Storage Heater MCB rating

[bobknot]

I've just noticed that our storage heating consumer unit is fitted with 32amp MCBs. They are MK Sentry RCBO type (four, running four storage heaters). This was all installed around 18 months ago along with a new 'economy 7' meter (the old one switched tariffs when it felt like it!). The work was done by a qualified electrician, along with a couple of other wiring jobs.

Looking at a few posts on the forum, it seems that 16 or 20amp MCBs are more the norm. Are the ones fitted safe?

I should probably mention the largest heater is 3.4kw.

 

[JohnW2]

I've just noticed that our storage heating consumer unit is fitted with 32amp MCBs. They are MK Sentry RCBO type (four, running four storage heaters). This was all installed around 18 months ago along with a new 'economy 7' meter .... The work was done by a qualified electrician, along with a couple of other wiring jobs. ... Looking at a few posts on the forum, it seems that 16 or 20amp MCBs are more the norm. Are the ones fitted safe? ... I should probably mention the largest heater is 3.4kw.

Welcome to the forum. I think your suspicions are probably correct. A 3.4kW heater requires under 15A - so, as you say, 16A or 20A RCBOs (or MCBs) would be adequate for any of your heaters.

The MCB/RCBO is there primarily to protect the cable from excessive current, and it is the size of the cable that therefore dictates the maximum size of MCB or RCBO a circuit should have. One would normally expect a circuit for a single storage heater to use 2.5mm² (maybe even 1.5mm²) cable. If that is the case, then a 32A MCB or RCBO is too big to adequately protect that cable. If the cable were 4mm², then the 32A MCB or RCBO would probably be OK, but I would think it pretty unlikely that cable that large will have been used.

The reality is that a storage heater is extremely unlikely to ever overload a cable, so to say that the situation you have is unsafe would probably be putting it a bit strongly (and some might even try to argue that this would make the RCBOs compliant with regulations), but if (as is likely) the cable is 2.5mm² (or smaller), then you really should have the devices reduced to 20A or 16A ones. Since the circuits were installed relatively recently, I would be inclined to suggest that you speak to the electrician who did this work and indicate your concerns.

Kind Regards, John

 

[sparkwright]

Exactly as John says.

Would only expect 32 amp breakers to be used if 4 mm2 cable is used - and then I would expect to see 13 amp switched fused spurs used at the heaters - as opposed to the more usual 20 amp UNfused switches.

But then a 13 amp switched fused spur would be just a bit too small for the 3.4 kW heater - which at a guess is somewhere around 14 amp.

Photos may help, if only to give us an idea of the quality of the install.

 

[securespark]

We love photos!

 

[bobknot]

Thanks for the quick replies.

I think the work was carried out to a good enough standard (or at least looks that way), but we did have problems with the electrician in that he disappeared for weeks at a time. He eventually arranged for a different fella to come and finish the work.

I have a fair bit of experience with domestic electrics, having, in the past, been involved in building and fitting kitchens and the like. Which is why alarm bells rang for me after I noticed 32amp MCBs running to DP switches rather than fused spurs. Sadly, I can't really get hold of the original electrician now as he has since got very ill and retired, and, believe it or not, I never had the contact details of the fella that finished off the work!

It is possible that heavier than normal cable was used as, at the same time, we had power laid on to an adjoining workshop, which is fed from a 40amp MCB at the consumer unit (he might have used the same cable?). Either way, I reckon I need to have a look to see if I can figure out what cable IS used for the heating.

I've uploaded some images of the installation, but am struggling to insert them!

 

[bobknot]

View media item 84165
View media item 84166
Ahhhh ...success!

 

[sparkwright]

Is the wiring for the storage heaters buried in the wall, or run surface, perhaps in trunking?

If the wiring is concealed in the wall at less than 50 mm, and this was only 18 months ago, an RCD should have been fitted to protect the wiring.

If possible, a picture of some of the actual wiring would be helpful here...

 

[JohnW2]

It is possible that heavier than normal cable was used as, at the same time, we had power laid on to an adjoining workshop, which is fed from a 40amp MCB at the consumer unit (he might have used the same cable?). Either way, I reckon I need to have a look to see if I can figure out what cable IS used for the heating.

I suppose that's possible, but 4mm² cable would have been way OTT for the purpose for which it was being installed. If the electrician had installed heavy cable 'simply because he had it', one might have expected him to still have used 16A or 20A MCBs, even though it would have been acceptable and safe with the 32A ones.

As you say, you need to find out what the cable is and, if it's smaller than 4mm², you would then be advised to get the MCBs changed. Your pics obviously don't help us to answer this question.

Kind Regards, John

 

[flameport]

They are 32A RCBOs - separate RCD not required.
Changing them won't be cheap.

You need to remove the front cover so that the wiring size can be seen.

 

[sparkwright]

So they are, didn't notice they were RCBOs - well done.

 

[JohnW2]

Is the wiring for the storage heaters buried in the wall, or run surface, perhaps in trunking? ... If the wiring is concealed in the wall at less than 50 mm, and this was only 18 months ago, an RCD should have been fitted to protect the wiring.

Indeed - but it that trunking below the CU? (of course, even if it is, that doesn't tell us about the whole cable runs).

Kind Regards, John

 

[AndyPRK]

If the wiring is concealed in the wall at less than 50 mm, and this was only 18 months ago, an RCD should have been fitted to protect the wiring.

They are RCBO's thankfully.


Looks tidy, shame about the lack of labeling.

Maybe removing the front of the trunking, we will be able to see the cable size

 

[JohnW2]

Sadly, I can't really get hold of the original electrician now as he has since got very ill and retired, and, believe it or not, I never had the contact details of the fella that finished off the work!

Did you not get any paperwork at the time? You should have an installation/test certificate which ought at least indicate what size cable was used.

Kind Regards, John

 

[sparkwright]

Is the wiring for the storage heaters buried in the wall, or run surface, perhaps in trunking? ... If the wiring is concealed in the wall at less than 50 mm, and this was only 18 months ago, an RCD should have been fitted to protect the wiring.

Indeed - but it that trunking below the CU? (of course, even if it is, that doesn't tell us about the whole cable runs).

Kind Regards, John

The trunking looks like it MAY only be for the tails. (RCBOs thankfully).

 

[JohnW2]

The trunking looks like it MAY only be for the tails. (RCBOs thankfully).

Indeed - but, as you say, the fact that they are RCBOs (which I missed, too!) makes the question of cable installation method moot.

Kind Regards, John