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Home / Technical Articles / Electrical Thumb Rules You MUST Follow (1)
Electrical Thumb Rules You MUST Follow (Part 1)
Electrical Thumb Rules You MUST Follow (Part 1)

Electrical Thumb Rules For:

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

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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)

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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 Distance18 to 22mm/KV (Moderate Polluted Air) or
  • Creepage Distance = 25 to 33mm/KV (Highly Polluted Air)

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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.

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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.

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Lighting Arrestor

Arrestor have Two Rating:

  1. MCOV=Max. Continuous Line to Ground Operating Voltage.
  2. Duty Cycle Voltage. (Duty Cycle Voltage > MCOV).

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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.

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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.

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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).

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Quick Electrical Calculation

1HP = 0.746KWStar Connection
1KW = 1.36HPLine Voltage = √3 Phase Voltage
1Watt = 0.846 Kla/HrLine Current = Phase Current
1Watt = 3.41 BTU/HrDelta Connection
1KWH = 3.6 MJLine Voltage = Phase Voltage
1Cal = 4.186 JLine Current = √3 Phase Current
1Tone = 3530 BTU
85 Sq.ft Floor Area = 1200 BTU
1Kcal = 4186 Joule
1KWH = 860 Kcal
1Cal = 4.183 Joule

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author-pic

Jignesh Parmar

Jignesh Parmar has completed M.Tech (Power System Control), B.E (Electrical). He is member of Institution of Engineers (MIE), India. He has more than 13 years experience in transmission & distribution-energy theft detection and maintenance electrical projects.

114 Comments


  1. Sudhakar
    Dec 11, 2020

    Hi Sir,

    In a substation we use different connectors, like, bus support, couplers, body & cap connector etc, I would like to know what each component is rated for with respect to ampacity. How can we calculate that, is there a way to do that.


  2. Chandraprakash G
    Nov 29, 2020

    Excellent and neat write up, keep the great work !!


  3. murali nagarajan
    Nov 19, 2020

    3phase 40A energy meter 17Nos in one apparetment.
    How much amps switch rating and transformer is required?


  4. saranraj.M
    Sep 17, 2020

    Typically the local utility provides transformers rated up to 1250kVA For maximum connected load of 150kW.
    Power factor*KVA=KW
    0.8*1250=1000KW
    But here answer is 150KW
    How it will calculate?


  5. Hemant Samal
    Jul 16, 2020

    Sir, one of my client wants to take connection having his required load 70 KW. In fact I have maximum feeder capacity in the existing panel 100 AMP TPN,MCCB-16 KA. Please suggest me the Feeder specification, so that I can install it in my panel in the spare box. also please eleborate the formula to select.


  6. Rama Sinha Adhikari
    Nov 26, 2019

    Thank you sir. This is very needful to us.


  7. Raj kumar
    Oct 27, 2019

    Sir,

    I need to know the load calculations of
    2core 1sqmm al ar cable = Amps
    1sqmm cu cable = Amps


  8. Subhro
    Jul 24, 2019

    Tor electrical calculation should we take 1HP= 735.5W or 746W


    • Anup Kumar
      Oct 13, 2020

      1HP=746W


  9. C P
    Jul 15, 2019

    Dear sir
    Twin core flat aluminium pvc insulated cable when we are supply current one core than earthing second core why ?


  10. Abhishek Robinson
    Jul 09, 2019

    Dear sir

    I need your expertise ON TRIPOING OF 220KV EHV transmission line, during lightening.

    Need your help. Can I have your contact number please.


  11. udeerna kV
    Jul 08, 2019

    Very very nice ..very very useful informations &Calculations..thank u very much


  12. V PRASHANTH
    Jul 04, 2019

    For 4Cx 6sqmm cu at cable, you can go with 4Cx 25 sqmm Al AR XLPE cable. And for 3Cx4sqmm you go with 16sqmm if the length is less than 80 to 100mts.


  13. Nanhey lal chauhan
    Jan 13, 2019

    load calculation of3.5*300 sq mm al. cable


  14. Pramod
    Dec 31, 2018

    For single core house wiring to 1.5 sq. Mm. How much current take ?


  15. N Satyanarayana
    Nov 16, 2018

    2800KW, 12Amps 1 phase AC load and distance between incoming supply source to load point is 1500mtr. Cable laid in under ground and HDPE pipe. now can you please suggest cable size and voltage drop calculation

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