Cable Sizing Calculation

Posted: November 16, 2011 in Electrical Engineering

After designing or obtaining the lighting and the electrical equipment loads, cables has to be selected.

There are 3 main factors that effecting our calculation in selecting the right cable for each of our electrical equipment..

1) Temperature correction factor

2) Group de-rating factor

3) K factor

**These 3 factors are obtained from the cable vendor that we intended to use.

Then, we had other requirement to be meet which could be considered the next important factors after the main 3 factors..

1) 5% Allowable Voltage Drop

2) Cable Length of the respected cables.

3) Let through energy for Cable > Let through energy for Circuit Breaker/Fuse

4) Cable site rating > FLC + Safety margin

**Cable site rating = Max. current can flow in the cable

**FLC = Full Load Current

  1. sudhakar says:

    helo sir,

    i want an example of manual cal of cable size and voltage drop ,short circuit calculations. I am searching for a designer job electrical in india. . . .pls help me

    • ridhwanali says:

      I thought a designer do not need to calculate all of those, your expertise should be on AutoCAD/PDMS? the calculations usually done by the Electrical Engineers?

  2. Bernard says:

    hello my name is bernard and im studying for my master electrical exam for phila. pa.
    Im having some issues in calculating kva per phases or plugging in values into the formula for solving very fustrating to me. can you point me in a direction and give me infomation or way to easy understand the relationship if the formulas

  3. sujeesh says:

    i have a load of 25kw load ,with the distance of 250 mts, how calculate the cable size

    • ridhwanali says:

      Hello Suresh! Sorry for a late reply. I’ve been quite busy nowdays. I did a simple calculation for your question. My answer would be 50mm2 and above. Nevertheless, there are several assumptions that I need to made to able me to come out with this answer. Thanks.

  4. Aditya says:

    Hi i am aditya, i want to build a transformer at home i am studying intermediate and this i want to do it as my old laptop charger is lost and i dont want spend much money on it and i am very much intrested in doing something cool – the specific out put is 19 volts and 8 amps – i dont know much about electronics i ve just googled some information about transformers – can u please guide me step by step for making a transformer its dimensions and primary and secondary coil windings and gauge (size) of wire how to determine them – please

    • ridhwanali says:

      Hello Aditya! Wow.. that is a genuine engineer there!hehe.. Be careful as what you are doing is very risky as you are trying to step down 240V from your household socket outlet to 19V. Plus, if you mention about electronics. I guess you need an access to the labs equipments to able you to at least test your breadboard or PCB. I would suggest you to open your broken charger and look at the design and how they (the company brand) do it. It might look simple but definitely it is not a transformer there.

      My advice would be: You could keep your energy and time and use it during your final year project at university. There you could design, test and do whatever you had in mind. And with a proper guidance and helps needed. 🙂

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