Electrical Thumb Rules-(Part 1)

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

§  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=0.746KW	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