Site logo

Electrical Engineering Academy

Cover image

Electrical Useful Equations.

ELECTRICAL USEFUL EQUATIONS

  • 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. * Short Circuit Level of Cable in KA (Isc)=(0.094xCable Dia in Sq.mm)/√ Short Circuit Time (Sec) * Cable Voltage Drop(%)=(1.732xcurrentx(RcosǾ+jsinǾ)x1.732xLength (km)x100)/(Volt(L-L)x Cable Run.

  • SIZE OF CABLE ACCORDING TO SHORT CIRCUIT (FOR 11KV,3.3KV ONLY)

  • Short circuit verification is performed by using following formula: * Cross Section area of Cable (mm2)S = I x√t / K * Where: * t = fault duration (S) * I = effective short circuit current (kA) * K = 0.094 for aluminum conductor insulated with XLPE * Example: Fault duration(t)= 0.25sec,Fault Current (I) = 26.24 kA * Cross Section area of Cable = 26.24 x √ (0.25) / 0.094= 139.6 sq. mm * The selected cross sectional area is 185 sq. mm.

  • 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 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). * Protection radius of Lighting Arrestor = √hx (2D-h) + (2D+L). Where h= height of L.A, D-distance of equipment (20, 40, 60 Meter), L=Vxt (V=1m/ms, t=Discharge Time). * Size of Lighting Arrestor= 1.5x Phase to Earth Voltage or 1.5x (System Voltage/1.732).

  • 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. * Fault Level at TC Secondary=TC (VA) x100 / Transformer Secondary (V) x Impedance (%)

  • 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 curren…