2x 9Kw Electric Boilers

RickyB1990 · Mar 4, 2023

Good morning Guys

Looking for some advice with regard to a current job.

Building company I'm doing work for are installing Electric heating on a property.

Customer has no access to Gas so there are 2x 9kW Electric boilers being installed. (Plumbers and Builders recommendation).The incoming supply was initially 60amps

I made customer and company aware that the incoming supply wasn't big enough ( Electric shower and electric Cooker currently in place aswell). I advised that they would have to upgrade the fuse but that the maximum was 100 Amp for a domestic property.

I was then told that the fuse was upgraded and the works have now begun. The main Fuse sticker is showing 100Amp.(So I'm presuming that's what been installed) When I arrived on site I noticed the boilers were 9kW and made a calculation of around 78 Amps on full load. The manufacturer informed me both boilers won't run together initially at same time. Only one will come on initially and then once heat has reached the other will kick in.

So my question is, I'm a bit worried about the maximum demand. And where do I stand in terms of the installer.

Has anyone been on this situation before

Many Thanks

ferg · Mar 4, 2023

You can get relays for the heating boilers so that only one runs at a time.
Same for the electric shower.
You can also use current sensing relays to make your own.

Rockingit · Mar 4, 2023

As in, electric system boilers? Personally I think this would be fine on 2no. 32A radials (yes, I know the maths says 39A....) however, what you could do instead would be to install a 63A feed (I'm saying 63A because (a) I think that'll be fine and (b) you'll struggle to get more than a 63A MCB for a domestic board) to a simple twin module CU that's sited with the boilers (use a four way and you've got space for all your control circuits...) and have two local 40A breakers at that point.

It's always worth remembering in these situations that a 100A 88:2 or whatever it may be will sit at 110A for...... almost forever. So whilst it can be frowned upon as not entirely legit to the letter of our dictat, at an engineering level it's fine unless you expect people to actually be in the shower, boiling four pans on the hob, kettle on for a coffee AND setting the heating to 30degs, simultaneously for 20hrs. That's why we have the principal of diversity. You also have to consider that when the DNO came along and upgraded the supply, they didn't change the cable in the road, they just put a bigger fuse on the end of it! @Julie. will know this more than me but from memory I think the DNO's work out their own diversity as something like 8A per dwelling - it's a really low number!