Shower questions

[Philip Thompson]

My son's 7Kw shower has just packed in and we are looking round for a new one. There seem to be very few 7Kw ones but loads of 8.5Kw.

First question is can we run an 8.5Kw on his existing 6mm cable? Length is about 12 meters, some behind plasterboard and some clipped in the roof.

The fuse is rated 32A so presume that's a bit on the low side.

Other point is, looking at some makes of showers, the water feed seems to be pushfit on a lot of them whereas I'm used to seeing the compression elbow input. Does this mean we have to replace the feed with plastic or will the pushfit device take the existing 15mm copper.

When we have more time, we will upgrade the cable to 10mm but for a while yet, we will have to make do... Thanks for any help

[Lurch]

On Sun, 15 Jul 2007 21:10:44 +0100, "Philip Thompson" mused:

Without seeing it I would say that should be OK for 8.5kw.

Maybe, but I don't put anything more than a 32A on any 6mm and I haven't had a problem yet.

The existing copper should be fine on the pushfit. Best bet would be to RTFM.

[John Rumm]

Possibly, we can do some sums.

What type of earthing does the house have?[1]

Is there a RCD protecting the shower circuit?

[1] The fuse is rated 32A so presume that's a bit on the low side.

It is unlikely to blow (since a 32A cartridge fuse will take getting on for 5A indefinitely), but 40A MCB would be a better choice.

What type of fuse - cartridge or re-wireable? (I presume the former as you said 32A rather than 30A)

It will take the copper.

There is probably no need.

If you went with a 8.5kW shower at a nominal 230V, that gives 37A

Voltage drop at 37A and 12m should be well inside the 9V limit. The current rating for the cable is 46A for the installation method you have described assuming there is no insulating material behind the plasterboard.

With regards to disconnect time there is a very good chance that it will be within spec since a 40A MCB will only require Zs of 1.2 ohms to disconnect in 5 secs, and the cable will only contribute 0.13 ohms of that. We need to check the CPC is of adequate size. Say we take worst case as being TN-S installation with Ze of 0.8 ohms (this is probably pessimistically high), and add to that the round trip phase to CPC on a

6mm cable 10.49 mOhms/m we get 0.92 ohms loop impedance, so an earth fault current of 250A. That should disconnect in 0.1 secs, so we get sqrt( 250^2 x 0.1 ) / 115 = 0.7mm required minimum CPC size - well inside the 2.5mm^2 provided by the 6mm^2 T&E. That's not allowing anything for supply impedance (but is being pessimistic on earth fault loop), so chances are that will fine. If it does have a RCD then there is certainly no problem.

(the CPC size is still just ok without a RCD if you redo the sums for a

40A cartridge fuse as well)

[Philip Thompson]

Wonderful Thanks for all the information. The house is only about 3 years old so the electrics are pretty much up to standard.

I had got it in my mind that 6mm cable was rated at 32 amps ish and that was my concern with using an 8.5 KW shower.

Anyway, at least we can get him up and running with a new one fairly quickly..

Thanks very much guys :)

[Dave Plowman (News)]

This will give you something to play with:-