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).