Having an electric shower replaced

I've just had a quote for an electric shower replacement and unusually it has come in cheaper than expected. Not such a bad thing but just wanted to check it is being done properly.

The quote specifies a mains pressure 8.5kw shower. It will also reuse the current cable and fuse. I'm not 100% sure what size the current cable is but I'm guessing it is 6mm. The fuse is 32amp.

I did question it as I thought the cable and the fuse would need to be upgraded to 10mm and 40amp. I was told that although they are called 8.5kw they only run at 7.8kw. He also said that the short run of cable meant it didn't need to be higher. The wire runs for about 2.5m in a cavity (no cavity wall insulation). It then goes about 40cm under the loft insulation, then goes down about 50cm in the airing cupboard.

Does that all sound ok to be on what I think is 6mm cable and a 32 amp fuse?

I don't really like to question as they are the expert and it will save me money but just wanted to make sure before I go for it.

 

He's very probably right about that. The 8.5 kW is very probably quoted at 240V - in which case he is correct is saying that that would equate to about 7.8 kW at 230V (the nominal supply voltage, used for calculations).

It's a bit iffy. 7.8 kW at 230V is about 33.9A, so a bit over 32A. As for the cable, there is nothing I am aware of in the regs relating to a short run of cable being between a (presumably plasterboard) ceiling and insulation. However, if it were all between a plasterboard ceiling and insulation no deeper than 100mm, then it would have a current-carrying capacity of 34A, or if it were in a studd wall between plasterboard and insulation it would be 35A, hence in both cases just adequate for a 7.8 kW shower.

Opinions may vary, and it sounds as if your electrician may well be exercising 'common sense'! The cable business is really all about 'regulations', since the short duration of use of showers and the short length of cable covered by insulation are such that I personally would not regard there as being any (electrical) problem. Can the short bit of cable under the loft insulation not be raised up so that it would be above the insulation. The 'fuse' (I imagine it is actually an MCB) rating is perhaps a bit different, since one is not really meant to subject an MCB to even just 'a little' more than its rating - but, again, maybe your electrician is exercising 'discretion'.

Kind Regards, John

 

Yes it is an MCB, the consumer unit is fairly modern, we had it replaced about 10 years ago.

Sounds like it will be ok then.

The cable probably could sit above the insulation, I'll ask on that one. If the cable is all going to be ok, I guess they could always swap the MCB over to a 40 amp?

 

Fair enough - if you can do that, you might as well, particularly given ...

From what I understand of your description of it's route, if you get get that bit of cable above the insulation (such that none was then covered with insulation), then the 6 mm² cable should be good for 47A, in which case it would be fine with a 40A MCB (which, as I implied, is what you really should have with a 7.8 kW shower). If any of the cable remained under insulation, it would be much less certain that, strictly speaking, a 40A MCB could/should be used.

If you did get the cable above insulation and change to a 40A MCB, then you could have a load up to 9.2 kW at 230V which would probably translate to an 'advertised' power (at 240V) of about 10 kW - it might therefore be possible for you to discuss the possibility of having a larger shower than the "8.5 kW" one.

Kind Regards, John

 

If you think the cable would be alright under the insulation with the 32A MCB, then it will be with a 40A; no more current will flow.

 

Very true, electrically. However, strict adherence with the regs (if one is concerned about that) requires that In>Iz, as well as Iz>Ib.

Kind Regards, John

 

Then the 32A MCB is not compliant.

 

Indeed - which is what I said, maybe not exactly in those words ....

and ...

... but that's all about In>Ib - the cable (Iz) is a separate issue, and has to 'fit in between' In and Ib.

Kind Regards, John

 

Edit - made a mistake if you read it that quickly.


Correction - 433, Omission of overload

 

You did that just in time

For the record, before that edit I was starting to type that, when I was at school, 33.9 was greater than 32!!

Kind Regards, John

 

I'm getting confused, or ...

32A is the MCB < 33.9 so not compliant. That's what we said although the electrician is happy with it.

The cable is fine - no matter what MCB is fitted - 32A or 40A.

 

I'm pretty sure that for a cable run with a mixture of installation methods you use the highest derating factors and appy it to the whole thing.

Not to be taken as a claim that it would happen in this case, but there'd be little consolation in saying that the rest of the cable was just fine if a short stretch was on fire...

 

Indeed, and were this not so 'public', I suspect that some of us would also say that we were 'happy' with it - for a dedicated load used only infrequently for short periods.

Only if the OP can rearrange things so that the entire cable run was Method C - otherwise the CCC would (probably) be <40A, hence not compliant with a 40A MCB.

Kind Regards, John

 

In the specific case of a cable which has a short length totally surrounded by thermal insulation, Table 52.2 (not sure why it's not in Appendix 4) gives 'reduced' de-rating factors for lengths of cable within insulation of less than 500mm. However, I'm not aware of any other similar specified 'reductions' for lesser degrees of association with thermal insulation.

 

No, it only has to be greater than 33.9A - which you said it was - the shower cannot overload the cable.