40A MCB for 9.5kW shower?

I have an old Wylex consumer unit, like this:

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've bought an 9.5kW electric shower, but I've just discovered that Wylex don't make 45A MCBs for these consumer units anymore, the highest rated they produce is 40A. At 230v, 9.5kW draws 41.3A. Will I get away with a 40A MCB or will it be forever tripping? (I have 10mm cable, by the way, so 40A won't be a problem there).

Reply to
Martin Pentreath
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Although marketed as a 9.5kW shower, this might be at a nominal 240V. Do check. If so, then it is fine on a 40A anyway, as it would draw only 39.6A on a 240V supply and even less on a 230V one.

Your shower should show the voltage at which it is rated somewhere on it.

Christian.

Reply to
Christian McArdle

The reason is most likely that the fusebox is only rated 60A in total, and you may find your main fuse is also 60A in that case. Sounds like you need to check if your existing supply can support the additional 9.5kW load you are intending to add. You may need the supply (or at least the main fuse) upgrading to 100A and a new fusebox.

Reply to
Andrew Gabriel

You're right, the main fuse is a 60A one. The cooker is gas though, and the water is heated by a combi, so there's not that much demand for electricity. It's a three bedroom terrace. The shower is for occasional use only, predominantly in case the combi breaks down.

Can upgrading the supply be as simple as getting the main fuse changed? Presumably the cable from the meter to the CU has to be heavy enough to take 100A. I've no idea what size the current cable is, except that it looks bloody thick! The only problem with changing that is that it the fusebox is boxed in in a place that makes access to the incoming cable very difficult.

Reply to
Martin Pentreath

You must have upgraded the fusebox to a 100A one before they will upgrade the fuse, and make sure earthing is up to current regs, etc. I haven't had a fuse upgraded for over 10 years, but it used to be free (at least if supply cable was OK for 100A). Someone here may have had it done more recently and might be able to confirm if it's still free or not -- with increasing split-up of the supply industry, it may well not be.

Reply to
Andrew Gabriel
[snip]

Just a reminder of the importance of the supplementary bonding, when you're fitting an electric shower, especially if it's over a metal bath. At the very least make sure you have a 4 mm^2 bond between the shower's earth terminal and the bath.

Without bonding, if your shower heater happens to develop an earth fault the shower metalwork may rise to over 160 volts[*], relative to other 'earthy' objects such as the bath. This voltage will persist until the fuse blows or MCB trips, and that could take up to 5 seconds, during which time you could be receiving a particularly nasty shock. The local supplementary bonding will greatly reduce this fault voltage and thus keep you safe.

[*] 10 mm^2 cable has a 4 mm^2 CPC, so the resistance in the CPC is 2.5 times as high as in the phase conductor. So with a phase-CPC s/c at the shower and no bonding you'll have 230 * 2.5 / 3.5 V = 164 V developed across the CPC - nasty!
Reply to
Andy Wade

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