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Thread: Effect of transformer size on main breaker

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    Effect of transformer size on main breaker

    Does the transformer size Eskom is installing have anything to do with how i should determine the size of my main breaker to feed?

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    Hi

    The transformer size has everything to do with the sizing of your Main Breaker.

    Without knowing if
    Is it a new supply , are you upgrading the supply , are you taking LT or MV supply ,3 phase or single phase , who is responsible for the cable work from the transformer to the main C/Breaker, or distances involved.

    You would obviously need to determine the power allocated/applied for, to your site as a starting point ( 300 Amp , 1500 Amp )
    Eskom should then give you the fault level that is expected at the supply point which will determine what KA rating the main C/Breaker should be.From the Main Breaker you would then need to determine the fault level ( KA rating ) for the balance of the installation.

    If we assume that you have taken a 400KVA, 400v supply and your Main C/breaker is close to the terminals of the transformer, and there is no protection between the transformer and your Main C/Breaker.
    Eskom install a 400Kva transformer on the original installation date you would more than likely have a 15,5KA fault level at the Main C/Breaker - You would more then likely have a C/breaker rated at 25 KA 600 Amp
    If Eskom now come along and change the transformer out with say a 1 600 KVA transformer and the above still applies you would then need to ensure that your main C/Breaker is now rated around 40 KA 600 Amp

    If you leave your C/Breaker rated at 25KA when the actual short circuit fault level is around 40KA and you have a serious short circuit fault the C/Breaker could explode causing untold damage and possible loss of life.

    The fault level calculations,design and ratings of C/breakers and cables is a critical part of the original design stage.

    Have a look at the video of a C/Breaker under duress ( https://www.youtube.com/watch?v=qu-ssfQRRJg ) - Good reason to double check the original design against what is being changed

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    What would be the ideal main breaker if supplied with a 25kVA transformer?

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    Hi Maseral

    A 25KVA transformer should have a low fault level and reasonable low amperage .

    I picked up on a previous post that you are a semi skilled - Get a registered person involved to assist you on site.

    You need to take into account Transformer impedance , input voltage , step down voltages ,earthing arrangements , cabling arrangements , time settings and fault levels further back that may have an influence

    You could end up killing somebody before you fully start your path in life.

    In this electrical contracting sphere that we play in don't take chances , rather employee the services of an expert within the field and stay safe - learn the correct manner of approaching similar tasks from that expert.

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    Quote Originally Posted by maseral View Post
    Does the transformer size Eskom is installing have anything to do with how i should determine the size of my main breaker to feed?
    Hi Maseral
    Yes by all means.
    When applying for supply. The supply Authority make the provison for you for the power supply you the require with a suitable circuit breaker (MCB)that you applied for. Example: if you request a 3phase 40Amp supply you will be issued with a 25 KVA supply becuase thats what a 25 KVA can deliver. However if you wish to upgrade your supply infuture you have to file a request to the supply authority ofwhich they will upgrade the supplying trfr accordingly if there is a need to a bigger size if a reticulation system can handle.
    I hope this addresses your concern.

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    That addresses my problem absolutely, I was so shocked that Eskom will request a COC before they put a new transformer BUT refuse to connect temporary power for the COC to be thoroughly done...anyways the COC was issued with lots and lots of N/As and Eskom was happy and now the building is powered up

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