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.


--
Cheers,

John.

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