Electrical Thumb Rules(Part 1).
July 27, 2013 67 Comments

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=PhaseGround Voltage, U=PhasePhase 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 (LL)) / (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 (PhasePhase)
 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 
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