Electrical Thumb Rules-(Part 1).
July 27, 2013 60 Comments
- 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 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 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.
- Arrestor have Two Rating=
- (1) MCOV=Max. Continuous Line to Ground Operating Voltage.
- (2) Duty Cycle Voltage. (Duty Cycle Voltage>MCOV).
- 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 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.
- 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
|1KW=1.36HP||Line Voltage=√3 Phase Voltage|
|1Watt=0.846 Kla/Hr||Line Current=Phase Current|
|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|