John Rumm wrote in
o.uk:

Subject: Do I need to update my house's fuse box?
From: John Rumm
Newsgroups: uk.d-i-y

On 26/06/2015 09:12, Jim x321x wrote:
wrote in
:

On Wednesday, 24 June 2015 10:59:29 UTC+1, Jim x321x wrote:
Can someone tell me if I can get away with a 45A MCB for a 8.5kW
shower without endagering life and limb? The manual tells me to use
a 40A one, but I just happen to have a 45A one. I ask, because
those things aren't cheap.


Probably, as you're not relying on the MCB for overload protection
(as the load is fixed and unlikely to draw a higher than rated
current), merely for short circuit in the event of fault.

What is the cable size an mounting method.?

Owain



It's a 6mm^ T&E cable that is surface mounted all the way to the
8.5kW shower. (about 4ft goes through surface-mounted conduit.

Most of that is Method C, but the section of trunking means you need
to treat it all as Method B. That gives the cable a continuous rating
of 38A.

Your design current is 8500 / 230 = 37A, so you are ok there (just).

Let's assume you are TN-C-S, and we will take the default 0.35 ohms as
the supply & EL impedance.

The cable round trip resistance will be 6.16 mOhms / metre.

Did you say the total run was 12m? If so that gives a total round trip
(by calculation - may be lower by measurement) 0.35 + 12 x 0.00616 =
0.42 ohms.

That gives a prospective fault current of 230 / 0.42 = 542A. The 0.1ms
trip threshold for a B type MCB is 5x In, or 5 x 45 = 225A in this
case. So we are safely into the magnetic or "instant" response part of
the curve for the MCB.

For piece of mind, treat the design as a non RCD one (even though a
RCD is required for other reasons), and ensure the fault withstand
capability of the cable is ok with that (nice to know the able won't
fail if the RCD does):

s = sqrt( 542^2 x 0.1 ) / 115 = 1.5mm^2 of CPC required (you have
2.5mm^2 of CPC in a 6mm^ T&E, so that is ok as well).

(where 115 is the k factor for PVC clad cable)

So based on a few assumptions - you are good to go. You will have no
overload protection for the cable, but that is not required in this
case due to the nature of the load.


Many thanks for the detailed reply. So what would be the ideal rating of
MCB to use? I just ordered a 40A one, because that's what the shower
manual recommends. The 45A one I had wasn't ideal because it wasn't a
Wylex one, and didn't fit without removing the base plate.

Jim