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Friday, 2 August 2013

Electrical thumb rule

Cable current Capacity:
  • For Copper  Wire Current Capacity (Up to 25 Sq.mm) = 5-6X Size of Wire in Sq.mm
  • Ex. For 4 Sq.mm=6×4=24 Amp, 
  • For Aluminum wire Current Capacity = 4X Size of Cable in Sq.mm ,upto 10sqmm
  • Ex. For 1.5 Sq.mm=4×1.5=6 Amp.
Nomenclature for cable Rating 
Uo=Phase-Ground Voltage, U=Phase-Phase Voltage, Um=Highest Permissible Voltage
Current Capacity of Equipments:
  • 1 Phase Motor draws Current=6-7Amp per HP.
  • 3 Phase Motor draws Current=1.7Amp per KW.
  • Full Load Current of 3 Phase Motor=HPx1.3 from 10 HP onward
  • Full Load Current of 1 Phase Motor=5 to 6 times of HP
  • No Load Current of 3 Phase Motor =25-30% 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)

Insulation Resistance:
  • IR Value for Rotating Machine= (KV+1) MΩ.
  • IR Value for Motor (IS 732) = ((20xVoltage (L-L)) / (1000+ (2xKW)).
  • IR Value for Equipment (<1KV) = Minimum 1 MΩ.
  • IR Value for Equipment (>1KV) = KV 1 MΩ per 1KV.
  • IR 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=KVAx1.35
  • Short Circuit Current of Generator/transformer
= 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 Transformer 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).


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