Electrical Thumb Rules-(Part 1).
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