Electrical Cable/Wire Sizing

This time i want shared about how to sizing the electrical cable.This topic is important because it’s involved with safety issue,cost and time line  for the electrical installation project.

Cable or wire sizing matter already discussed for a long time ago and it’s a hot topic for Electricians and Engineers.So i hope from this post,it can give a clear view and  understanding how to select current rating for wire,cable rating,voltage drop, and wire sizing for wiring project application

.From cable/wire sizing calculation we can estimate the proper size for cable to our wiring installation project.One thing that we need to prepared before cable or wire sizing  is several data or information about wiring project includes :

How to sizing the electrical cable/wire?

1) Type of load : For feeder,control panel,distribution board (DB) or motor

2)  Power Source : AC /DC power for Three phase or single phase

3) Full load current (Ampere)

4) Circuit Breaker size

5) Types of cable ( PVC,XLPE,Fire proved)

6) Installation Condition ( Underground, trefoil,trunking,tray or conduit)

7) Distance / length of cable run from source to load


Calculation for sizing electrical cable/wire

Ib ≤ In ≤ Iz

Ib = Amperage  load

In = Circuit Breaker size

Iz = Current rating for cable/wire


Example :

We plan to install 1 unit induction motor 75 hp for running a water pump.Power factor value is 0.8 and motor efficiency is 0.85.Power supply for this motor is 415 volt 3 phase 50Hz.Distance from panel to motor is 75 meter.This panel and pump is located at outside of factory.Please sizing the suitable circuit breaker,electrical cable and also type of cable for this application.


Ib ≤ In ≤ Iz

Ib = ( hp x 746) / (√3 x Volt x PF x Efficiency)

Ib = (75 x 746) / (√3 x 415 x 0.8 x 0.85)

Ib = 114.5 Ampere


In = Ib ≤ breaker sizing

In = 114 ≤ 150 ampere

In = 150 ampere (we decide to used MCCB 3 pole with 150 ampere rating)


Iz = In ≤ Iz

Iz = 150 ≤ Iz

Iz = 187 ampere ( carried capacity for 50mm² cable size)

So we decide to used cable size = 50 mm² with 4 core cable and clipped direct to cable tray.To confirm this cable size,we need to calculate the voltage drop.

* For full table of cable size,please download cable size chart / table below.Clink link to download PDF file for Cable Current Rating Table

Voltage Drop Calculation

Below is a voltage drop calculation that i made for cable power supply from panel to induction motor.
From Panel to Induction motor = 75 meter
mV = 0.87 ( refer table below )

Formula voltage drop :  Vd = Ib x length(meter) x mV / 1000 

Vd = (114.5 x 75 x 0.87) / 1000

Vd = 7.47 ( Acceptable refer to IEE 17th edition )

* Voltage drop acceptable value is 4% x 415 volt = 16.6 Volt


From this calculation, we fully confident to laying cable size 50mm² with 150 ampere of MCCB,using multicore PVC Armored cable (due to outdoor wiring application) and also cable laying with cable tray.

That why all Electrician and Electrical Engineer need to know about cable sizing calculation to make sure wiring installation project successful and meet the requirement of Electrical Safety and Regulation without any disaster.


  1. Mohamed Shawer says:

    Thanks for distributing these useful technical information

  2. Your blog very useful. thank you very much
    really appreciated what u did

  3. John Smith says:

    I have gone through your site which is much relevant to my topic. Being a professional writer I want to contribute one guest article.
    The article would be informative and hope your visitors like the article and shares on social platforms. The deal would be mutually beneficial to both of us and add value on both ends.

    Looking forward to your reply.


  4. saleem rasheed says:

    hi. i need cable sizing formula & star delta connection with wiring diagram…. take care

  5. Radwan says:

    thanks for valuable information
    I’m wondering why you didn’t choose a conductors of 35 mm2? , It gives you Ib ≤ In ≤ Iz

  6. Thanks for information which are very important to us. We never consider the size of cables and wires – Eutex International INC

  7. Govind kumar says:

    Hello bhai

  8. Idowu Afekhare says:

    you guys did a beutifull job.it just solve my problem.good job

  9. joseph says:

    how do i calculate for the circuit breaker required give the formular please

  10. Hello Sir

    I want the complete procedure for cable selection

  11. K.Niranjan says:

    Thanks a lot sir giving this technical information…

  12. Arichandran says:


  13. Thanks

  14. Maninder Preet singh says:

    Sir kindly provide the formula for single phase motor also

  15. Arshad khalil says:

    Kya hum 22 Gage ke jaga 23 ya 24 number ka gage istamal nahi ker sakte help me brother

  16. Arshad khalil says:

    Kya hum 22 number Gage ke jaga 23 ya 24 number ka gage istamal nahi ker sakte help me brother

  17. James Abalah says:

    30 air-conditioning units wired in parallel with an average of 10amps each……
    Help me calculate the cable size needed to supply 3phase power to a distribution board n the probable MCB size……..

  18. James Abalah says:

    30 air-conditioning units wired in parallel with an average of 10amps each……
    Help me calculate the cable size needed to supply 3phase power to a distribution board n the probable MCB size……..
    N/B air-conditioning r 24000btu size

  19. Somanna says:

    Thank you so much sir given information… about cable detection…

  20. mohammaed hoossain says:

    air condition unit 4.5 TR I need cabal siz and isolater siz com amp
    air condition unit 11.5 TR INEED CABAL SIZ and isolater com amp
    air condition uni 8.5 TR nidd cabal siz and isolaters com amp

  21. hemant says:

    Current size k hissab SE or voltage drop k liye table yaad krni padegi kya sir

  22. richard says:

    hi,i really like the calculation but i request for full procedure of how to calculation.

  23. syed bilal ahmed says:

    how did you get the answer equals to 187 ???? please reply

  24. premperumal says:

    Thank you for your free message of electrical techniques

  25. Sabir ansari says:

    20 air-conditioning units wired in parallel with an average of 15amps each……
    Help me calculate the cable size needed to supply 3phase power to a distribution board n the probable MCB size……..
    N/B air-conditionin and cabal size

  26. odidi oluwafemi Gabriel says:

    thanks for the information

  27. What is 75HP Motor Earthing Wire Size

  28. Md. Aminul Islam says:

    What will be capacity of neutral cable for 3000 KVA transformer? What is the Neutral cable size percentage of three phase wiring respect to phase?

  29. Abdolgabar Ahmed Mahmood says:

    thank you for your discreption methode , but I didn`t understand how did you geat the 150 amps for the mccb circuit breaker .
    can you please explane that for me .
    best regaurds.
    eng. Abdolgabar Ahmed Mahmood

  30. Abdolgabar Ahmed Mahmood says:

    Thank you for your explanation, as the mater of fact the calculated current is 114.5 amps, and from the table, we get cable size of 25mm2 and current carrying capacity is 124 amps which is the nearest current value from the calculated one.
    You used a cable size 50mm2 and a current carrying capacity is 187 amps which is completely different and far away from the value in the table.
    please explain to me.
    Best Regards.
    Eng. Abdolgabar Ahmed .

  31. can we use below formula
    P=1.723 x V x I X COSphi

Speak Your Mind


This site uses Akismet to reduce spam. Learn how your comment data is processed.