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TX to switchboard cable sizing

danny wells

Hi all, new to the forum 

hoping for some help with my following questions.

I have been asked to take on a project that has had a new 800kva transformer installed by northern power grid and the supply conductors have been installed from the new switchboard underground in 2x sets of 400mm AWA per phase, 1 x 400mm AWA neutral and 240mm cpc. The sets of phase conductors are installed in 2 x 4m of ducting in trefoil along with neutral conductor in single duct between the above two ducts and cpc in separate duct. 
the sizing of the conductors appear to have been sized for an 800amp 3phase fused supply from the transformer although I have asked for 500amp fuses as at present 800 amps is not required and I believe it was to allow for future development of the site. The site is going to house 120 residential park homes with ev charger on all homes.

my question is will the neutral need to be larger size for future expansion to allow for an 800 amp supply and should the cpc be larger than 240mm.

From my calculations the phase conductors are fine but correct me if I am wrong just trying to make sure all is correct before I take on the project.

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  • 0

    I  suspect that the neutral will be fine for the increase in load to 800 amps per phase. The neutral only needs to carry the unbalanced current, and this tends not to increase much with extra load. A rule of thumb is that a half size neutral is fine in all but unlikely cases in which substantial neutral currents are expected.

    Confirm by calculation that the neutral is still protected against fault currents by 800 amp fuses.

  • 0 in reply to broadgage

    Hi Broadgage thanks for the reply, I did believe it would be ok and will check calculations but had a guy joining the HV cable underground near  me and he popped over and started telling me he had always connected two neutrals on all the transformers he had connected which I found strange as the transformer in question only has capabilities for 7 connections as it is below 800 KVA and so placed doubt in my mind.

    accidently pressed the up down arrows at bottom of your reply and not sure what they do so hopefully nothing negative.

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  • 0 in reply to broadgage

    Hi Broadgage thanks for the reply, I did believe it would be ok and will check calculations but had a guy joining the HV cable underground near  me and he popped over and started telling me he had always connected two neutrals on all the transformers he had connected which I found strange as the transformer in question only has capabilities for 7 connections as it is below 800 KVA and so placed doubt in my mind.

    accidently pressed the up down arrows at bottom of your reply and not sure what they do so hopefully nothing negative.

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