shower mcb

7 Aug 2008
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United Kingdom
hi all - just had a shower replaced in the same place as the original using the same wiring as the old one. ( 45a switch moved postition ). the old shower was a 9.5kw and was using a 32a wylex mcb in the consumer unit and never gave any problems.

i am renewing the shower with only an 8.5kw shower but i think i need a 40a mcb? even though my new shower is less powerful? :confused:

my question is if i need a 40a mcb can i simply replace the old mcb with the new one of the correct rating?

i think the wiring is 10mm.

my mcbs look like an older style than the wylex currently on sale so will these fit in the unit o.k?

many thanks
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Your old shower should have been protected by a 45A MCB. I'm quite suprised you didn't ever have any problems with tripping, although if you only ever used the shower for short periods of time then the breaker may have held in. Either that, or it's faulty.

Having not seen your consumer unit, I can only assume it's a Wylex standard with plug-in MCBs, in which case the newer models designed for their NH range will not fit.

I'd be inclined to believe that you'll have a 6mm feed to the shower if the entire installation is fairly old. If you have 10mm then it will likely be adequate for your new shower, but there are derating factors and cable length to take into account, so I can't give you a solid answer.

Either way, I personally wouldn't want to fit such a large load onto an outdated consumer unit. It's probably overdue replacement - have you got any pictures?
hi there - thanks for the reply

heres a photo of said consumer unit - i think if i have uploaded them correctly ! :confused:
not sure if i loaded the photos correctly or not but i think you get them by clicking on my username ? :confused:
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May not actually be as old as I expected - you can still purchase these, although I can't imagine many electricians would fit them these days other than for a direct replacement. I believe MCBs and carriers up to 40A are also available to fit, although you really need to enlist the help of a competent electrician before you simply start making changes.

A new MCB could completely alter the characteristics of the circuit, you need to be sure the cable is adequately rated, and that the new protective device will comply in terms of minimum disconnection times. You also don't appear to have an RCD, and this is a requirement specified in the instructions for probably every shower on the UK market today.

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