Electrical Thumb Rules

Electrical Thumb Rules

Title: Illuminating Insights: Electrical Basics and General Thumb Rules

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.

  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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


•   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


•   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=0.746KWStar Connection
1KW=1.36HPLine Voltage=√3 Phase Voltage
1Watt=0.846 Kla/HrLine Current=Phase Current
1Watt=3.41 BTU/HrDelta Connection
1KWH=3.6 MJLine Voltage=Phase Voltage
1Cal=4.186 JLine 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


•   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


•   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 KVA5% of KVA Rating
315 KVA to 1000 KVA6% of KVA Rating
>1000 KVA8% of KVA Rating

Earthing Resistance value:

Earthing Resistance Value

Power Station0.5 Ω
Sub Station Major1.0 Ω
Sub Station Minor2.0 Ω
Distribution Transformer5.0 Ω
Transmission Line10 Ω
Single Isolate Earth Pit5.0 Ω
Earthing Grid0.5 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:

Class A105°C
Class E120°C
Class B130°C
Class F155°C
Class H180°C
Class N200°C

Standard Voltage Limit:

VoltageIEC (60038)IECIndian Elect.
ELV< 50 V  
LV50 V to 1 KV<=1 KV< 250 V
MV <= 35 KV250 V to 650 V
HV> 1KV<= 230 KV650 V to 33 KV
EHV > 230 KV> 33 KV

Standard Electrical Connection (As per GERC):

As per Type of Connection
LT Connection<=440V
HT connection440V to 66KV
EHT connection>= 66KV
As per Electrical Load Demand
Up 6W Load demand1 Phase 230V Supply
6W to 100KVA(100KW)3 Phase 440V Supply
100KVA to 2500KVA11KV,22KV,33KV
Above 2500KVA66KV
HT Connection Earthing
H.T Connection’s Earthing Strip20mmX4mm Cu. Strip
CT & PT bodies2Nos
PT Secondary1Nos
CT Secondary1Nos
I/C and O/G Cable+ Cubicle Body2Nos

Standard Meter Room Size (As per GERC):

Meter Box HeightUpper level does not beyond 1.7 meter and Lower level should not below 1.2 meter from ground.
Facing of Meter BoxMeter Box should be at front area of Building at Ground Floor.
Meter Room / Closed Shade4 meter square Size

Approximate Load as per Sq.ft Area (As per DHBVN):

Sq.ft AreaRequired Load (Connected)
< 900 Sq.ft8 KW
901 Sq.ft to 1600 Sq.ft16 KW
1601 Sq.ft to 2500 Sq.ft20 KW
> 2500 Sq.ft24 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 Load500 watt per 100 Sq. foot of the constructed area.
For Commercial1500 watt per 100 Sq. foot of the constructed area
  Other Common LoadFor lift, water lifting pump, streetlight if any, corridor/campus lighting and other common facilities, actual load shall be calculated
Staircase Light11KW/Flat Ex: 200Flat=200×11=2.2KW
Sanctioned Load for Building
Up to 50 kWThe 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
VoltageSub Station CapacityShort Circuit Current
400 KVUp to 1000 MVA40 KA  (1 to 3 Sec)
220 KVUp to 320 MVA40 KA  (1 to 3 Sec)
132 KVUp to 150 MVA32 KA  (1 to 3 Sec)
66 KVUp to 80 MVA25 KA  (1 to 3 Sec)
33 KV1.5 MVA to 5 MVA35 KA (Urban) (1 to 3 Sec)
11 KV150 KVA to 1.5 MVA25 KA (Rural) (1 to 3 Sec)
415 V6  KVA to 150 KVA10 KA  (1 to 3 Sec)
220 VUp to 6 KVA6 KA  (1 to 3 Sec)

Sub Station Capacity & Short Circuit Current Capacity:

As per Central Electricity Authority
VoltageSub Station CapacityShort Circuit Current
765 KV4500 MVA31.5 KA for 1 Sec
400 KV1500 MVA31.5 KA for 1 Sec
220 KV500 MVA40 KA for 1 Sec
110/132 KV150 MVA40 KA or 50 KA for 1 Sec
66 KV75 MVA40 KA or 50 KA for 1 Sec

Minimum Ground Clearance and Fault Clearing Time:

VoltageMin. Ground ClearanceFault Clear Time
400 KV8.8 Meter100 mille second
220 KV8.0 Meter120 mille second
132 KV6.1 Meter160 mille second
66 KV5.1 Meter300 mille second
33 KV3.7 Meter 
11 KV2.7 Meter 

Bus bar Ampere Rating:

For Phase Bus barAluminium 130 Amp / Sq.cm or 800Amp / Sq.inch.
For Phase Bus barCopper 160 Amp / Sq.cm or 1000Amp / Sq.inch
For Neutral Bus barSame 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 Earth26mm (Min)
Between Phase and Phase32mm (Min)

Bus bar Support between Two



Sound Level of Diesel Generator (ANSI 89.2&NEMA


KVAMax. Sound Level
<9 KVA40 DB
10 KVA to 50 KVA45 DB
51 KVA to 150 KVA50 DB
151 KVA to 300 KVA55 DB
301 KVA to 500 KVA60 DB

IR Value of Transformer:

IR Value of Transformer
22KV to 33KV500MΩ250MΩ125MΩ
6.6KV to 11KV400MΩ200MΩ100MΩ

Standard Size of MCB/MCCB/ELCB/RCCB/SFU/Fuse:

MCBUp to 63 Amp (80Amp and 100 Amp aper Request)
MCCBUp to 1600 Amp (2000 Amp as per Request)
ACBAbove 1000 Amp
MCB Rating6A,10A,16A,20A,32A,40A,50A,63A
MCCB Rating0.5A,1A,2A,4A,6A,10A,16A,20A,32A,40A,50A,63A,80A,100A (Domestic Max 6A)
Sen. of ELCB30ma (Domestic),100ma (Industrial),300ma
DPIC (Double5A,15A,30 A for 250V
Pole Iron Clad) main switch
TPIC (Triple30A, 60A, 100A, 200 A For 500 V
Pole Iron Clad) main switch
DPMCB5A, 10A, 16A, 32A and 63 A for 250V
TPMCCB100A,200A, 300Aand 500 A For 660 V
TPN main switch30A, 60A, 100A, 200A, 300 A For 500 V
TPNMCB16A, 32A,63A For 500 V, beyond this TPNMCCB: 100A, 200A, 300A, 500 A For 660 V
TPN Fuse Unit16A,32A,63A,100A,200A
Change over32A,63A,100A,200A,300A,400A,630A,800A
switch (Off Load)
SFU (Switch Fuse32A,63A,100A,125A,160A,200A,250A,315A,400A,630A
HRC Fuse TPN125A,160A,200A,250A,400A.630A
HRC Fuse DPN16A,32A,63A
MCB/MCCB/ELCB Termination Wire / Cable
Up to 20A MCBMax. 25 Sq.mm
20A to 63A MCBMax. 35 Sq.mm
MCCBMax. 25 Sq.mm
6A to 45A ELCB16 Sq.mm
24A to 63A35 Sq.mm
80A to 100A50 Sq.mm

Electrical Thumb Rules-(Part-5)

September 1, 2013  4 Comments

Standard Size of Transformer (IEEE/ANSI 57.120):

Single Phase TransformerThree Phase Transformer
5KVA,10 KVA,15 KVA,253 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
  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

Standard Size of Motor (HP):

Electrical Motor (HP)



Approximate RPM of Motor

< 10 HP750 RPM
10 HP to 30 HP600 RPM
30 HP to 125 HP500 RPM
125 HP to 300 HP375 RPM

Standard Size of Motor (HP):

Electrical Motor (HP)



Motor Line Voltage:

Motor (KW)Line Voltage
< 250 KW440 V (LV)
150 KW to 3000KW2.5 KV to 4.1 KV (HV)
200 KW to 3000KW3.3 KV to 7.2 KV (HV)
1000 KW to 1500KW6.6 KV to 13.8 KV (HV)

Motor Starting Current:

SupplySize of MotorMax. Starting Current
1 Phase< 1 HP6 X Motor Full Load Current
1 Phase1 HP to 10 HP3 X Motor Full Load Current
3 Phase10 HP2 X Motor Full Load Current
3 Phase10 HP to 152 X Motor Full Load
3 Phase> 15 HP1.5 X Motor Full Load Current

Motor Starter:

StarterHP or KWStarting CurrentTorque
DOL<13 HP(11KW)7 X Full Load CurrentGood
Star-Delta13 HP to 48 HP3 X Full Load CurrentPoor
Auto TC> 48 HP (37 KW)4 X Full Load CurrentGood/ Average
VSD 0.5 to 1.5 X Full Load CurrentExcellent
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 MVA5%
1 MVA to 2.5 MVA6%
2.5 MVA to 5 MVA7%
5 MVA to 7 MVA8%
7 MVA to 12 MVA9%
12 MVA to 30 MVA10%
> 30 MVA12.5%

Standard Size of Transformer:

Standard Size of TransformerKVA
Power Transformer (Urban)3,6,8,10,16
Power Transformer (Rural)1,1.6,3.15,5
Distribution Transformer25,50,63,100,250,315,400,500,630

Electrical Thumb Rules-(Part-6)

September 5, 2013  9 Comments

Transformer Earthing Wire / Strip Size:

Size of T.C or DGBody EarthingNeutral Earthing
<315 KVA25×3 mm Cu / 40×6 mm GI25×3 mm Cu Strip
315 KVA to 50025×3 mm Cu / 40×6 mm GI25×3 mm Cu Strip
500 KVA to 75025×3 mm Cu / 40×6 mm GI40×3 mm Cu Strip
750 KVA to 100025×3 mm Cu / 40×6 mm GI50×3 mm Cu Strip

Motor Earthing Wire / Strip Size:

Size of MotorBody Earthing
< 5.5 KW85 SWG GI Wire
5.5 KW to 22 KW25×6 mm GI Strip
22 KW to 55 KW40×6 mm GI Strip
>55 KW50×6 mm GI Strip

Panel Earthing Wire / Strip Size:

Type of PanelBody Earthing
Lighting & Local Panel25×6 mm GI Strip
Control & Relay Panel25×6 mm GI Strip
D.G & Exciter Panel50×6 mm GI Strip
D.G & T/C Neutral50×6 mm Cu Strip

Electrical Equipment Earthing:

EquipmentBody Earthing
LA (5KA,9KA)25×3 mm Cu Strip
HT Switchgear50×6 mm GI Strip
Structure50×6 mm GI Strip
Cable Tray50×6 mm GI Strip
Fence / Rail Gate50×6 mm GI Strip

Earthing Wire (As per BS 7671)

Cross Section Area of Phase, Neutral Conductor(S) mm2Minimum Cross Section area of Earthing Conductor (mm2)
S<=16S (Not less than 2.5 mm2)

Area for Transformer Room: (As per NBC-2005):

Transformer SizeMin. Transformer Room Area (M2)Min. Total Sub Station Area( Incoming HV,LV Panel, T.C Roof) (M2)Min. Space Width (Meter)

•    The Capacitor Bank should be automatic Switched type for Sub Station of 5MVA and


•   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):

VoltageNormal Span
765 KV400 to 450 Meter
400 KV400 Meter
220 KV335,350,375 Meter
132 KV315,325,335 Meter
66 KV240,250,275 Meter
1 HP45 Amp
1.5 HP50 Amp
2 HP65 Amp
3 HP90 Amp
5 HP135 Amp
7.5 HP200 Amp
10 HP260 Amp

Three Phase Motor Code (NEMA)

<1 HPL
1.5 to 2.0 HPL,M
>15 HPG

Service Factor of Motor:

  HPSynchronous Speed (RPM)
3600 RPM1800 RPM1200 RPM900 RPM720 RPM600 RPM514 RPM
1 HP1.
1.5 to 1.25  HP1.
150 HP1.
200 HP1.
> 200 HP11.1511111

Type of Contactor:

AC1Non Inductive Load or Slightly Inductive Load
AC2Slip Ring Motor, Starting, Switching OFF
AC3Squirrel Cage Motor
AC4,AC5,AC5a, AC5b,AC6aRapid Start & Rapid Stop
AC 5aAuxiliary Control circuit
AC 5bElectrical discharge Lamp
AC 6aElectrical Incandescent Lamp
AC 6bTransformer Switching
AC 7aSwitching of Capacitor Bank
AC 7bSlightly Inductive Load in Household
AC 5aMotor Load in Household
AC 8aHermetic refrigerant compressor motor with Manual Reset O/L Relay
AC 8bHermetic refrigerant compressor motor with Automatic Reset O/L Relay
AC 12Control of Resistive Load & Solid State Load
AC 13Control of Resistive Load & Solid State Load with Transformer Isolation
AC 14Control of small Electro Magnetic Load (<72 VA)
AC 15Control of Electro Magnetic Load (>72 VA)

Contactor Coil:

Coil VoltageSuffix
24 VoltT
48 VoltW
110 to 127 VoltA
220 to 240 VoltB
277 VoltH
380 to 415 VoltL

Electrical Thumb Rules-(Part-7)

September 16, 2013  10 Comments

Overhead Conductor /Cable Size:

VoltageOverhead ConductorCable Size
33 KVACSR-Panther/Wolf/Dog , AAAC150,185,300,400,240 mm2 Cable
11 KVACSR-Dog/Recon/Rabbit , AAAC120, 150,185,300 mm2 Cable
LTACSR-Dog/Recon/Rabbit ,95,120, 150,185,300 mm2 Cable
SpanHeight of Tower
400KV=400 Meter400KV=30Meter (Base 8.8 Meter)
220KV=350 Meter220KV=23Meter (Base 5.2 Meter)
132KV=335 Meter220KV Double Circuit=28 Meter
66KV=210 Meter66KV=13Meter
Conductor AmpereVoltage wise Conductor
Dog=300Amp400KV=Moose ACSR=500MVA Load
Panther=514Amp220KV=Zebra ACSR=200MVA Load
Zebra=720Amp132KV=Panther ACSR=75MVA Load
Rabbit=208Amp66KV=Dog ACSR=50MVA Load

Type of Tower:

ASuspension TowerUp to 2°
BSmall Angle Tower2° to 15°
CMedium Angle Tower15° to 30°
DLarge Angle / Dead End Tower30° to 60° & Dead End

Tower Swing Angle Clearance (Metal Part to Live Part):

Swing AngleLive Part to Metal Part Clearance (mm)

Cable Coding (IS 1554) 🙁 A2XFY / FRLS / FRPVC / FRLA


FFlat Armoured
WWire Armoured
YOuter PVC Insulation Sheath
WSteel Round Wire
WWSteel double round wire Armoured
YYSteel double Strip Armoured
FRFire Retardation
LSLow Smoke
LALow Acid Gas Emission
WANon Magnetic round wire Armoured
FANon Magnetic Flat wire Armoured
FFDouble Steel Round Wire Armoured

Corona Ring Size:

<170 KV160mm Ring put at HV end
>170 KV350mm Ring put at HV end
>275 KV450mm Ring put at HV end & 350 mm Ring put at Earth end

Load as per Sq.Ft:

Type of LoadLoad/Sq.FtDiversity Factor
Industrial1000 Watt/Sq.Ft0.5
Commercial30 Watt/Sq.Ft0.8
Domestic15 Watt/Sq.Ft0.4
Lighting15 Watt/Sq.Ft0.8

Size of Ventilation Shaft:

Height of Building in meterSize of ventilation shaft in sq meterMinimum size of shaft in meter
15 and above4.01.5

Electrical Thumb Rules-(Part-8)

October 1, 2013  6 Comments

Accuracy Class of Metering CT:

Metering Class CT
0.1 To 0.2Precision measurements
0.5High grade kilowatt hour meters for commercial grade kilowatt hour meters
3General industrial measurements
3 OR 5Approximate measurements

Accuracy Class Letter of CT:

Metering Class CT
Accuracy ClassApplications
BMetering Purpose
Protection Class CT
CCT has low leakage flux.
TCT can have significant leakage flux.
HCT accuracy is applicable within the entire range of secondary currents from 5 to 20 times the nominal CT rating. (Typically wound CTs.)
LCT 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:

10P5Instantaneous over current relays & trip coils: 2.5VA
10P10Thermal inverse time relays: 7.5VA
10P10Low consumption Relay: 2.5VA
10P10/5Inverse definite min. time relays (IDMT) over current
10P10IDMT Earth fault relays with approximate time grading:15VA
5P10IDMT 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


•   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


•   Required Over Load Relay Plug Setting= Over Load Current (In) / CT Primary


•   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


•   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


(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


•   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


•    (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


•    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


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


•    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 LoadCalculationDiversity
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


Type of LoadCalculationDiversity
Domestic (Constructed area)15 watt / sq. Foot0.4
Commercial(Constructed area)30 watt / sq. Foot0.8
Industrial (Constructed area)100 watt/ 1 sq. Foot0.5
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 CostRs 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 Work1.3%23 Rs / Sq.Foot
Electrical Work (Internal)4.1%76 Rs / Sq.Foot
Lift Work4.4%82 Rs / Sq.Foot
Street Light Costing (CPWD-2012)
Fluorescent Lamp95 Rs/Sq.Meter
With HPMV Lamp130 Rs/Sq.Meter
With HPSV Lamp165 Rs/Sq.Meter
Electrical Sinage85 Rs/Sq.Meter
Other Electrical Cost
Area Required for Solar Light  10 Watt/Sq.Foot
Solar Power Installation1.5 Lacs Rs/1Kw
HVAC Cost18 Watt/Sq.Foot

Distribution Losses (Gujarat Electricity Board)

Voltage (Point of Injection)  At 11 KVPoint of Energy Delivered
11KV / 22KV / 33KV10%1082%
400 Volt16.77%
Rate Analysis (CPWD-2012)
Sub Station Equipment7000 Rs/ KVA
D.G Set with installation1000 Rs / KVA
  UPS with 30min Breakup20000 Rs / KVA add 8000 Rs / KVA additional each 30 min
Solar Power Generation1.25 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 Plant75000 RS / Ton
VRF / VRV System55000 Rs / HP
Air condition System11000 Rs / Ton
CCTV System300 Rs / Sq Meter
Access Control system200 Rs / Sq Meter
Hydropenumatic Water system2000 Rs / LPM
  Building Management System300 Rs / Sq Meter add 100 Rs / Sq Meter additional area beyond 10000 Sq Meter
Rate Analysis (Rs per Sq. Meter) (CPWD-2012)
Fire Fighting (with Wet Riser)500500500500
Fire Fighting (with Sprinkler)750750750750
Fire Alarm (Manually)300
Fire Alarm (Automatic)500500500500
Pressurized  Mechanical Ventilation650650650650
Rate Analysis (% of Total Project Cost) (CPWD-2012)
Internal Water Supply & Sanitary  4%  10%  5%  12%
Internal Electrical Installation12.5%12.5%12.5%12.5%
Lift Speed (Indian Army Manual)
No of FloorLift Speed
4 to 50.5 to 0.7 meter/Sec
  6 to 120.75 to 1.5 meter/Sec
3 to 201.5 to 2.5 meter/Sec
  Above 20Above 2.5 meter/Sec
Lift  Details (CPWD-2012)
  Type of Lift  Persons  WeightSpeed  M/SecTravelPriceAdd Rs /Floor
Passenger Lift8 Person544 Kg1.0G+418 Lacs1.25 Lacs
Passenger Lift13 Person844 Kg1.5G+422 Lacs1.25 Lacs
Passenger Lift16 Person1088 Kg1.0G+428 Lacs1.50 Lacs
Passenger Lift120 Person1360 Kg1.5G+424 Lacs1.50 Lacs
MCB Class according to Appliances
ApplianceCapacity / wattMCB RatingMCB Class
    Air Conditioner1.0 Tone10AC Class
1.5 Tone16AC Class
2.0 Tone20AC Class
  Freeze165 Liter3 AC Class
350 Liter4 AC Class
  Oven /Grill4500 Watt32 AB Class
1750 Watt10 AB Class
Oven / Hotplate750 Watt6 AB Class
 2000 Watt10 AB Class
  Room Heater1000 Watt6 AB Class
2000 Watt10 AB Class
Washing Machine300 Watt2 AC Class
1300 Watt8 AC Class
    Water Heater1000 Watt6 AB Class
2000 Watt10 AB Class
3000 Watt16 AB Class
6000 Watt32 AB Class
  Iron750 Watt6 AB Class
1250 Watt8 AB Class
  Toaster1200 Watt8 AB Class
1500 Watt10 AB 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 LoadEconomical Voltage
Up to 750 KVA415 V
750 KVA to 2500 KVA3.3 KV
2500 KVA to 5000 KVA6.6 KV
Above 5000 KVA11 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)


•   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)

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


•   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 LevelGround Clearance
<=33KV5.2  Meter
66KV5.49 Meter
132KV6.10 Meter
220KV7.0   Meter
400KV8.84  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%.


•   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


•   Total Load KVA (new) for Connection(1)= KW /New Power Factor= 41.1 /0.98=


•   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


•   Total Load KVA (new) for Connection(1)= KW /New Power Factor= 16.66 /0.98=


•   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=


•   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=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/


•   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


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


•   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.


•    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

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