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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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    120 homes in total is of course 40 homes per phase, presuming a single phase supply to each one. at 500 amps in total that is 12.5 amps each, or at the increased total of 800 amps it is 20 amps per home. Maybe a little less if any of the supply capacity is to be used for any common facilities such as a shop, residents club, or estate lighting.

    12.5 amps per home sounds a bit marginal to me, presuming 100% electric. DNOs often assume about 10 amps per house, after diversity in the absence of gas for heating. An EV charger for each home would tend to increase that a bit, and most park homes prohibit use of bottled gas or solid fuel, which tends to increase demand for electricity. Also 40 homes per phase might be a bit low for the DNO assumption of 10 amps per home to be valid.

    20 amps per home sounds more realistic after allowing for electric vehicles and a prohibition on alternative fuels. A lot will depend on the use made of the EV charging, A 32 amp charger per home and with 10% of these used at the same time is another 3.2 amps per home on average. 

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    120 homes in total is of course 40 homes per phase, presuming a single phase supply to each one. at 500 amps in total that is 12.5 amps each, or at the increased total of 800 amps it is 20 amps per home. Maybe a little less if any of the supply capacity is to be used for any common facilities such as a shop, residents club, or estate lighting.

    12.5 amps per home sounds a bit marginal to me, presuming 100% electric. DNOs often assume about 10 amps per house, after diversity in the absence of gas for heating. An EV charger for each home would tend to increase that a bit, and most park homes prohibit use of bottled gas or solid fuel, which tends to increase demand for electricity. Also 40 homes per phase might be a bit low for the DNO assumption of 10 amps per home to be valid.

    20 amps per home sounds more realistic after allowing for electric vehicles and a prohibition on alternative fuels. A lot will depend on the use made of the EV charging, A 32 amp charger per home and with 10% of these used at the same time is another 3.2 amps per home on average. 

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