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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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  • Former Community Member
    0 Former Community Member
    I suspect 2 key issues here:


    1 - The cable sizing is based on the nominal setting of the 1600A device - it is likely that this is actually set (or should be) to a value around the TX FLC - ie 1333A


    2 - The multicore cables are buried in ducts - so they will have a certain natural spacing - which suggests the grouping factor and buried direct factors may be a little less onerous - also the determination of depth of lay and ground resistivity will have a reasonable impact


    If we take the FLC as being 1333A, and a reasonable correction factor for all conditions of grouping, ground temperature, depth of lay, spacing etc of say 0.75, then we need (for 5no multicore XLPE run to 90C) a conductor CCC of 1333A/0.75 = 1777/5 = 355A - which is just about credible for a 240mm2 conductor


    regards


    OMS
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  • Former Community Member
    0 Former Community Member
    I suspect 2 key issues here:


    1 - The cable sizing is based on the nominal setting of the 1600A device - it is likely that this is actually set (or should be) to a value around the TX FLC - ie 1333A


    2 - The multicore cables are buried in ducts - so they will have a certain natural spacing - which suggests the grouping factor and buried direct factors may be a little less onerous - also the determination of depth of lay and ground resistivity will have a reasonable impact


    If we take the FLC as being 1333A, and a reasonable correction factor for all conditions of grouping, ground temperature, depth of lay, spacing etc of say 0.75, then we need (for 5no multicore XLPE run to 90C) a conductor CCC of 1333A/0.75 = 1777/5 = 355A - which is just about credible for a 240mm2 conductor


    regards


    OMS
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