Replacing 32a mcb with a 40a

[BromleyRob]

Hi all, I'm new here, and shall we say, electrically challenged.

We had an extension built 3 years ago, and had an electric shower installed (8.5kw) it runs through a 32a type b mob on a Schneider CU, and recently kept tripping out.

Aqualisa, advised us that the heating element had probably gone, and would cause the man to trip, so they all but replaced the unit with a 9.5kw one. Problem is, it still trips out straight away, so after a little research, I found that it should run through a 40a mcb... any ways I've now got a 40a type b mcb, and needed to know two things:

1) is it a case of simply replacing the mcb (not "simple" for me but you know what I mean) and
2) is replacing a single pole mcb a job that can be done myself or should it be carried out by an electrician?

Thank you in advance!

Rob

 

[LearningSpark]

Firstly, what size cable feeds the shower and what route does it take from the consumer unit to the shower?

 

[BromleyRob]

The cable runs under the floor (floating floor) then is only buried under the plaster/behind tiles to where the shower is positioned on the wall. And as far as I know it is 6mm cable

 

[Taylortwocities]

And as far as I know it is 6mm cable

You cannot guess at this. If you have a 40A MCB the cable must be at least 6mm²
You may be able to gauge it using this guide //www.diynot.com/wiki/Electrics:flatpvccables

Also, you should check that the EFLI is within limits before upgrading the MCB.

 

[ban-all-sheds]

Hi all, I'm new here, and shall we say, electrically challenged.

Then the answer to your last question ought to be "by an electrician"

We had an extension built 3 years ago, and had an electric shower installed (8.5kw) it runs through a 32a type b mob on a Schneider CU, and recently kept tripping out.

Not surprised - it draws more than 32A, so the breaker is probably shot by now.

What kind of "electrician" was involved with the extension? The shower should never have been installed like that.

1) is it a case of simply replacing the mcb (not "simple" for me but you know what I mean) and
2) is replacing a single pole mcb a job that can be done myself or should it be carried out by an electrician?

As TTC said, the cable size needs to be known. And the fault-loop impedance checked - here's why: //www.diynot.com/wiki/Electrics:MCB2

More info re cable sizes etc:



//www.diynot.com/wiki/Electrics:flatpvccables

//www.diynot.com/wiki/Electrics:MCBtoCable

http://www.batt.co.uk/upload/files/4d5.pdf

http://electrical.theiet.org/wiring-matters/28/appendix4.cfm?type=pdf

http://itshosting.me/aaes/twin-and-earth-te/

 

[ericmark]

If a MCB is overloaded on a regular basis then over time it start to trip and a lower and lower amount of energy used. So draw 40A from a 32A MCB and on day one likely 20 minutes before it trips.

4 years down the line and that time is reduced to 5 minutes.

As said you should not overload either the cable or trip, but likely replace the 32A trip for a new one of same type and for another 4 year you will get away with it.

The main point is you want the trip to disconnect before the cable is damaged. So unless you know exactly how the cable is run plus earth loop impedance etc the only option is replace with same size MCB and accept in 4 years time it will likely need changing again.

 

[JohnW2]

... As said you should not overload either the cable or trip, but likely replace the 32A trip for a new one of same type and for another 4 year you will get away with it.

That may well be true, but I'm not sure that we should be advising (or even suggesting) that anyone should deliberately overload an MCB (even if the cable is adequately protected) - not the least because it is non-compliant with regulations.

Kind Regards, John

 

[ericmark]

... As said you should not overload either the cable or trip, but likely replace the 32A trip for a new one of same type and for another 4 year you will get away with it.

That may well be true, but I'm not sure that we should be advising (or even suggesting) that anyone should deliberately overload an MCB (even if the cable is adequately protected) - not the least because it is non-compliant with regulations.

Kind Regards, John

OK I can see your point, however better some one replaces an old MCB with a new one of same rating, than with a new one of a higher rating. I agree there is a risk, however assessing that risk I felt it better they replace like for like than uprate the MCB. On assessing the risk I considered the likely hood that the poster would replace the cable, I considered this was rather remote, I also considered if they would buy a smaller shower unit, I also considered that as remote. So I thought replacing like for like safer than using a bigger MCB. I admit it is wrong, but do you really think up to date we have really persuaded them to go to the more expensive root?

 

[EFLImpudence]

Wouldn't it be wisest to let someone who knows have a look and then it may be possible to fit a 40A MCB and all is well with the world - well, apart from the Middle East, oh and ...

 

[JohnW2]

OK I can see your point, however better some one replaces an old MCB with a new one of same rating, than with a new one of a higher rating. I agree there is a risk, however assessing that risk I felt it better they replace like for like than uprate the MCB.

Yes, but your risk assessment was based on inadequate information. I suspect that even 'cowboys' probably don't often wire showers in 4mm² cable, so the chances are that it is at least 6mm² (as suggested by OP), and very probably with an adequate EFLI (for a B40). If, as I think likely, that proved to be the case, then your risk assessment would be totally wrong - since it would then be correct, compliant and 'safe' to replace the MCB with a 40A one, but incorrect, non-compliant and arguably not even 'safe' (risk of overheating MCB) to install another 32A one.

Kind Regards, John

 

[BromleyRob]

Thank you for the advice everyone, I think it's going to be a case of getting an electrician in to check it over prior to any changes, just to be sure then, I'd rather not cause more damage if possible!

 

[JohnW2]

Thank you for the advice everyone, I think it's going to be a case of getting an electrician in to check it over prior to any changes, just to be sure then, I'd rather not cause more damage if possible!

Indeed. As I've said, I think it pretty likely that an electrician would be able to confirm that your original plan (to replace the 32A MCB with a 40A one) would not only be OK, but would actually be the only correct and regulation-compliant thing to do - but you do really need that confirmation before you do it.

It would be nice if you could let us know what the electrician says/does.

Kind Regards, John

 

[scousespark]

If a MCB is overloaded on a regular basis then over time it start to trip and a lower and lower amount of energy used. So draw 40A from a 32A MCB and on day one likely 20 minutes before it trips.

4 years down the line and that time is reduced to 5 minutes.

As said you should not overload either the cable or trip, but likely replace the 32A trip for a new one of same type and for another 4 year you will get away with it.

The main point is you want the trip to disconnect before the cable is damaged. So unless you know exactly how the cable is run plus earth loop impedance etc the only option is replace with same size MCB and accept in 4 years time it will likely need changing again.

For the benefit of the the OP, this 'advice' is absolute nonsense. Regardless of what was in place originally, a 32A MCB is not suitable for 9.5Kw. If, as Rob intially stated the shower was rated at 8.5Kw, there should have been a 40A MCB from day 1. That's elecrical installation 101. To advise Rob to stick with the undersized MCB and accept it will need replacing in a few years is ridiculous.

 

[scousespark]

Thank you for the advice everyone, I think it's going to be a case of getting an electrician in to check it over prior to any changes, just to be sure then, I'd rather not cause more damage if possible!

Indeed. As I've said, I think it pretty likely that an electrician would be able to confirm that your original plan (to replace the 32A MCB with a 40A one) would not only be OK, but would actually be the only correct and regulation-compliant thing to do - but you do really need that confirmation before you do it.

It would be nice if you could let us know what the electrician says/does.

Kind Regards, John

Uprating the MCB may fix the problem, but it's just as likely it won't be that simple. We don't know why the electrician selected the 32A MCB, but he may have installed 4mm cable. If this is the case, a 40A MCB will not be compliant (more importantly - not safe)

I'd be concerned that the shower guy uprated the elements and walked away leaving it in a non-working state. The 32A MCB may indeed be shot, but that's not necessarily the (only) problem.

I don't post very often because of threads like this. Rob has a real-life problem and was advised to get it checked. The early responders didn't suggest the cause or solution, as this needs investiagtion and cannot be resolved over the internet. It then it turned into the equivalent of a C&G classroom question and Rob gets the 'preferred' answer and asked to let us know how he got on. Is that so we can see you are right again?

You tell us that you have this job writing major reports for medical companies. I assume this involves a lot of investigation and little/no guesswork, but your posts in this thread are guesses based on assumptions.

 

[JohnW2]

As I've said, I think it pretty likely that an electrician would be able to confirm that your original plan (to replace the 32A MCB with a 40A one) would not only be OK, but would actually be the only correct and regulation-compliant thing to do - but you do really need that confirmation before you do it. It would be nice if you could let us know what the electrician says/does.

Good to see you after some time, but you do, yet again, seem to be singling me out for 'finding fault' with things I've written - on this occasion I don't think with much justification.

Uprating the MCB may fix the problem, but it's just as likely it won't be that simple. We don't know why the electrician selected the 32A MCB, but he may have installed 4mm cable. If this is the case, a 40A MCB will not be compliant (more importantly - not safe)

All true, but so what? - nothing I wrote was inconsistent with that. I previously suggested that its fairly unlikley that anyone would wire a shower circuit in 4mm² cable, but it's not impossible, which is why I only said that it is "pretty likely" that an electrician would be able to confirm that a 40A MCB would be appropriate (and required for the shower) - but adding that it was necessary to obtain that confirmation before 'up-rating' the MCB.

I don't post very often because of threads like this. Rob has a real-life problem and was advised to get it checked. The early responders didn't suggest the cause or solution, as this needs investigation and cannot be resolved over the internet.

Quite so - which is why I said that an electrician needs to investigate and determine whether a 40A MCB should/could be fitted (and whether the cable is adequate for the shower). My only real contributions to this thread have been that indication that an electrician's investigation is needed and to respond to eric's suggestion that the OP should continue using a 32A MCB for a 9.5kW shower with:

That may well be true, but I'm not sure that we should be advising (or even suggesting) that anyone should deliberately overload an MCB (even if the cable is adequately protected) - not the least because it is non-compliant with regulations.

... and I'm a little surprised that even you should criticise me for any of that!

It then it turned into the equivalent of a C&G classroom question and Rob gets the 'preferred' answer and asked to let us know how he got on. Is that so we can see you are right again?

You really seem to 'have it in' for me I've told the OP that an electrician needs to investigate and have told him what, IMO, is the 'likely' outcome of that investigation. Is it not reasonable that I should be interested in discovering whether or not I am correct in that opinion?

You tell us that you have this job writing major reports for medical companies. I assume this involves a lot of investigation and little/no guesswork, but your posts in this thread are guesses based on assumptions.

I've never 'told you' anything particularly specific about my professional life. It is certainly true that some aspects of my working life involve the writing of 'major reports' (or involvement in the writing thereof), for a spectrum of types of client. Some of that is purely mechanistic, but one of the important things a client often pays me for is my personal opinion (based on my knowledge, skill and experience) as to the 'likely' interpretation of the available information - you may call that "guesses and assumptions", but it's not how my clients see it!

Kind Regards, John