Replacing 32a mcb with a 40a

[ericmark]

If you see some one about to climb a cliff face likely the best advice would be don't climb the cliff better to walk in a large circle so you can get to top without climbing the face. However likely he would take no notice and climb it anyway, so better advice would be use rope and other climbing aides to remove some of the danger. It's just possible he will listen to that advice.

In a DIY forum it is unlikely anyone will do it the safe way. They can't possibly do everything they should and still save money.
So here will look at what he should do, work out the cable route, work out the cable size, and from published data select a MCB which will protect the cable. One should not assume, but assuming original MCB was selected to protect cable if the MCB is not changed the cable is still protected. If you have a 10A MCB B type and you draw 10A then you can do that for as long as one likes, as one starts to draw over 10 amps the time you can draw it for will reduce until between 30 and 50 amp when it will trip within milliseconds. However drawing 25 amp will not cause a fire because the MCB will open before the cable over heats. Unless one can get a cold 10A overload device as with changing a fuse then you can't over heat the cable as the trip will open first.

It is clearly not a good idea to draw 13 amp on a regular basis from a 10A MCB but it will work for a long time without any cable damage. However as soon as you change that 10 amp MCB for a 16 amp you have lost the protection.

In the case in question we are reasonable sure that with a 32A trip before damage happens the trip will open. It does not matter if the shower is 7.5kW or 12kW as long as the trip is 32A the cable will not be damaged. Although with regular tripping the 32A MCB will likely degrade. It is near impossible for anyone to know the installation method without renewing the cable so to swap to 40A has rather too much danger involved. To reduce the shower to 7.5kW or renew cable and MCB is the only real answer. However the whole idea of swapping to a 40A MCB fills me with dread.

Yet it seems people are willing to advise checking what cable is installed and route and if the rating is up to 40A renewing the MCB. In a factory where the cables are on cable tray this could be an option but a house is not a factory and in most cases it means lifting floor boards to work out the route, so likely the person doing the work will take a chance.

The 7.5kW shower is best answer, but assuming he will not go out and get one then it's down to comparing risk. Swapping to a new 32A MCB is very little risk swapping to a 40A MCB is high risk so best option is a new 32A MCB if the new shower unit is not going to be bought or and new cable is not going to be installed.

 

[ban-all-sheds]

Eric - you can't put an 8k5 shower on a 32A device, and that's all there is to it.

 

[sparkwright]

Extremely unlikely, as in 2013 the typical shower circuit was run in ten mm cable on a 40amp breaker (most dedicated shower consumer units of that era had a B40 breaker in them).

I don't think it's that typical that 3 years ago 10 mm2 was often used on shower wiring.

Many electricians unfortunately try to get away with using 6 mm2 cable, as it's cheaper and easier to use.

However, around 35 years ago it was not unheard to wire showers in 4 mm2 as showers were less powerful.

It is unlikely that only 3 years ago anyone would consider using 4 mm2 for a shower. A lot of electricians don't even regularly buy or use 4 mm2.

 

[scousespark]

I made an observation that there is nothing in this thread to support an asumption that the cable will likely be 6mm.

How likely do you think it is that 3 years ago an electrician would have installed 4mm² for a shower circuit?

Hi BAS

You are missing my point. I have no clue what size cable is in place. We don't know for certain that the poster had an 8.5Kw shower (given his statement that he has no elecrrical knowledge). For an 8.5Kw shower the likely circuit would have a 40A MCB and be at least 6mm. Given the MCB is 32A, it's a pure guess as to whether the cable is 6mm.

 

[ban-all-sheds]

I made an observation that there is nothing in this thread to support an asumption that the cable will likely be 6mm.

How likely do you think it is that 3 years ago an electrician would have installed 4mm² for a shower circuit?

Hi BAS

You are missing my point. I have no clue what size cable is in place. We don't know for certain that the poster had an 8.5Kw shower (given his statement that he has no elecrrical knowledge). For an 8.5Kw shower the likely circuit would have a 40A MCB and be at least 6mm. Given the MCB is 32A, it's a pure guess as to whether the cable is 6mm.

I was going to try and explain again, but then I realised that you simply do not have the capacity to differentiate between saying that something is likely, and assuming that it is.

 

[scousespark]

I made an observation that there is nothing in this thread to support an asumption that the cable will likely be 6mm.

How likely do you think it is that 3 years ago an electrician would have installed 4mm² for a shower circuit?

Hi BAS

You are missing my point. I have no clue what size cable is in place. We don't know for certain that the poster had an 8.5Kw shower (given his statement that he has no elecrrical knowledge). For an 8.5Kw shower the likely circuit would have a 40A MCB and be at least 6mm. Given the MCB is 32A, it's a pure guess as to whether the cable is 6mm.

I was going to try and explain again, but then I realised that you simply do not have the capacity to differentiate between saying that something is likely, and assuming that it is.

Of course I know the difference between assuming and saying something is likely. I am not there and I have no idea what size cable is fitted. I would not assume it's 6mm. It may be or it may not be.

 

[JohnW2]

Of course I know the difference between assuming and saying something is likely.

We all seem agreed, then.

I am not there and I have no idea what size cable is fitted. I would not assume it's 6mm. It may be or it may not be.

Exactly - which is why I merely expressed my opinion that it was 'likely' that a shower circuit installed 3 years ago was wired in at least 6mm² cable, without in any way 'assuming' that it was. However, for some reason you seemed unhappy when I expressed that opinion.

Kind Regards, John