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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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    Morning all, thank you for your input. 


    So the process of getting to the 2900 A (ish) value was by using the protective device value (1600 A) and dividing it by the assumed rating factors, which were (I don't have my book on me rn) I think 0.6 (Cg) and 0.9 (Cc I think, the value is from a note in the regs). I'm not sure about the existing install so I've assumed it was installed to keep the effects on the cables to a minimum.  

    Using the tabulated current from my initial post of 324 A (T 4E4A ref D) that would give a total of 1620 A, which I understand is rated more than the protective device on paper but the ref d column in the table must be a ballpark figure? 

    I may be incorrect about the transformer voltage, I assumed the value. The cables are able to carry the transformers max operational current but Iz should be greater than In... which it is by 20 A. 


    Thanks, Adam.
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  • 0
    Morning all, thank you for your input. 


    So the process of getting to the 2900 A (ish) value was by using the protective device value (1600 A) and dividing it by the assumed rating factors, which were (I don't have my book on me rn) I think 0.6 (Cg) and 0.9 (Cc I think, the value is from a note in the regs). I'm not sure about the existing install so I've assumed it was installed to keep the effects on the cables to a minimum.  

    Using the tabulated current from my initial post of 324 A (T 4E4A ref D) that would give a total of 1620 A, which I understand is rated more than the protective device on paper but the ref d column in the table must be a ballpark figure? 

    I may be incorrect about the transformer voltage, I assumed the value. The cables are able to carry the transformers max operational current but Iz should be greater than In... which it is by 20 A. 


    Thanks, Adam.
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