Shower 32amp breaker

The degree of unpleasentness rather depends on the true consumption of the shower, vs the breaker rating, - 8.5kW at 230V (36 amps), or at 240V ( 35A) or indeed a shower that would have been 35A if the supply was 240V but in a rural location where the supply reaching the shower is more like 220, then 33A. . 

So is it safe ? The breaker may trip, if the user stays in the shower for a very long time, but I think for the cable size it feels nicer to have something giving closer protection than a 40 or 50 amp breaker. In a house with an 80 or 60A company fuse, it probably seems to be a better fit to the maximum demand, though really it makes no odds.  (and you will occasionally find the odd shower has been added in on Henley blocks without anything other than the company fuse for top cover, but that really is much worse than the undersized breaker.)

To the letter it is a regs fail as you are designing for a small overload in normal operation. In practice, it seems to work just fine. And a shower on 4mm cable would probably be OK too.

I suspect the folk who do it always used to fit a 30A fuse, and have not read the label on the box of the shower, or only had a 32A breaker in the kit of parts.

Or the shower is an upgrade for an original 7.2kW model...

   - Andy.

Agree with last two replies and would add that in a majority of instances that 6mm cable will be method 100. In practice it is a trade off, but protect the cable in first instance.

433.1 forbids a small overload (which this is) for a long duration (which it probably is not). I assume that this is about a prolonged overload which will never trip the MCB, so I don't think that it applies in this case.


In answer to the OP, I suspect that it is done by electricians who work by rule of thumb rather than by proper design. ?

There is nothing to stop anyone sticking any old shower onto an original shower circuit. Happens all the time with plumber/builder/DIY electrical replacement [delete as applicable]. There always seems to be a default position on this new forum to blame an electrician. If you are an electrician with more than a few years experience, it being your primary occupation, you fully understand what happens in reality rather than in the cossetted world of BS7671 perusal.

With an arrangement as the OP, the B32 may become trigger happy over time. So it progressively will not tolerate 33-35 amps for shortish   durations. That is when an electrician gets the call. Personally, I see this happen more obviously when we get to 9kW plus. Even then, it is usually at least 6 to 12 months after the bigger shower is fitted. All that happens is one day they find the MCB will go off after 3 or 4 minutes of use.

In ye olden days 6mm and 7 kW was the norm. I would suggest that an electrician wiring a new shower circuit in the last 20/25 years would have defaulted to 10mm, even if the original fitted shower was 7kW or so.

At home I have 8kW on a 10mm on a B32. No problems, no issue for many years.

Showers last 5 to 10 years. They will be replaced often in a lifetime of the final circuit. So someone gets the call fix/replace a shower.  More often than not the client wants bigger bang for bucks, so will want the most powerful fitted. The circuit is not given any consideration, unless an electrician is involved. Of course, then the client may not like the answer, so goes back to the builder/ Plumber/ DIYr for the big bang for bucks.

If you feel the 8.5kW on the B32 is of concern, check all the connections for overheating. Check the MCB and the conductor for signs of heat damage. If there are no signs and it is reasonable that this has been in service for at least a few months, there is no problem to worry about.

Of course, things can go the other way. Someone may decide to change MCB for a 40 amp, but did they then consider what the installation method for this shower cable really is? Insulation is progressively now everywhere, especially with the extra layer thrown over the top on a grant. It is very rare that the shower cable is not going to be covered by something in an attic; with new builds in the last 20 years this insulation will also be in floor voids. So you could, in an effort you feel is required "to comply" with one bit of BS7671, fall foul of a more onerous consideration of BS7671.

All things considered, excepting the unknown installation method and unknown length of circuit, I would be quite comfortable with the existing situation. You would also accept that there is a very small chance, in the next one or two years, that you may need to replace the MCB.

I'd change if it is looking distressed - brown plastic, or melted looking, but for a modest overload that is probably not going to be on long enough to do anything like that, so if it looks in good shape, and is not tripping in normal use of the shower, then changing it is one of those kick into long grass jobs  - they are designed to trip after all - but if it does it too frequently it is not a good thing  one day it will trip off and stay off. 

Generally the main thing it may do is make the neighbouring MCBs a bit more sensitive by providing a degree of pre-heat, so perhaps your 6A lights are actually protected at 5 and a bit...


edit crossed in post with above, which expresses the same ideas but more fully.

My tendency would be to ignore it if the run is hard to change... but upgrade if it's easy. My own home has had an 8.5kw shower which runs between 38 and 39 amps  (the standing voltage is about 244-247 usually, due to being next door to the substation). This has been sat on a 32A breaker for 18 yrs, and more recently a B32 RCBO. Because it was 6mm2 I didn't upgrade the OCPD to a B40 when changing to RCBO's. No signs of overheating. along the route or in the connections when i upgraded to the RCBOs in 2018.


We tend to take shortish showers though (10 min MAX). A household with a teenager who likes half hour showers may have alternate experiences.


Ideally I'd upgrade, but not possible with the routing :(


Edit: the final section of the cable is 10mm2 from the isolator to the shower unit because I had the opportunity to do so. The original 6mm was done by an 'electrician' who causes me to use scare quotes round the word.  He bodged TRS (black rubber) cable into 30A jb's and replaced the first 2m from the board to the roofspace (bungalow), while I was resident in the USA and my elderly mother was in charge.


There are words for what I think of him but they're not fit for this forum.