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Parallel cable sizing

ARW182
Hi, I'm currently completing my project report for BEng based on an existing install at work. The install is between a 1000 kVA trans to main section board, the existing cable is 5x 240mm 4C SWA in duct buried. All ACB on the circuit are 1600A, I calc the max output of the trans at 1443A. 

I'm trying to prove the existing install using the BS 18th regs as the installer hasn't supplied any, and this is where I'm having an issue. 

From my understanding rating factors must be applied to In (protective device) and that value must be less than or equal to It (tabulated current carrying capacity of a cable). I've got a 'new' In value of 2962A which Ive divided by 5 (number of cores per phase) which gives 592A. Am I right in thinking that I then need to specify a cable using the 592A value? If that is correct than the existing install is incorrect? 

In table 4E4A (18th regs) the max current for 240 SWA multi core, ref D (buried) is 324A. 

Have I gone wrong at some point? 


Any help is very much appreciated, thanks Adam.
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  • 0
    you may find it does not meet the ratings as tabulated in the regs, especially if you are using grouping factors to generate the higher effective current.

    I agree with the 1440 amps per phase, s that is 1000VA /230V = 4.3kA , then split into 3 phases  call it 1450A per phase as a round number.


    If that current is split into n cores per phase then each core carries 1450 /n


    If n is 5, then 1450 becomes 290A per phase per cable

    Then if you apply  the right grouping and environmental factors, you will see that you need a larger cable than the 290A would suggest, but please describe how the cables and cooling are organised, as it is unlikely to be as bad as the factor of 2 you seem to be using, and I'm not sure if you are rolling in a factor of 1.45 you do not need, or one of the C factors is counted twice..
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  • 0
    you may find it does not meet the ratings as tabulated in the regs, especially if you are using grouping factors to generate the higher effective current.

    I agree with the 1440 amps per phase, s that is 1000VA /230V = 4.3kA , then split into 3 phases  call it 1450A per phase as a round number.


    If that current is split into n cores per phase then each core carries 1450 /n


    If n is 5, then 1450 becomes 290A per phase per cable

    Then if you apply  the right grouping and environmental factors, you will see that you need a larger cable than the 290A would suggest, but please describe how the cables and cooling are organised, as it is unlikely to be as bad as the factor of 2 you seem to be using, and I'm not sure if you are rolling in a factor of 1.45 you do not need, or one of the C factors is counted twice..
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