Keep electrical cords away from heat sources, water, and sharp objects.
- Regularly inspect electrical cords, outlets, and switches for signs of wear or damage.
- General Thumb Rules:
- The National Electrical Code (NEC) recommends that electrical outlets should be spaced no more than 12 feet apart in residential buildings.
- For safety, it’s generally advised to limit the load on a circuit to 80% of its maximum capacity.
- The standard voltage for residential buildings in the United States is 120 volts AC, while commercial buildings often use 240 volts AC.
- The resistance of a wire increases with its length and decreases with its cross-sectional area.
• Cable Capacity:
• For Cu Wire Current Capacity (Up to 30 Sq.mm) = 6X Size of Wire in Sq.mm
• Ex. For 2.5 Sq.mm=6×2.5=15 Amp, For 1 Sq.mm=6×1=6 Amp, For 1.5 Sq.mm=6×1.5=9
Amp
• For Cable Current Capacity = 4X Size of Cable in Sq.mm ,Ex. For 2.5
Sq.mm=4×2.5=9 Amp.
• Nomenclature for cable Rating = Uo/U
• where Uo=Phase-Ground Voltage, U=Phase-Phase Voltage, Um=Highest Permissible
Voltage
• Current Capacity of Equipments:
• 1 Phase Motor draws Current=7Amp per HP.
• 3 Phase Motor draws Current=1.25Amp per HP.
• Full Load Current of 3 Phase Motor=HPx1.5
• Full Load Current of 1 Phase Motor=HPx6
• No Load Current of 3 Phase Motor =30% of FLC
• KW Rating of Motor=HPx0.75
• Full Load Current of equipment =1.39xKVA (for 3 Phase 415Volt)
• Full Load Current of equipment =1.74xKw (for 3 Phase 415Volt)
• Earthing Resistance:
• Earthing Resistance for Single Pit=5Ω ,Earthing Grid=0.5Ω
• As per NEC 1985 Earthing Resistance should be <5Ω.
• Voltage between Neutral and Earth <=2 Volts
• Resistance between Neutral and Earth <=1Ω
• Creepage Distance=18 to 22mm/KV (Moderate Polluted Air) or
• Creepage Distance=25 to 33mm/KV (Highly Polluted Air)
• Minimum Bending Radius:
• Minimum Bending Radius for LT Power Cable=12xDia of Cable.
• Minimum Bending Radius for HT Power Cable=20xDia of Cable.
• Minimum Bending Radius for Control Cable=10xDia of Cable.
• Insulation Resistance:
• Insulation Resistance Value for Rotating Machine= (KV+1) MΩ.
• Insulation Resistance Value for Motor (IS 732) = ((20xVoltage (L-L)) / (1000+ (2xKW)).
• Insulation Resistance Value for Equipment (<1KV) = Minimum 1 MΩ.
• Insulation Resistance Value for Equipment (>1KV) = KV 1 MΩ per 1KV.
• Insulation Resistance Value for Panel = 2 x KV rating of the panel.
• Min Insulation Resistance Value (Domestic) = 50 MΩ / No of Points. (All Electrical
Points with Electrical fitting & Plugs). Should be less than 0.5 MΩ
• Min Insulation Resistance Value (Commercial) = 100 MΩ / No of Points. (All
Electrical Points without fitting & Plugs).Should be less than 0.5 MΩ.
• Test Voltage (A.C) for Meggering = (2X Name Plate Voltage) +1000
• Test Voltage (D.C) for Meggering = (2X Name Plate Voltage).
• Submersible Pump Take 0.4 KWH of extra Energy at 1 meter drop of Water.
• Lighting Arrestor:
• Arrestor have Two Rating=
• (1) MCOV=Max. Continuous Line to Ground Operating Voltage.
• (2) Duty Cycle Voltage. (Duty Cycle Voltage>MCOV).
• Transformer:
• Current Rating of Transformer=KVAx1.4
• Short Circuit Current of T.C /Generator= Current Rating / % Impedance
• No Load Current of Transformer=<2% of Transformer Rated current
• Capacitor Current (Ic)=KVAR / 1.732xVolt (Phase-Phase)
• Typically the local utility provides transformers rated up to 500kVA For maximum connected load of 99kW,
• Typically the local utility provides transformers rated up to 1250kVA For maximum
connected load of 150kW.
• The diversity they would apply to apartments is around 60%
• Maximum HT (11kV) connected load will be around 4.5MVA per circuit.
• 4No. earth pits per transformer (2No. for body and 2No. for neutral earthing),
• Clearances, approx.1000mm around TC allow for transformer movement for replacement.
• Diesel Generator:
• Diesel Generator Set Produces=3.87 Units (KWH) in 1 Litter of Diesel.
• Requirement Area of Diesel Generator = for 25KW to 48KW=56 Sq.meter,
100KW=65 Sq.meter.
• DG less than or equal to 1000kVA must be in a canopy.
• DG greater 1000kVA can either be in a canopy or skid mounted in an acoustically treated room
• DG noise levels to be less than 75dBA @ 1meter.
• DG fuel storage tanks should be a maximum of 990 Litter per unit Storage tanks above this level will trigger more stringent explosion protection provision.
• Current Transformer:
• Nomenclature of CT:
• Ratio: input / output current ratio
• Burden (VA): total burden including pilot wires. (2.5, 5, 10, 15 and 30VA.)
• Class: Accuracy required for operation (Metering: 0.2, 0.5, 1 or 3, Protection: 5, 10, 15,
20, 30).
• Accuracy Limit Factor:
• Nomenclature of CT: Ratio, VA Burden, Accuracy Class, Accuracy Limit
Factor.Example: 1600/5, 15VA 5P10 (Ratio: 1600/5, Burden: 15VA, Accuracy Class:
5P, ALF: 10)
• As per IEEE Metering CT: 0.3B0.1 rated Metering CT is accurate to 0.3 percent if the connected secondary burden if impedance does not exceed 0.1 ohms.
• As per IEEE Relaying (Protection) CT: 2.5C100 Relaying CT is accurate within 2.5
percent if the secondary burden is less than 1.0 ohm (100 volts/100A).
| Quick Electrical Calculation | |
| 1HP=0.746KW | Star Connection |
| 1KW=1.36HP | Line Voltage=√3 Phase Voltage |
| 1Watt=0.846 Kla/Hr | Line Current=Phase Current |
| 1Watt=3.41 BTU/Hr | Delta Connection |
| 1KWH=3.6 MJ | Line Voltage=Phase Voltage |
| 1Cal=4.186 J | Line Current=√3 Phase Current |
| 1Tone= 3530 BTU | |
| 85 Sq.ft Floor Area=1200 BTU | |
| 1Kcal=4186 Joule | |
| 1KWH=860 Kcal | |
| 1Cal=4.183 Joule | |
Electrical Thumb Rules (Part-2)
August 6, 2013 14 Comments
Useful Equations:
• For Sinusoidal Current : Form Factor = RMS Value/Average Value=1.11
• For Sinusoidal Current : Peak Factor = Max Value/RMS Value =1.414
• Average Value of Sinusoidal Current(Iav)=0.637xIm (Im= Max.Value)
• RMS Value of Sinusoidal Current(Irms)=0.707xIm (Im= Max.Value)
• A.C Current=D.C Current/0.636.
• Phase Difference between Phase= 360/ No of Phase (1
Phase=230/1=360°,2Phase=360/2=180°)
• Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit
Time (Sec)
• Max.Cross Section Area of Earthing Strip(mm2) =√(Fault Current x Fault Current x
Operating Time of Disconnected Device ) / K
• K=Material Factor, K for Cu=159, K for Alu=105, K for steel=58 , K for GI=80
• Most Economical Voltage at given Distance=5.5x√ ((km/1.6)+(kw/100))
• Cable Voltage Drop(%)=(1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength
(km)x100)/(Volt(L-L)x Cable Run.
• Spacing of Conductor in Transmission Line (mm) = 500 + 18x (P-P Volt) + (2x (Span in Length)/50).
• Protection radius of Lighting Arrestor = √hx (2D-h) + (2D+L). Where h= height of
L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time).
• Size of Lighting Arrestor= 1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).
• Maximum Voltage of the System= 1.1xRated Voltage (Ex. 66KV=1.1×66=72.6KV)
• Load Factor=Average Power/Peak Power
• If Load Factor is 1 or 100% = This is best situation for System and Consumer both.
• If Load Factor is Low (0 or 25%) =you are paying maximum amount of KWH
consumption. Load Factor may be increased by switching or use of your Electrical
Application.
• Demand Factor= Maximum Demand / Total Connected Load (Demand Factor <1)
• Demand factor should be applied for Group Load
• Diversity Factor= Sum of Maximum Power Demand / Maximum Demand (Demand
Factor >1)
• Diversity factor should be consider for individual Load
• Plant Factor(Plant Capacity)= Average Load / Capacity of Plant
• Fusing Factor=Minimum Fusing Current / Current Rating (Fusing Factor>1).
• Voltage Variation(1 to 1.5%)= ((Average Voltage-Min Voltage)x100)/Average Voltage
• Ex: 462V, 463V, 455V, Voltage Variation= ((460-455) x100)/455=1.1%.
• Current Variation(10%)= ((Average Current-Min Current)x100)/Average Current
• Ex:30A,35A,30A, Current Variation=((35-31.7)x100)/31.7=10.4%
• Fault Level at TC Secondary=TC (VA) x100 / Transformer Secondary (V) x
Impedance (%)
• Motor Full Load Current= Kw /1.732xKVxP.FxEfficiency
Electrical Thumb Rules-(Part-3)
August 8, 2013 15 Comments
Size of Capacitor for P.F Correction:
| For Motor | |
| Size of Capacitor = 1/3 Hp of Motor ( 0.12x KW of Motor) | |
| For Transformer | |
| < 315 KVA | 5% of KVA Rating |
| 315 KVA to 1000 KVA | 6% of KVA Rating |
| >1000 KVA | 8% of KVA Rating |
Earthing Resistance value:
Earthing Resistance Value
| Power Station | 0.5 Ω |
| Sub Station Major | 1.0 Ω |
| Sub Station Minor | 2.0 Ω |
| Distribution Transformer | 5.0 Ω |
| Transmission Line | 10 Ω |
| Single Isolate Earth Pit | 5.0 Ω |
| Earthing Grid | 0.5 Ω |
| As per NEC Earthing Resistance should be <5.0 Ω | |
Voltage Limit (As per CPWD & Kerala Elect.Board):
| Voltage Limit (As Per CPWD) | |
| 240V | < 5 KW |
| 415V | <100 KVA |
| 11KV | <3 MVA |
| 22KV | <6 MVA |
| 33KV | <12 MVA |
| 66KV | <20 MVA |
| 110KV | <40 MVA |
| 220KV | >40 MVA |
Voltage Variation
| > 33 KV | (-) 12.5% to (+) 10% |
| < 33 KV | (-) 9% to (+) 6% |
| Low Voltage | (-) 6% to (+) 6% |
Insulation Class:
| Insulation | Temperature |
| Class A | 105°C |
| Class E | 120°C |
| Class B | 130°C |
| Class F | 155°C |
| Class H | 180°C |
| Class N | 200°C |
Standard Voltage Limit:
| Voltage | IEC (60038) | IEC | Indian Elect. |
| (6100:3.6) | Rule | ||
| ELV | < 50 V | ||
| LV | 50 V to 1 KV | <=1 KV | < 250 V |
| MV | <= 35 KV | 250 V to 650 V | |
| HV | > 1KV | <= 230 KV | 650 V to 33 KV |
| EHV | > 230 KV | > 33 KV |
Standard Electrical Connection (As per GERC):
| As per Type of Connection | |
| Connection | Voltage |
| LT Connection | <=440V |
| HT connection | 440V to 66KV |
| EHT connection | >= 66KV |
| As per Electrical Load Demand | |
| Up 6W Load demand | 1 Phase 230V Supply |
| 6W to 100KVA(100KW) | 3 Phase 440V Supply |
| 100KVA to 2500KVA | 11KV,22KV,33KV |
| Above 2500KVA | 66KV |
| HT Connection Earthing | |
| H.T Connection’s Earthing Strip | 20mmX4mm Cu. Strip |
| CT & PT bodies | 2Nos |
| PT Secondary | 1Nos |
| CT Secondary | 1Nos |
| I/C and O/G Cable+ Cubicle Body | 2Nos |
Standard Meter Room Size (As per GERC):
| Meter Box Height | Upper level does not beyond 1.7 meter and Lower level should not below 1.2 meter from ground. |
| Facing of Meter Box | Meter Box should be at front area of Building at Ground Floor. |
| Meter Room / Closed Shade | 4 meter square Size |
Approximate Load as per Sq.ft Area (As per DHBVN):
| Sq.ft Area | Required Load (Connected) |
| < 900 Sq.ft | 8 KW |
| 901 Sq.ft to 1600 Sq.ft | 16 KW |
| 1601 Sq.ft to 2500 Sq.ft | 20 KW |
| > 2500 Sq.ft | 24 KW |
| For Flats :100 Sq.ft / 1 KW | |
| For Flats USS /TC: 100 Sq.ft / 23 KVA | |
Contracted Load in case of High-rise Building:
| For Domestic Load | 500 watt per 100 Sq. foot of the constructed area. |
| For Commercial | 1500 watt per 100 Sq. foot of the constructed area |
| Other Common Load | For lift, water lifting pump, streetlight if any, corridor/campus lighting and other common facilities, actual load shall be calculated |
| Staircase Light | 11KW/Flat Ex: 200Flat=200×11=2.2KW |
| Sanctioned Load for Building | |
| Up to 50 kW | The L.T. existing mains shall be strengthened. |
| 50 kW to 450 kW (500 kVA) | 11 kV existing feeders shall be extended if spare capacity is available otherwise, new 11 kV feeders shall be constructed. |
| 450 kW to 2550 kW (3000 kVA) | 11 kV feeder shall be constructed from the nearest 33 kV or 110 kV substation |
| 2550 kW to 8500 kW (10,000 kVA) | 33kV feeder from 33 kV or 110 kV sub station |
| 8500 kW (10,000 kVA) | 110 kV feeder from nearest 110 kV or 220 kV sub- station |
Electrical Thumb Rules-(Part-4)
August 18, 2013 7 Comments
Sub Station Capacity & Short Circuit Current Capacity:
| As per GERC | ||
| Voltage | Sub Station Capacity | Short Circuit Current |
| 400 KV | Up to 1000 MVA | 40 KA (1 to 3 Sec) |
| 220 KV | Up to 320 MVA | 40 KA (1 to 3 Sec) |
| 132 KV | Up to 150 MVA | 32 KA (1 to 3 Sec) |
| 66 KV | Up to 80 MVA | 25 KA (1 to 3 Sec) |
| 33 KV | 1.5 MVA to 5 MVA | 35 KA (Urban) (1 to 3 Sec) |
| 11 KV | 150 KVA to 1.5 MVA | 25 KA (Rural) (1 to 3 Sec) |
| 415 V | 6 KVA to 150 KVA | 10 KA (1 to 3 Sec) |
| 220 V | Up to 6 KVA | 6 KA (1 to 3 Sec) |
Sub Station Capacity & Short Circuit Current Capacity:
| As per Central Electricity Authority | ||
| Voltage | Sub Station Capacity | Short Circuit Current |
| 765 KV | 4500 MVA | 31.5 KA for 1 Sec |
| 400 KV | 1500 MVA | 31.5 KA for 1 Sec |
| 220 KV | 500 MVA | 40 KA for 1 Sec |
| 110/132 KV | 150 MVA | 40 KA or 50 KA for 1 Sec |
| 66 KV | 75 MVA | 40 KA or 50 KA for 1 Sec |
Minimum Ground Clearance and Fault Clearing Time:
| Voltage | Min. Ground Clearance | Fault Clear Time |
| 400 KV | 8.8 Meter | 100 mille second |
| 220 KV | 8.0 Meter | 120 mille second |
| 132 KV | 6.1 Meter | 160 mille second |
| 66 KV | 5.1 Meter | 300 mille second |
| 33 KV | 3.7 Meter | |
| 11 KV | 2.7 Meter |
Bus bar Ampere Rating:
| For Phase Bus bar | Aluminium 130 Amp / Sq.cm or 800Amp / Sq.inch. |
| For Phase Bus bar | Copper 160 Amp / Sq.cm or 1000Amp / Sq.inch |
| For Neutral Bus bar | Same as Phase Bus bar up to 200 Amp than Size of Neutral Bus bar is at least half of Phase Bus bar. |
Bus bar Spacing:
| Between Phase and Earth | 26mm (Min) |
| Between Phase and Phase | 32mm (Min) |
Bus bar Support between Two
Insulator
250mm.
Sound Level of Diesel Generator (ANSI 89.2&NEMA
51.20):
| KVA | Max. Sound Level |
| <9 KVA | 40 DB |
| 10 KVA to 50 KVA | 45 DB |
| 51 KVA to 150 KVA | 50 DB |
| 151 KVA to 300 KVA | 55 DB |
| 301 KVA to 500 KVA | 60 DB |
IR Value of Transformer:
| IR Value of Transformer | |||
| Voltage | 30°C | 40°C | 50°C |
| >66KV | 600MΩ | 300MΩ | 150MΩ |
| 22KV to 33KV | 500MΩ | 250MΩ | 125MΩ |
| 6.6KV to 11KV | 400MΩ | 200MΩ | 100MΩ |
| <6.6KV | 200MΩ | 100MΩ | 50MΩ |
| 415V | 100MΩ | 50MΩ | 20MΩ |
Standard Size of MCB/MCCB/ELCB/RCCB/SFU/Fuse:
| MCB | Up to 63 Amp (80Amp and 100 Amp a | per Request) | |
| MCCB | Up to 1600 Amp (2000 Amp as per Request) | ||
| ACB | Above 1000 Amp | ||
| MCB Rating | 6A,10A,16A,20A,32A,40A,50A,63A | ||
| MCCB Rating | 0.5A,1A,2A,4A,6A,10A,16A,20A,32A,40A,50A,63A,80A,100A (Domestic Max 6A) | ||
| RCCB/ELCB | 6A,10A,16A,20A,32A,40A,50A,63A,80A,100A | ||
| Sen. of ELCB | 30ma (Domestic),100ma (Industrial),300ma | ||
| DPIC (Double | 5A,15A,30 A for 250V | ||
| Pole Iron Clad) main switch | |||
| TPIC (Triple | 30A, 60A, 100A, 200 A For 500 V | ||
| Pole Iron Clad) main switch | |||
| DPMCB | 5A, 10A, 16A, 32A and 63 A for 250V | ||
| TPMCCB | 100A,200A, 300Aand 500 A For 660 V | ||
| TPN main switch | 30A, 60A, 100A, 200A, 300 A For 500 V |
| TPNMCB | 16A, 32A,63A For 500 V, beyond this TPNMCCB: 100A, 200A, 300A, 500 A For 660 V |
| TPN Fuse Unit | 16A,32A,63A,100A,200A |
| (Rewirable) | |
| Change over | 32A,63A,100A,200A,300A,400A,630A,800A |
| switch (Off Load) | |
| SFU (Switch Fuse | 32A,63A,100A,125A,160A,200A,250A,315A,400A,630A |
| Unit) | |
| HRC Fuse TPN | 125A,160A,200A,250A,400A.630A |
| (Bakelite) | |
| HRC Fuse DPN | 16A,32A,63A |
| (Bakelite) | |
| MCB/MCCB/ELCB Termination Wire / Cable | |
| Up to 20A MCB | Max. 25 Sq.mm |
| 20A to 63A MCB | Max. 35 Sq.mm |
| MCCB | Max. 25 Sq.mm |
| 6A to 45A ELCB | 16 Sq.mm |
| 24A to 63A | 35 Sq.mm |
| ELCB | |
| 80A to 100A | 50 Sq.mm |
| ELCB | |
Electrical Thumb Rules-(Part-5)
September 1, 2013 4 Comments
Standard Size of Transformer (IEEE/ANSI 57.120):
| Single Phase Transformer | Three Phase Transformer |
| 5KVA,10 KVA,15 KVA,25 | 3 KVA,5 KVA,9 KVA,15 KVA,30 KVA,45 KVA,75 KVA,112.5 KVA,150 KVA,225 KVA,300 KVA,500 KVA,750 KVA,1MVA,1.5 MVA,2 MVA,2.5 MVA,3.7 MVA,5 MVA,7.5MVA, 10MVA ,12MVA,15MVA,20MVA ,25MVA, 30MVA,37.5MVA ,50MVA ,60MVA,75MVA,100MVA |
| KVA,37.5 KVA,50 KVA,75 | |
| KVA,100 KVA,167 KVA,250 | |
| KVA, | |
| 333 KVA,500 KVA,833 KVA,1.25 | |
| KVA,1.66 KVA,2.5 KVA,3.33 | |
| KVA,5.0 KVA,6.6 KVA,8.3 | |
| KVA,10.0 KVA,12.5 KVA,16.6 | |
| KVA,20.8 KVA,25.0 KVA,33.33 | |
| KVA |
Standard Size of Motor (HP):
Electrical Motor (HP)
1,1.5,2,3,5,7.5,10,15,20,30,40,50,60,75,100,125,150,200,250,300,400,450,500,600,700,
800,900,1000,1250,1250,1500,1750,2000,2250,3000,3500,4000
Approximate RPM of Motor
| HP | RPM |
| < 10 HP | 750 RPM |
| 10 HP to 30 HP | 600 RPM |
| 30 HP to 125 HP | 500 RPM |
| 125 HP to 300 HP | 375 RPM |
Standard Size of Motor (HP):
Electrical Motor (HP)
1,1.5,2,3,5,7.5,10,15,20,30,40,50,60,75,100,125,150,200,250,300,400,450,500,600,700,
800,900,1000,1250,1250,1500,1750,2000,2250,3000,3500,4000
Motor Line Voltage:
| Motor (KW) | Line Voltage |
| < 250 KW | 440 V (LV) |
| 150 KW to 3000KW | 2.5 KV to 4.1 KV (HV) |
| 200 KW to 3000KW | 3.3 KV to 7.2 KV (HV) |
| 1000 KW to 1500KW | 6.6 KV to 13.8 KV (HV) |
Motor Starting Current:
| Supply | Size of Motor | Max. Starting Current |
| 1 Phase | < 1 HP | 6 X Motor Full Load Current |
| 1 Phase | 1 HP to 10 HP | 3 X Motor Full Load Current |
| 3 Phase | 10 HP | 2 X Motor Full Load Current |
| 3 Phase | 10 HP to 15 | 2 X Motor Full Load |
| HP | Current | |
| 3 Phase | > 15 HP | 1.5 X Motor Full Load Current |
Motor Starter:
| Starter | HP or KW | Starting Current | Torque |
| DOL | <13 HP(11KW) | 7 X Full Load Current | Good |
| Star-Delta | 13 HP to 48 HP | 3 X Full Load Current | Poor |
| Auto TC | > 48 HP (37 KW) | 4 X Full Load Current | Good/ Average |
| VSD | 0.5 to 1.5 X Full Load Current | Excellent | |
| Motor > 2.2KW Should not connect direct to supply voltage if it is in Delta winding | |||
Impedance of Transformer (As per IS 2026):
| MVA | % Impedance |
| < 1 MVA | 5% |
| 1 MVA to 2.5 MVA | 6% |
| 2.5 MVA to 5 MVA | 7% |
| 5 MVA to 7 MVA | 8% |
| 7 MVA to 12 MVA | 9% |
| 12 MVA to 30 MVA | 10% |
| > 30 MVA | 12.5% |
Standard Size of Transformer:
| Standard Size of Transformer | KVA |
| Power Transformer (Urban) | 3,6,8,10,16 |
| Power Transformer (Rural) | 1,1.6,3.15,5 |
| Distribution Transformer | 25,50,63,100,250,315,400,500,630 |
Electrical Thumb Rules-(Part-6)
September 5, 2013 9 Comments
Transformer Earthing Wire / Strip Size:
| Size of T.C or DG | Body Earthing | Neutral Earthing |
| <315 KVA | 25×3 mm Cu / 40×6 mm GI | 25×3 mm Cu Strip |
| Strip | ||
| 315 KVA to 500 | 25×3 mm Cu / 40×6 mm GI | 25×3 mm Cu Strip |
| KVA | Strip | |
| 500 KVA to 750 | 25×3 mm Cu / 40×6 mm GI | 40×3 mm Cu Strip |
| KVA | Strip | |
| 750 KVA to 1000 | 25×3 mm Cu / 40×6 mm GI | 50×3 mm Cu Strip |
| KVA | Strip |
Motor Earthing Wire / Strip Size:
| Size of Motor | Body Earthing |
| < 5.5 KW | 85 SWG GI Wire |
| 5.5 KW to 22 KW | 25×6 mm GI Strip |
| 22 KW to 55 KW | 40×6 mm GI Strip |
| >55 KW | 50×6 mm GI Strip |
Panel Earthing Wire / Strip Size:
| Type of Panel | Body Earthing |
| Lighting & Local Panel | 25×6 mm GI Strip |
| Control & Relay Panel | 25×6 mm GI Strip |
| D.G & Exciter Panel | 50×6 mm GI Strip |
| D.G & T/C Neutral | 50×6 mm Cu Strip |
Electrical Equipment Earthing:
| Equipment | Body Earthing |
| LA (5KA,9KA) | 25×3 mm Cu Strip |
| HT Switchgear | 50×6 mm GI Strip |
| Structure | 50×6 mm GI Strip |
| Cable Tray | 50×6 mm GI Strip |
| Fence / Rail Gate | 50×6 mm GI Strip |
Earthing Wire (As per BS 7671)
| Cross Section Area of Phase, Neutral Conductor(S) mm2 | Minimum Cross Section area of Earthing Conductor (mm2) |
| S<=16 | S (Not less than 2.5 mm2) |
| 16<S<=35 | 16 |
| S>35 | S/2 |
Area for Transformer Room: (As per NBC-2005):
| Transformer Size | Min. Transformer Room Area (M2) | Min. Total Sub Station Area( Incoming HV,LV Panel, T.C Roof) (M2) | Min. Space Width (Meter) |
| 1X160 | 14 | 90 | 9 |
| 2X160 | 28 | 118 | 13.5 |
| 1X250 | 15 | 91 | 9 |
| 2X250 | 30 | 121 | 13.5 |
| 1X400 | 16.5 | 93 | 9 |
| 2X400 | 33 | 125 | 13.5 |
| 3X400 | 49.5 | 167 | 18 |
| 2X500 | 36 | 130 | 14.5 |
| 3X500 | 54 | 172 | 19 |
| 2X630 | 36 | 132 | 14.5 |
| 3X630 | 54 | 176 | 19 |
| 2X800 | 39 | 135 | 14.5 |
| 3X800 | 58 | 181 | 14 |
| 2X1000 | 39 | 149 | 14.5 |
| 3X1000 | 58 | 197 | 19 |
• The Capacitor Bank should be automatic Switched type for Sub Station of 5MVA and
Higher.
• Transformer up to 25KVA can be mounted direct on Pole.
• Transformer from 25KVA to 250KVA can be mounted either on “H” Frame of Plinth.
• Transformer above 250KVA can be mounted Plinth only.
• Transformer above 100MVA shall be protected by Drop out Fuse or Circuit Breaker.
Span of Transmission Line (Central Electricity Authority):
| Voltage | Normal Span |
| 765 KV | 400 to 450 Meter |
| 400 KV | 400 Meter |
| 220 KV | 335,350,375 Meter |
| 132 KV | 315,325,335 Meter |
| 66 KV | 240,250,275 Meter |
| HP | Amp |
| 1 HP | 45 Amp |
| 1.5 HP | 50 Amp |
| 2 HP | 65 Amp |
| 3 HP | 90 Amp |
| 5 HP | 135 Amp |
| 7.5 HP | 200 Amp |
| 10 HP | 260 Amp |
Three Phase Motor Code (NEMA)
| HP | Code |
| <1 HP | L |
| 1.5 to 2.0 HP | L,M |
| 3 HP | K |
| 5 HP | J |
| 7 to 10 HPPHPHPHHHHH | H |
| >15 HP | G |
Service Factor of Motor:
| HP | Synchronous Speed (RPM) | ||||||
| 3600 RPM | 1800 RPM | 1200 RPM | 900 RPM | 720 RPM | 600 RPM | 514 RPM | |
| 1 HP | 1.25 | 1.15 | 1.15 | 1.15 | 1 | 1 | 1 |
| 1.5 to 1.25 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 |
| 150 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1 |
| 200 HP | 1.15 | 1.15 | 1.15 | 1.15 | 1.15 | 1 | 1 |
| > 200 HP | 1 | 1.15 | 1 | 1 | 1 | 1 | 1 |
Type of Contactor:
| Type | Application |
| AC1 | Non Inductive Load or Slightly Inductive Load |
| AC2 | Slip Ring Motor, Starting, Switching OFF |
| AC3 | Squirrel Cage Motor |
| AC4,AC5,AC5a, AC5b,AC6a | Rapid Start & Rapid Stop |
| AC 5a | Auxiliary Control circuit |
| AC 5b | Electrical discharge Lamp |
| AC 6a | Electrical Incandescent Lamp |
| AC 6b | Transformer Switching |
| AC 7a | Switching of Capacitor Bank |
| AC 7b | Slightly Inductive Load in Household |
| AC 5a | Motor Load in Household |
| AC 8a | Hermetic refrigerant compressor motor with Manual Reset O/L Relay |
| AC 8b | Hermetic refrigerant compressor motor with Automatic Reset O/L Relay |
| AC 12 | Control of Resistive Load & Solid State Load |
| AC 13 | Control of Resistive Load & Solid State Load with Transformer Isolation |
| AC 14 | Control of small Electro Magnetic Load (<72 VA) |
| AC 15 | Control of Electro Magnetic Load (>72 VA) |
Contactor Coil:
| Coil Voltage | Suffix |
| 24 Volt | T |
| 48 Volt | W |
| 110 to 127 Volt | A |
| 220 to 240 Volt | B |
| 277 Volt | H |
| 380 to 415 Volt | L |
Electrical Thumb Rules-(Part-7)
September 16, 2013 10 Comments
Overhead Conductor /Cable Size:
| Voltage | Overhead Conductor | Cable Size | |
| 33 KV | ACSR-Panther/Wolf/Dog , AAAC | 150,185,300,400,240 mm2 Cable | |
| 11 KV | ACSR-Dog/Recon/Rabbit , AAAC | 120, 150,185,300 mm2 Cable | |
| LT | ACSR-Dog/Recon/Rabbit , | 95,120, 150,185,300 mm2 Cable | |
| AAC,AAAC |
| Span | Height of Tower |
| 400KV=400 Meter | 400KV=30Meter (Base 8.8 Meter) |
| 220KV=350 Meter | 220KV=23Meter (Base 5.2 Meter) |
| 132KV=335 Meter | 220KV Double Circuit=28 Meter |
| 66KV=210 Meter | 66KV=13Meter |
| Conductor Ampere | Voltage wise Conductor |
| Dog=300Amp | 400KV=Moose ACSR=500MVA Load |
| Panther=514Amp | 220KV=Zebra ACSR=200MVA Load |
| Zebra=720Amp | 132KV=Panther ACSR=75MVA Load |
| Rabbit=208Amp | 66KV=Dog ACSR=50MVA Load |
| Moose=218Amp |
Type of Tower:
| Type | Used | Angle/Deviation |
| A | Suspension Tower | Up to 2° |
| B | Small Angle Tower | 2° to 15° |
| C | Medium Angle Tower | 15° to 30° |
| D | Large Angle / Dead End Tower | 30° to 60° & Dead End |
Tower Swing Angle Clearance (Metal Part to Live Part):
| Swing Angle | Live Part to Metal Part Clearance (mm) | |||
| 66KV | 132KV | 220KV | 400KV | |
| 0° | 915mm | 1530mm | 2130mm | 3050mm |
| 15° | 915mm | 1530mm | 2130mm | – |
| 22° | – | – | – | 3050mm |
| 30° | 760mm | 1370mm | 1830mm | – |
| 44° | – | – | – | 1860mm |
| 44° | 610mm | 1220mm | 1675mm | – |
Cable Coding (IS 1554) 🙁 A2XFY / FRLS / FRPVC / FRLA
/ PILC)
| A | Aluminium |
| 2X | XLPE |
| F | Flat Armoured |
| W | Wire Armoured |
| Y | Outer PVC Insulation Sheath |
| W | Steel Round Wire |
| WW | Steel double round wire Armoured |
| YY | Steel double Strip Armoured |
| FR | Fire Retardation |
| LS | Low Smoke |
| LA | Low Acid Gas Emission |
| WA | Non Magnetic round wire Armoured |
| FA | Non Magnetic Flat wire Armoured |
| FF | Double Steel Round Wire Armoured |
Corona Ring Size:
| Voltage | Size |
| <170 KV | 160mm Ring put at HV end |
| >170 KV | 350mm Ring put at HV end |
| >275 KV | 450mm Ring put at HV end & 350 mm Ring put at Earth end |
Load as per Sq.Ft:
| Type of Load | Load/Sq.Ft | Diversity Factor |
| Industrial | 1000 Watt/Sq.Ft | 0.5 |
| Commercial | 30 Watt/Sq.Ft | 0.8 |
| Domestic | 15 Watt/Sq.Ft | 0.4 |
| Lighting | 15 Watt/Sq.Ft | 0.8 |
Size of Ventilation Shaft:
| Height of Building in meter | Size of ventilation shaft in sq meter | Minimum size of shaft in meter |
| 9.0 | 1.5 | 1.0 |
| 12.5 | 3.0 | 1.2 |
| 15 and above | 4.0 | 1.5 |
Electrical Thumb Rules-(Part-8)
October 1, 2013 6 Comments
Accuracy Class of Metering CT:
| Metering Class CT | |
| Class | Applications |
| 0.1 To 0.2 | Precision measurements |
| 0.5 | High grade kilowatt hour meters for commercial grade kilowatt hour meters |
| 3 | General industrial measurements |
| 3 OR 5 | Approximate measurements |
Accuracy Class Letter of CT:
| Metering Class CT | |
| Accuracy Class | Applications |
| B | Metering Purpose |
| Protection Class CT | |
| C | CT has low leakage flux. |
| T | CT can have significant leakage flux. |
| H | CT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.) |
| L | CT accuracy applies at the maximum rated secondary burden at 20 time rated only. The ratio accuracy can be up to four times greater than the listed value, depending on connected burden and fault current. (Typically window, busing, or bar-type CTs.) |
Accuracy Class of Protection CT:
| Class | Applications |
| 10P5 | Instantaneous over current relays & trip coils: 2.5VA |
| 10P10 | Thermal inverse time relays: 7.5VA |
| 10P10 | Low consumption Relay: 2.5VA |
| 10P10/5 | Inverse definite min. time relays (IDMT) over current |
| 10P10 | IDMT Earth fault relays with approximate time grading:15VA |
| 5P10 | IDMT Earth fault relays with phase fault stability or accurate time grading: 15VA |
Calculate IDMT over Current Relay
Setting (50/51)
October 11, 2013 18 Comments
• Calculate setting of IDMT over Current Relay for following Feeder and CT Detail
• Feeder Detail: Feeder Load Current 384 Amp, Feeder Fault current Min11KA and Max
22KA.
• CT Detail: CT installed on feeder is 600/1 Amp. Relay Error 7.5%, CT Error 10.0%, CT
over shoot 0.05 Sec, CT interrupting Time is 0.17 Sec and Safety is 0.33 Sec.
• IDMT Relay Detail:
• IDMT Relay Low Current setting: Over Load Current setting is 125%, Plug setting of
Relay is 0.8 Amp and Time Delay (TMS) is 0.125 Sec, Relay Curve is selected as
Normal Inverse Type.
• IDMT Relay High Current setting :Plug setting of Relay is 2.5 Amp and Time Delay
(TMS) is 0.100 Sec, Relay Curve is selected as Normal Inverse Type
Calculation of Over Current Relay Setting:
(1) Low over Current Setting: (I>)
• Over Load Current (In) = Feeder Load Current X Relay setting = 384 X 125% =480
Amp
• Required Over Load Relay Plug Setting= Over Load Current (In) / CT Primary
Current
• Required Over Load Relay Plug Setting = 480 / 600 = 0.8
• Pick up Setting of Over Current Relay (PMS) (I>)= CT Secondary Current X Relay
Plug Setting
• Pick up Setting of Over Current Relay (PMS) (I>)= 1 X 0.8 = 0.8 Amp
• Plug Setting Multiplier (PSM) = Min. Feeder Fault Current / (PMS X (CT Pri.
Current / CT Sec. Current))
• Plug Setting Multiplier (PSM) = 11000 / (0.8 X (600 / 1)) = 22.92
• Operation Time of Relay as per it’s Curve
• Operating Time of Relay for Very Inverse Curve (t) =13.5 / ((PSM)-1).
• Operating Time of Relay for Extreme Inverse Curve (t) =80/ ((PSM)2 -1).
• Operating Time of Relay for Long Time Inverse Curve (t) =120 / ((PSM) -1).
• Operating Time of Relay for Normal Inverse Curve (t) =0.14 / ((PSM) 0.02 -1).
• Operating Time of Relay for Normal Inverse Curve (t)=0.14 / ( (22.92)0.02-1) = 2.17
Amp
• Here Time Delay of Relay (TMS) is 0.125 Sec so
• Actual operating Time of Relay (t>) = Operating Time of Relay X TMS =2.17 X
0.125 =0.271 Sec
• Grading Time of Relay = [((2XRelay Error)+CT Error)XTMS]+ Over shoot+ CB Interrupting Time+ Safety
• Total Grading Time of Relay=[((2X7.5)+10)X0.125]+0.05+0.17+0.33 = 0.58 Sec
• Operating Time of Previous upstream Relay = Actual operating Time of Relay+ Total Grading Time Operating Time of Previous up Stream Relay = 0.271 + 0.58 = 0.85
Sec
(2) High over Current Setting: (I>>)
• Pick up Setting of Over Current Relay (PMS) (I>>)= CT Secondary Current X Relay Plug Setting
• Pick up Setting of Over Current Relay (PMS) (I>)= 1 X 2.5 = 2.5 Amp
• Plug Setting Multiplier (PSM) = Min. Feeder Fault Current / (PMS X (CT Pri.
Current / CT Sec. Current))
• Plug Setting Multiplier (PSM) = 11000 / (2.5 X (600 / 1)) = 7.33
• Operation Time of Relay as per it’s Curve
• Operating Time of Relay for Very Inverse Curve (t) =13.5 / ((PSM)-1).
• Operating Time of Relay for Extreme Inverse Curve (t) =80/ ((PSM)2 -1).
• Operating Time of Relay for Long Time Inverse Curve (t) =120 / ((PSM) -1).
• Operating Time of Relay for Normal Inverse Curve (t) =0.14 / ((PSM) 0.02 -1).
• Operating Time of Relay for Normal Inverse Curve (t)=0.14 / ( (7.33)0.02-1) = 3.44
Amp
• Here Time Delay of Relay (TMS) is 0.100 Sec so
• Actual operating Time of Relay (t>) = Operating Time of Relay X TMS =3.44 X
0.100 =0.34 Sec
• Grading Time of Relay = [((2XRelay Error)+CT Error)XTMS]+ Over shoot+ CB Interrupting Time+ Safety
• Total Grading Time of Relay=[((2X7.5)+10)X0.100]+0.05+0.17+0.33 = 0.58 Sec
• Operating Time of Previous upstream Relay = Actual operating Time of Relay+ Total Grading Time.
• Operating Time of Previous up Stream Relay = 0.34 + 0.58 = 0.85 Sec
Conclusion of Calculation:
• Pickup Setting of over current Relay (PMS) (I>) should be satisfied following Two
Condition.
• (1) Pickup Setting of over current Relay (PMS)(I>) >= Over Load Current (In) / CT Primary Current
• (2) TMS <= Minimum Fault Current / CT Primary Current
• For Condition (1) 0.8 > =(480/600) = 0.8 >= 0.8, Which found OK
• For Condition (2) 0.125 <= 11000/600 = 0.125 <= 18.33, Which found OK
• Here Condition (1) and (2) are satisfied so
• Pickup Setting of Over Current Relay = OK
• Low Over Current Relay Setting: (I>) = 0.8A X In Amp
• Actual operating Time of Relay (t>) = 0.271 Sec
• High Over Current Relay Setting: (I>>) = 2.5A X In Amp
• Actual operating Time of Relay (t>>) = 0.34 Sec
Selection of 3P-TPN-4P MCB & Distribution Board
November 1, 2013 28 Comments
Type of breakers based on number of pole:
• Based on the number of poles, the breakers are classified as
1. SP – Single Pole
2. SPN – Single Pole and Neutral
3. DP – Double pole
4. TP – Triple Pole
5. TPN – Triple Pole and Neutral
6. 4P – Four Pole
1. SP ( Single Pole ) MCB:
• In Single Pole MCCB, switching & protection is affected in only one phase.
• Application: Single Phase Supply to break the Phase only.
2. DP ( Double Pole ) MCB:
• In Two Pole MCCB, switching & protection is affected in phases and the neutral.
• Application: Single Phase Supply to break the Phase and Neutral.
3. TP ( Triple Pole) MCB:
• In Three Pole MCB, switching & protection is affected in only three phases and the neutral is not part of the MCB.
• 3pole MCCB signifies for the connection of three wires for three phase system (R-Y-B Phase).
• Application: Three Phase Supply only (Without Neutral).
4. TPN (3P+N) MCB:
• In TPN MCB, Neutral is part of the MCB as a separate pole but without any protective given in the neutral pole (i.e.) neutral is only switched but has no protective element incorporated.
• TPN for Y (or star) the connection between ground and neutral is in many countries not allowed. Therefore the N is also switches.
• Application: Three Phase Supply with Neutral
5. 4 Pole MCB:
• 4pole MCCB for 4 wires connections, the one additional 4th pole for neutral wire connection so that between neutral and any of the other three will supply.
• In 4-Pole MCCBs the neutral pole is also having protective release as in the phase poles.
• Application: Three Phase Supply with Neutral
Difference between TPN and 4P (or SPN and DP):
• TPN means a 4 Pole device with 4th Pole as Neutral. In TPN opening & closing will open & close the Neutral.
• For TPN, protection applies to the current flows through only 3 poles (Three Phase) only; there is no protection for the current flow through the neutral pole. Neutral is just an isolating pole.
• TP MCB is used in 3phase 4wire system. It is denoted as TP+N which will mean a three pole device with external neutral link which can be isolated if required.
• For the 4 pole breakers, protection applies to current flow through all poles. However when breaker trips or manually opened, all poles are disconnected.
• Same type of difference also applies for SPN and DP.
Where to Use TP, TPN and 4P in Distribution panel:
• For any Distribution board, the protection system (MCB) must be used in the incomer.
For a three phase distribution panel either TP or TPN or 4P can be used as the incoming protection.
• TP MCB: It is most commonly used type in all ordinary three phase supply.
• TPN MCB: It is generally used where there are dual sources of incomer to the panel
(utility source and emergency generator source).
• 4P MCB: It is used where is the possibility of high neutral current (due to unbalance loads and /or 3rd and multiple of 3rd harmonics current etc) and Neutral / Earth Protection is provided on Neutral.
Where to use 4 Pole or TPN MCB instead of 3 Pole (TP) MCB.
• Multiple Incoming Power System:
• When we have a transformer or a stand-by generator feeding to a bus, it is mandatory that at least either of the Incomers or the bus coupler must be TPN or 4-Pole Breaker please
refers IS 3043.
• In multi incomer power feeding systems, we cannot mix up the neutrals of incoming
powers to other Power Source so we can use TPN or 4P breakers or MCB instead of TP MCB to isolate the Neutral of other power sources from the Neutral of incomer power in use.
• We can use 4 Pole ACB instead of TP for safety reasons .If there is power failure and DG
sets are in running condition to feed the loads, if there is some unbalance in loads(which
is practically unavoidable in L.V. distribution system ), depending of quantum of
unbalance, there will be flow of current through Neutral. During this time, if Power Supply Utility Technicians are working, and if they touch the neutral conductors(which is earthed at their point ) they will likely to get electric shock depending on the potential
rise in common neutral due flow of current through Neutral conductor as stated above. Even fatal accident may occur due the above reason. As such, it is a mandatory practice to isolate the two Neutrals.
• We can use 4-pole breakers or TPN Breakers when the system has two alternative sources and, in the event of power failure from the mains, change-over to the standby
generator is done. In such a case, it is a good practice to isolate the neutral also.
• 4 pole circuit breakers have advantages in the case when one of the poles of the device will get damage, and it also provides isolation from neutral voltage.
• Normally, Neutral is not allowed to break in any conditions, (except special applications)
for human & equipment safety. So for single incomer power fed systems, 3P breaker is
used, where only phases are isolated during breaking operations.
• Where We have dual Power like in DG & other electricity supply sources ,it is required
to isolate neutral, where neutral needs to be isolated in internal network TPN MCB or 4P MCB can be used.
Where to use 4 Pole MCB instead of TPN MCB
• Any Protection Relay used on Neutral (Ground Fault Protection of Double ended
System):
• The use of four poles or three poles CB will depend on system protection and system configuration.
• Normally in 3phase with neutral we just use 3pole CB and Neutral is connected on common Neutral Link but if application of 3pole will affect the operation of protective
relay then we must use 4pole CB.
• System evaluation has to be required to decide whether three-pole circuit breakers plus
neutral link can be used or four-pole breakers are required.
• If unrestricted ground fault protection is fitted to the transformer neutral, then the bus section circuit breaker should have 4-poles and preferably incomer circuit breakers should also have 4-poles because un cleared ground fault located at the load side of a
feeder have two return paths. As shown in fig a ground fault on a feeder at the bus section
“A” will have a current return path in both the incomers, thus tripping both Bus. The
sensitivity of the unrestricted ground fault relay is reduced due to the split current paths.
• For System Stability :
• In an unbalanced 3phase system or a system with non-linear loads, the neutral gives the safety to the unbalanced loads in the system and therefore It must not be neglected. In
perfectly balanced conditions the neutral functions as a safety conductor in the unforeseen short-circuit and fault conditions. Therefore by using 4-pole MCB will enhance the system stability.
• 4 Poles will be decided after knowing the Earthing Systems (TT, TN-S, TN-C, IT).
(1) IT (with distributed neutral) System:
• The Neutral should be switched on & off with phases.
• Required MCB: TPN or 4P MCB.
(2) IT (without distributed neutral) System:
• There is no neutral.
• Required MCB: TP MCB.
(3) TN-S System:
• Required MCB: TP MCB because even when neutral is cut off system remains connected with Ground.
(4) TN-C System:
• Required MCB: TPN or 4P only, because we cannot afford to cut neutral doing so will result in system loosing contact with Ground.
(5) TN-C-S System:
• Neutral and Ground cable are separate
• Required MCB: TP MCB Because Neutral and Ground cable are separate.
(6) TT System:
• Ground is provided locally
• Required MCB: TP MCB because ground is provided locally.
• Conclusion: Its compulsory to use TPN in TN-C system rest everywhere you can use
MCB.
Nomenclature of Distribution Board:
• Distribution Box can be decided by “way” means how many how many single phase
(single pole) distribution. Circuit and Neutral are used.
1) SPN Distribution Board (Incoming+ Outgoing)
• 4way (Row) SPN = 4 X 1SP= 4Nos (Module) of single pole MCB as outgoing feeders.
• 6way (Row) SPN = 6 X 1SP= 6Nos (Module) of single pole MCB as outgoing feeders.
• 8way (Row) SPN = 8 X 1SP= 8Nos (Module) of single pole MCB as outgoing feeders.
• 10way (Row) SPN = 10 X 1SP= 10Nos (Module) of single pole MCB as outgoing feeders.
• 12way (Row) SPN = 12 X 1SP= 12Nos (Module) of single pole MCB as outgoing
feeders.
• Normally single phase distribution is mainly used for small single phase loads at house wiring or industrial lighting wiring.
2) TPN Distribution Board (Incoming, Outgoing)
• 4way (Row) TPN = 4 X TP= 4nos of 3pole MCB as outgoing feeders =12 No of single pole MCB.
• 6way (Row) TPN = 6 X TP= 6nos of 3pole MCB as outgoing feeders =18 No of single pole MCB.
• 8way (Row) TPN = 8 X TP= 8nos of 3pole MCB as outgoing feeders =24 No of single pole MCB.
• 10way (Row) TPN = 10 X TP= 10nos of 3pole MCB as outgoing feeders =30 No of
single pole MCB.
• 12way (Row) TPN =12 X TP= 12nos of 3pole MCB as outgoing feeders =36 No of single pole MCB
33)Transformer Losses-Regulation-
Efficiency(TC Name Plate)
December 31, 2013 9 Comments
Calculate Transformer Losses- Regulation- Efficiency (From TC Name Plate Data)
• Calculate No Load Losses at various Loading of Transformer.
• Calculate Full Load Losses at various Loading of Transformer.
• Calculate Percentage Impedance
• Calculate Transformer regulation at various Power Factor.
• Calculate Transformer Efficiency at Unity P.F at various Loading condition.
• Calculate Transformer Efficiency at various P.F at various Loading condition
Calculate Size and Short Circuit Capacity of
D.G Synchronous Panel
January 17, 2014 7 Comments
• Calculate Size of D.G Synchronous Panel.
• Calculate Total Fault Current of D.G Synchronous Panel.
• Total Equivalent Impedance of D.G Synchronous Panel.
• Calculate Short Circuit Current of D.G Synchronous Panel.
Electrical Thumbs Rules (Part-9)
March 15, 2014 7 Comments
| Load in Multi-storied Building (Madhyanchal Vidyut Vitran Nigam) | ||
| Type of Load | Calculation | Diversity |
| Domestic (Without Common Area) | 50 watt / sq. meters | 0.5 |
| Commercial (Without Common Area) | 150 watt / sq. meters | 0.75 |
| Lift, Water Pump, Streetlight ,Campus Lighting ,Common Facilities, | Actual load shall be calculated | 0.75 |
Load in Multi-storied Building (Noida Power Company
Limited)
| Type of Load | Calculation | Diversity |
| Domestic (Constructed area) | 15 watt / sq. Foot | 0.4 |
| Commercial(Constructed area) | 30 watt / sq. Foot | 0.8 |
| Industrial (Constructed area) | 100 watt/ 1 sq. Foot | 0.5 |
| Lift, Water Pump, Streetlight ,Campus Lighting ,Common Facilities, | 0.5Kw / Flat | |
| Voltage Drop: 2% Voltage drop from Transformer to Consumer end. | ||
| T&D Losses: 2% T&D Losses from Transformer to Consumer end. | ||
| Approximate % Cost or Sq.Foot Cost | ||
| Project Item | % of Total Project Cost | Rs per Sq.Foot |
| Articheture (Consultancy) | 0.7% | 13.1 Rs / Sq.Foot |
| Structural (Consultancy) | 1.2% | 21.8 Rs / Sq.Foot |
| Service Design (Consultancy) | 0.4% | 7.2 Rs / Sq.Foot |
| Fire Fighting Work | 1.3% | 23 Rs / Sq.Foot |
| Electrical Work (Internal) | 4.1% | 76 Rs / Sq.Foot |
| Lift Work | 4.4% | 82 Rs / Sq.Foot |
| Street Light Costing (CPWD-2012) | |
| Fluorescent Lamp | 95 Rs/Sq.Meter |
| With HPMV Lamp | 130 Rs/Sq.Meter |
| With HPSV Lamp | 165 Rs/Sq.Meter |
| Electrical Sinage | 85 Rs/Sq.Meter |
| Other Electrical Cost | |
| Area Required for Solar Light | 10 Watt/Sq.Foot |
| Solar Power Installation | 1.5 Lacs Rs/1Kw |
| HVAC Cost | 18 Watt/Sq.Foot |
Distribution Losses (Gujarat Electricity Board)
| Voltage (Point of Injection) | At 11 KV | Point of Energy Delivered |
| 11KV / 22KV / 33KV | 10% | 1082% |
| 400 Volt | – | 16.77% |
| Rate Analysis (CPWD-2012) | |
| Description | Amount |
| Sub Station Equipment | 7000 Rs/ KVA |
| D.G Set with installation | 1000 Rs / KVA |
| UPS with 30min Breakup | 20000 Rs / KVA add 8000 Rs / KVA additional each 30 min |
| Solar Power Generation | 1.25 Lacs / KW |
| Solar Water System (200Liter/Day) | 46000 Rs |
| Solar Water System (300Liter/Day) | 64000 Rs |
| Solar Water System (1000Liter/Day) | 210000 Rs |
| Central AC Plant | 75000 RS / Ton |
| VRF / VRV System | 55000 Rs / HP |
| Air condition System | 11000 Rs / Ton |
| CCTV System | 300 Rs / Sq Meter |
| Access Control system | 200 Rs / Sq Meter |
| Hydropenumatic Water system | 2000 Rs / LPM |
| Building Management System | 300 Rs / Sq Meter add 100 Rs / Sq Meter additional area beyond 10000 Sq Meter |
| Rate Analysis (Rs per Sq. Meter) (CPWD-2012) | ||||
| Work | Office/College/Hospital | School | Hostel | Residence |
| Fire Fighting (with Wet Riser) | 500 | 500 | 500 | 500 |
| Fire Fighting (with Sprinkler) | 750 | 750 | 750 | 750 |
| Fire Alarm (Manually) | – | – | – | 300 |
| Fire Alarm (Automatic) | 500 | 500 | 500 | 500 |
| Pressurized Mechanical Ventilation | 650 | 650 | 650 | 650 |
| Rate Analysis (% of Total Project Cost) (CPWD-2012) | ||||
| Work | Office/College/Hospital | School | Hostel | Residence |
| Internal Water Supply & Sanitary | 4% | 10% | 5% | 12% |
| Internal Electrical Installation | 12.5% | 12.5% | 12.5% | 12.5% |
| Lift Speed (Indian Army Manual) | |
| No of Floor | Lift Speed |
| 4 to 5 | 0.5 to 0.7 meter/Sec |
| 6 to 12 | 0.75 to 1.5 meter/Sec |
| 3 to 20 | 1.5 to 2.5 meter/Sec |
| Above 20 | Above 2.5 meter/Sec |
| Lift Details (CPWD-2012) | ||||||
| Type of Lift | Persons | Weight | Speed M/Sec | Travel | Price | Add Rs /Floor |
| Passenger Lift | 8 Person | 544 Kg | 1.0 | G+4 | 18 Lacs | 1.25 Lacs |
| Passenger Lift | 13 Person | 844 Kg | 1.5 | G+4 | 22 Lacs | 1.25 Lacs |
| Passenger Lift | 16 Person | 1088 Kg | 1.0 | G+4 | 28 Lacs | 1.50 Lacs |
| Passenger Lift | 120 Person | 1360 Kg | 1.5 | G+4 | 24 Lacs | 1.50 Lacs |
| MCB Class according to Appliances | |||
| Appliance | Capacity / watt | MCB Rating | MCB Class |
| Air Conditioner | 1.0 Tone | 10A | C Class |
| 1.5 Tone | 16A | C Class | |
| 2.0 Tone | 20A | C Class | |
| Freeze | 165 Liter | 3 A | C Class |
| 350 Liter | 4 A | C Class | |
| Oven /Grill | 4500 Watt | 32 A | B Class |
| 1750 Watt | 10 A | B Class | |
| Oven / Hotplate | 750 Watt | 6 A | B Class |
| 2000 Watt | 10 A | B Class | |
| Room Heater | 1000 Watt | 6 A | B Class |
| 2000 Watt | 10 A | B Class | |
| Washing Machine | 300 Watt | 2 A | C Class |
| 1300 Watt | 8 A | C Class | |
| Water Heater | 1000 Watt | 6 A | B Class |
| 2000 Watt | 10 A | B Class | |
| 3000 Watt | 16 A | B Class | |
| 6000 Watt | 32 A | B Class | |
| Iron | 750 Watt | 6 A | B Class |
| 1250 Watt | 8 A | B Class | |
| Toaster | 1200 Watt | 8 A | B Class |
| 1500 Watt | 10 A | B Class |
Electrical Thumbs Rules (Part-10)
March 18, 2014 9 Comments
Economical Voltage for Power Transmission:
• Economic generation voltage is generally limited to following values (CBIP Manual).
| Economic generation voltage (CBIP Manual) | |
| Total Load | Economical Voltage |
| Up to 750 KVA | 415 V |
| 750 KVA to 2500 KVA | 3.3 KV |
| 2500 KVA to 5000 KVA | 6.6 KV |
| Above 5000 KVA | 11 KV or Higher |
• Generally terminal voltage of large generators is 11 kV in India. Step up voltage depends upon Length of transmission line for interconnection with the power system and Power to be transmitted.
• High voltage increases cost of insulation and support structures for increased clearance
for air insulation but decreases size and hence Cost of conductors and line losses.
• Many empirical relations have been evolved to approximately determine economic voltages for power evacuation. An important component in transmission lines is labor costs which are country specific.
• An empirical relation is given below.
• Voltage in kV (line to line) = 5.5x√0.62L + kVA/150
• where kVA is total power to be transmitted;
• L is length of transmission line in km.
• American practice for economic line to line voltage kV (based on empirical formulation)
is
• Voltage in kV line to line = 5.5x√0.62L + 3P/100
• For the purpose of standardization in India transmission lines may be classified for operating at 66 kV and above. 33 kV is sub transmission, 11 kV and below may be classified as distribution.
• Higher voltage system is used for transmitting higher amounts of power and longer lengths and its protection is important for power system security and requires complex
relay systems.
| Required Power Transfer (MW) | Distance (KM) | Economical Voltage Level (KM) |
| 3500 | 500 | 765 |
| 500 | 400 | 400 |
| 120 | 150 | 220 |
| 80 | 50 | 132 |
Factor affected on Voltage Level of system:
• Power carrying capability of transmission lines increases roughly as the square of the voltage. Accordingly disconnection of higher voltage class equipment from bus bars get increasingly less desirable with increase in voltage levels.
• High structures are not desirable in earthquake prone areas. Therefore in order to obtain lower structures and facilitate maintenance it is important to design such sub-stations
preferably with not more than two levels of bus bars.
Size of Cable according to Short circuit (for 11kV,3.3kV
only)
• Short circuit verification is performed by using following formula:
• Cross Section area of Cable (mm2)S = I x√t / K
• Where:
• t = fault duration (S)
• I = effective short circuit current (kA)
• K = 0.094 for aluminum conductor insulated with XLPE
• Example: Fault duration(t)= 0.25sec,Fault Current (I) = 26.24 kA
• Cross Section area of Cable = 26.24 x √ (0.25) / 0.094= 139.6 sq. mm
• The selected cross sectional area is 185 sq. mm.
Ground Clearance:
• Ground Clearance in Meter = 5.812 + 0.305 X K
• Where K= (Volt-33) / 33
| Voltage Level | Ground Clearance |
| <=33KV | 5.2 Meter |
| 66KV | 5.49 Meter |
| 132KV | 6.10 Meter |
| 220KV | 7.0 Meter |
| 400KV | 8.84 Meter |
Voltage Rise in Transformers due to Capacitor Bank:
• The voltage drop and rise on the power line and drop in the transformers. Every transformer will also experience a voltage rise from generating source to the capacitors. This rise is independent of load or power factor and may be determined as follows:
• % Voltage Rise in Transformer=(Kvar / Kva)x Z
• Kvar =Applied Kvar
• Kva = Kva of the transformer
• z = Transformer Reactance in %
• Example: 300 Kvar bank given to 1200 KVA transformer with 5.75% reactance.
• % Voltage Rise in Transformer=(300/1200)x 5.75 =1.43%
Calculate Size of Capacitor Bank / Annual
Saving & Payback Period
April 1, 2014 5 Comments
• Calculate Size of Capacitor Bank Annual Saving in Bills and Payback Period for
Capacitor Bank.
• Electrical Load of (1) 2 No’s of 18.5KW,415V motor ,90% efficiency,0.82 Power Factor
,(2) 2 No’s of 7.5KW,415V motor ,90% efficiency,0.82 Power Factor,(3) 10KW ,415V
Lighting Load. The Targeted Power Factor for System is 0.98.
• Electrical Load is connected 24 Hours, Electricity Charge is 100Rs/KVA and 10Rs/KW.
• Calculate size of Discharge Resistor for discharging of capacitor Bank. Discharge rate of
Capacitor is 50v in less than 1 minute.
• Also Calculate reduction in KVAR rating of Capacitor if Capacitor Bank is operated at frequency of 40Hz instead of 50Hz and If Operating Voltage 400V instead of 415V.
• Capacitor is connected in star Connection, Capacitor voltage 415V, Capacitor Cost is
60Rs/Kvar. Annual Deprecation Cost of Capacitor is 12%.
Calculation:
• For Connection (1):
• Total Load KW for Connection(1) =Kw / Efficiency=(18.5×2) / 90=41.1KW
• Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 41.1 /0.82=50.1
KVA
• Total Load KVA (new) for Connection(1)= KW /New Power Factor= 41.1 /0.98=
41.9KVA
• Total Load KVAR= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f])
• Total Load KVAR1=41.1x([(√1-(0.82)2) / 0.82]- [(√1-(0.98)2) / 0.98])
• Total Load KVAR1=20.35 KVAR
• OR
• tanǾ1=Arcos(0.82)=0.69
• tanǾ2=Arcos(0.98)=0.20
• Total Load KVAR1= KWX (tanǾ1- tanǾ2) =41.1(0.69-0.20)=20.35KVAR
• For Connection (2):
• Total Load KW for Connection(2) =Kw / Efficiency=(7.5×2) / 90=16.66KW
• Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 16.66 /0.83=20.08
KVA
• Total Load KVA (new) for Connection(1)= KW /New Power Factor= 16.66 /0.98=
17.01KVA
• Total Load KVAR2= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f])
• Total Load KVAR2=20.35x([(√1-(0.83)2) / 0.83]- [(√1-(0.98)2) / 0.98])
• Total Load KVAR2=7.82 KVAR
• For Connection (3):
• Total Load KW for Connection(2) =Kw =10KW
• Total Load KVA (old) for Connection(1)= KW /Old Power Factor= 10/0.85=11.76 KVA
• Total Load KVA (new) for Connection(1)= KW /New Power Factor= 10 /0.98=
10.20KVA
• Total Load KVAR3= KWX([(√1-(old p.f)2) / old p.f]- [(√1-(New p.f)2) / New p.f])
• Total Load KVAR3=20.35x([(√1-(0.85)2) / 0.85]- [(√1-(0.98)2) / 0.98])
• Total Load KVAR1=4.17 KVAR
• Total KVAR=KVAR1+ KVAR2+KVAR3
• Total KVAR=20.35+7.82+4.17
• Total KVAR=32 Kvar
Size of Capacitor Bank:
• Site of Capacitor Bank=32 Kvar.
• Leading KVAR supplied by each Phase= Kvar/No of Phase
• Leading KVAR supplied by each Phase =32/3=10.8Kvar/Phase
• Capacitor Charging Current (Ic)= (Kvar/Phase x1000)/Volt
• Capacitor Charging Current (Ic)= (10.8×1000)/(415/√3)
• Capacitor Charging Current (Ic)=44.9Amp
• Capacitance of Capacitor = Capacitor Charging Current (Ic)/ Xc
• Xc=2×3.14xfxv=2×3.14x50x(415/√3)=75362
• Capacitance of Capacitor=44.9/75362= 5.96µF
• Required 3 No’s of 10.8 Kvar Capacitors and
• Total Size of Capacitor Bank is 32Kvar
Protection of Capacitor Bank
Size of HRC Fuse for Capacitor Bank Protection:
• Size of the fuse =165% to 200% of Capacitor Charging current.
• Size of the fuse=2×44.9Amp
• Size of the fuse=90Amp
Size of Circuit Breaker for Capacitor Protection:
• Size of the Circuit Breaker =135% to 150% of Capacitor Charging current.
• Size of the Circuit Breaker=1.5×44.9Amp
• Size of the Circuit Breaker=67Amp
• Thermal relay setting between 1.3 and 1.5of Capacitor Charging current.
• Thermal relay setting of C.B=1.5×44.9 Amp
• Thermal relay setting of C.B=67 Amp
• Magnetic relay setting between 5 and 10 of Capacitor Charging current.
• Magnetic relay setting of C.B=10×44.9Amp
• Magnetic relay setting of C.B=449Amp
Sizing of cables for capacitor Connection:
• Capacitors can withstand a permanent over current of 30% +tolerance of 10% on capacitor Current.
• Cables size for Capacitor Connection= 1.3 x1.1 x nominal capacitor Current
• Cables size for Capacitor Connection = 1.43 x nominal capacitor Current
• Cables size for Capacitor Connection=1.43×44.9Amp
• Cables size for Capacitor Connection=64 Amp
Maximum size of discharge Resistor for Capacitor:
• Capacitors will be discharge by discharging resistors.
• After the capacitor is disconnected from the source of supply, discharge resistors are required for discharging each unit within 3 min to 75 V or less from initial nominal peak voltage (according IEC-standard 60831).
• Discharge resistors have to be connected directly to the capacitors. There shall be no switch, fuse cut-out or any other isolating device between the capacitor unit and the discharge resistors.
• Max. Discharge resistance Value (Star Connection) = Ct / Cn x Log (Un x√2/ Dv).
• Max. Discharge resistance Value (Delta Connection)= Ct / 1/3xCn x Log (Un x√2/
Dv)
• Where Ct =Capacitor Discharge Time (sec)
• Cn=Capacitance Farad.
• Un = Line Voltage
• Dv=Capacitor Discharge voltage.
• Maximum Discharge resistance =60 / ((5.96/1000000)x log ( 415x√2 /50)
• Maximum Discharge resistance=4087 KΩ
Effect of Decreasing Voltage & Frequency on Rating of Capacitor:
• The kvar of capacitor will not be same if voltage applied to the capacitor and frequency changes
• Reduced in Kvar size of Capacitor when operating 50 Hz unit at 40 Hz
• Actual KVAR = Rated KVAR x(Operating Frequency / Rated Frequency)
• Actual KVAR = Rated KVAR x(40/50)
• Actual KVAR = 80% of Rated KVAR
• Hence 32 Kvar Capacitor works as 80%x32Kvar= 26.6Kvar
• Reduced in Kvar size of Capacitor when operating 415V unit at 400V
• Actual KVAR = Rated KVAR x(Operating voltage / Rated voltage)^2
• Actual KVAR = Rated KVAR x(400/415)^2
• Actual KVAR=93% of Rated KVAR
• Hence 32 Kvar Capacitor works as 93%x32Kvar= 23.0Kvar
Annual Saving and Pay Back Period
Before Power Factor Correction:
• Total electrical load KVA (old)= KVA1+KVA2+KVA3
• Total electrical load= 50.1+20.08+11.76
• Total electrical load=82 KVA
• Total electrical Load KW=kW1+KW2+KW3
• Total electrical Load KW=37+15+10
• Total electrical Load KW =62kw
• Load Current=KVA/V=80×1000/(415/1.732)
• Load Current=114.1 Amp
• KVA Demand Charge=KVA X Charge
• KVA Demand Charge=82x60Rs
• KVA Demand Charge=8198 Rs
• Annual Unit Consumption=KWx Daily usesx365
• Annual Unit Consumption=62x24x365 =543120 Kwh
• Annual charges =543120×10=5431200 Rs
• Total Annual Cost= 8198+5431200
• Total Annual Cost before Power Factor Correction= 5439398 Rs
After Power Factor Correction:
• Total electrical load KVA (new)= KVA1+KVA2+KVA3
• Total electrical load= 41.95+17.01+10.20
• Total electrical load=69 KVA
• Total electrical Load KW=kW1+KW2+KW3
• Total electrical Load KW=37+15+10
• Total electrical Load KW =62kw
• Load Current=KVA/V=69×1000/(415/1.732)
• Load Current=96.2 Amp
• KVA Demand Charge=KVA X Charge
• KVA Demand Charge=69x60Rs =6916 Rs————-(1)
• Annual Unit Consumption=KWx Daily usesx365
• Annual Unit Consumption=62x24x365 =543120 Kwh
• Annual charges =543120×10=5431200 Rs—————–(2)
• Capital Cost of capacitor= Kvar x Capacitor cost/Kvar = 82 x 60= 4919 Rs—(3)
• Annual Interest and Deprecation Cost =4919 x 12%=590 Rs—–(4)
• Total Annual Cost= 6916+5431200+4919+590
• Total Annual Cost After Power Factor Correction =5438706 Rs
Pay Back Period:
• Total Annual Cost before Power Factor Correction= 5439398 Rs
• Total Annual Cost After Power Factor Correction =5438706 Rs
• Annual Saving= 5439398-5438706 Rs
• Annual Saving= 692 Rs
• Payback Period= Capital Cost of Capacitor / Annual Saving
• Payback Period= 4912 / 692
• Payback Period = 7.1 Years
Calculate No of Lighting Fixtures / Lumen for Indoor Lighting
April 8, 2014 6 Comments
• An office area is 20meter (Length) x 10meter (width) x 3 Meter (height). The ceiling to desk height is 2 meters. The area is to be illuminated to a general level of 250 lux using twin lamp 32 watt CFL luminaires with a SHR of 1.25. Each lamp has an initial output (Efficiency) of 85 lumen per watt. The lamps Maintenance Factor (MF) is 0.63
,Utilization Factor is 0.69 and space height ratio (SHR) is 1.25
Calculation:
Calculate Total Wattage of Fixtures:
• Total Wattage of Fixtures= No of Lamps X each Lamp’s Watt.
• Total Wattage of Fixtures=2×32=64Watt.
Calculate Lumen per Fixtures:
• Lumen per Fixtures = Lumen Efficiency(Lumen per Watt) x each Fixture’s Watt
• Lumen per Fixtures= 85 x 64 = 5440Lumen
Calculate No’s of Fixtures:
• Required No of Fixtures = Required Lux x Room Area / MFxUFx Lumen per
Fixture
• Required No of Fixtures =(250x20x10) / (0.63×0.69×5440)
• Required No of Fixtures =21 No’s
Calculate Minimum Spacing Between each Fixture:
• The ceiling to desk height is 2 meters and Space height Ratio is 1.25 so
• Maximum spacing between Fixtures =2×1.25=2.25meter.
Calculate No of Row Fixture’s Row Required along with width of Room:
• Number of Row required = width of Room / Max. Spacing= 10/2.25
• Number of Row required=4.
Calculate No of Fixture’s required in each Row:
• Number of Fixture Required in each Row = Total Fixtures / No of Row = 21/4
• Number of Fixture Required in each Row = 5 No’s:
Calculate Axial Spacing between each Fixture:
• Axial Spacing between Fixtures = Length of Room / Number of Fixture in each Row
• Axial Spacing between Fixtures =20 / 5 = 4 Meter
Calculate Transverse Spacing between each Fixture:
• Transverse Spacing between Fixtures = width of Room / Number of Fixture’s row
• Transverse Spacing between Fixtures = 10 / 4 = 2.5 Meter.
Conclusion:
• No of Row for Lighting Fixture’s= 4 No
• No of Lighting Fixtures in each Row= 5 No
• Axial Spacing between Fixtures= 4.0 Meter
• Transverse Spacing between Fixtures= 2.5 Meter
• Required No of Fixtures =21 No’s