TX to switchboard cable sizing

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.

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.

Sorry to go off piste, Danny, but can you shed any light on how the demand for the site was arrived at? 

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. 

Reduced neutrals are far less fashionable these days than they used to be, I think even DNOs use full sized neutrals these days by default in LV networks. In 3-phase systems N currents don't always cancel nicely - especially where a lot of switch-mode PSUs are involved (e.g. data centres or perhaps lots of EV chargers).

BS 7671 certainly still allows reduced neutrals, but does require overcurrent protection for the N (431.2) - which will likely mean a protective device that'll react to the current flowing in the N and then disconnect by opening all the line conductors (not necessarily the N as well, certainly not ever the N alone).

   - Andy.

Hi all

Have found out today that the reason for the future upgrade in the supply was that only 78 homes have been planned but may increase to 120 in the future. All homes will have gas heating and hobs direct feeds from main gas network, the original contractor who for some reason cannot now take on the installation of the park form the switchboard had calculated 468KVA  supply with diversity and allowed for all EV chargers to be no more than 16 amps. I have done my own calculations and fusing at 500 amp per phase in my opinion should be fine but happy to take on others opinions and calculations.

There will be no other common facilities on site and street lighting appears to be 36 x 7w LED bollard lights.

With gas heating and cooking, I would expect that 12.5 amps per home, average after diversity would be fine. Marginal without gas but fine with gas in my view.

Modern practice is generally towards full size neutrals in DNO street mains, and in those parts of a private network that serve a similar purpose to street mains.

Between transformer and switchgear, half size neutrals remain more popular.

I think folk have temporarily forgotten that at least until the next U turns, from 2025 new homes are not supposed to be heated by gas, and from circa 2035, neither are pre-existing ones. Now it may not come to pass, but in terms of sizing such things as street cables and transformers, that have lives of well over 50 years, by which time the gas will have run out and such pre-emptive govt policy will be moot,  it seems foolish not to allow for the inevitable load creep of all-electric heat, as well as more houses.

Mike.

Mike.

Agree, 12.5 amps per home should be fine AT PRESENT with gas used for heating and cooking, but might well prove inadequate in the future.