Supply for an outbuilding

JRave · Jun 18, 2012

Hi all,
I'm having some trouble designing an installation for an outbuilding.
It's about 110m from the nearest CU in the house, which is fed via 16mm T+E by the main db, a good 15m away.
The building will have something like a 9.5kw instantaneous water heater in it, as well as about 3kw worth of electric heating plus lighting and one power ring.
I've made some calculations using a design current of 80A, as I can imagine the shower pulling the water heater, while the heating is on full blast and the kettle is on.

So I'm getting back that I should use 70mm 2-core SWA, which is massive. I've got the problem of getting such a big cable dressed tidily around the house, and then terminating it with additional termination boxes at either end.

This is the first time I've had to come up with a supply like this and I'm worried I'm going overboard.

I've looked seriously at diversity, but with the scenario I described before, I don't think it will apply much here.

The quote I've had for the cable is £9.82p/m. The materials for this supply alone will hit the £1500 mark so I'm worried that's a bit steep also.

Another thing is that it's a tree house! So it's all wood. The walls will be clad either side with insulation between an I'm not finding anything to suggest I should use any specially rated cable. It's only a 4.5/6 meter room but I was thinking of going low smoke.

Any help/comments gratefully welcomed.

Thanks,

John

telectrix · Jun 18, 2012

Just wondering what your customer's home planet is.

topquark · Jun 18, 2012

Can't comment on the power you need, you need what you need. However you won't be supplying that additional 80A requirement from a 2nd CU fed by 16mm tails/cable surely? What do you have as the main supply type and size?

If it's voltage drop (because of the distance) forcing the cable size up; then try the calcs with say two or three smaller cables carrying your remote supply.

topquark · Jun 18, 2012

telectrix said:
Just wondering what your customer's home planet is.

Gallifrey?

Plonker 3 · Jun 18, 2012

Tell them to buy a bloody generator, your incoming supply probably wont be big enough to take that load anyway. If it isn't and that needs upgrading then maybe you could ask them to install a new supply in the outhouse too?

Engineer54 · Jun 18, 2012

Never heard of anything so ridiculous!!!

JRave · Jun 18, 2012

Yeah it's the volt drop, I'll try some other calcs, but would it make a lot of difference to the cost and complication?
The main supply is 100A TT.

For the calcs, do I just work out with half the load, twice, to get 2 smaller cables?

topquark · Jun 18, 2012

I'm out.

JRave · Jun 18, 2012

It's going to be a tree house B&B! Has planning go-ahead.
I made some inquires to western power for a new supply this morning, but, as you'd expect, there's a massive process to go through before they'll talk costs to you.
Perhaps it is a bit ambitious to power from the house anyway. I didn't check to see what they use for heating and showers in the house. But yes, I'm thinking now if they have any other big pullers then new supply it is.