does everything in front of the meter need to be split across the 2 supplies or can it be considered as a general parallel supply?

If you're thinking of connecting the two supplies together to make one 800A supply, I'd say definitely not - isolation considerations alone (pull the fuses on one and it's still live from the other) would be a nightmare let alone ensuring equal sharing of the current when the two supply cables aren't necessarily quite the same lengths.

If it was some large industrial setup with private transformers you might be looking at bus couplers to parallel the supplies and switch-over systems to allow either supply to feed the entire installation (perhaps with load shedding),  if one supply went down, but that sort of thing seems a bit OTT for a school situation.

  - Andy. 

does everything in front of the meter need to be split across the 2 supplies or can it be considered as a general parallel supply?

If you're thinking of connecting the two supplies together to make one 800A supply, I'd say definitely not - isolation considerations alone (pull the fuses on one and it's still live from the other) would be a nightmare let alone ensuring equal sharing of the current when the two supply cables aren't necessarily quite the same lengths.

If it was some large industrial setup with private transformers you might be looking at bus couplers to parallel the supplies and switch-over systems to allow either supply to feed the entire installation (perhaps with load shedding),  if one supply went down, but that sort of thing seems a bit OTT for a school situation.

  - Andy. 

seconded on the 'they may not share well' comment, even if co-located.I presume they would be two independently metered supplies that get added together on the bill,. If the DNO are not prepared to parallel them up before they get to you, then you should assume they can't be paralleled afterwards.

So if possible divide the loads in a way that has some logic and so the labelling of isolators can be understood- so that the whole of one block is on one supply, and some other more or less equally  large zone is on the other, It may just be worth arranging the ability to flip a large load (kitchens  maybe ?) from one to the other but only if it is not divvying up well.

It may also be worth asking the DNO engineers how they are doing this, and why, as a supply upgrade more commonly involves the arrival of one new fatter cable, and a disconnection of the old thinner  one - it seems a bit odd, and there will be a reason.

Mike

as a supply upgrade more commonly involves the arrival of one new fatter cable, and a disconnection of the old thinner  one

I suspect it's a case of they just don't do LV connections above 400A (sometimes 315A) - they don't have the gear and trying to get loop impedances right for lager fuses might be a challenge. For very large customers they'd go for supply at HV and let the customer sort out the transformers, but that's probably a bit too demanding for school, so they've come up with a bit of a compromise.

   - Andy.

I think thats it - they can't give us anything more than 400A from an LV outgoing way on the Tx. I'm very much new to this particular project but given the age of the existing switchgear it'll be quite tricky (and costly) to alter to suit the 2 separate supplies. It might actually be easier to upgrade the Tx and have a single point of supply.

Thanks for the responses Andy/Mike

Perhaps a silly question,  but what is driving the need for the supply upgrade at all - is a lot of new buildings or new loads being addded ? it may be easier to do the split ' old stuff' / newstuff on on the two supplies  ?

Mike.

Hi Mike, we're installing an ASHP as well as a WWHP - both really load heavy pieces equipment (>150kW). 

maybe that is the logical load split for now then...

Mike.

A job lot of thick woolly jumpers would be a lot cheaper!

Smithers46 said:

Hi Mike, we're installing an ASHP as well as a WWHP - both really load heavy pieces equipment (>150kW).