Electrical Thumb Rules For:

Electrical Thumb Rules For:

• Cable Capacity
• Current Capacity of Equipment
• Earthing Resistance
• Minimum Bending Radius
• Insulation Resistance
• Lighting Arrestor
• Transformer
• Diesel Generator
• Current Transformer
• Quick Electrical Calculation

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 Equipment

• 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 Volt
• 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 = 12 x Dia of Cable.
• Minimum Bending Radius for HT Power Cable = 20 x Dia of Cable.
• Minimum Bending Radius for Control Cable = 10 x Dia 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 = KVA x 1.4
• 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 at 1 meter.
• 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).