MCB too small?

[kp7722]

Hi guys,

About 2 months ago I used a reputable electrician/company to replace an old Wylex fuse board with a new consumer unit. We've got a 8.5kW electric shower which is fed from a 6mm^2 cable which, if I remember correctly, was connected to a 30amp fuse. This has now been replaced with a 32amp MCB.

I've just had an electrician round to quote on moving the shower/installing an extractor fan and he reckons that the MCB is too small for this load. I've worked out that the shower (switched on to the hottest setting) will pull 35amp so he's correct in saying this. He then said that it's dangerous as the cable has the ability to get warmer without the MCB tripping? Surely this is the wrong way round and that the MCB will actually trip sooner and probably won't allow us to use the shower on max? (i.e. without any compromise on safety).

He did only have a brief look and said to check it out but I'm confused...

On a side note, we have an isolation switch for shower on the landing. He said that a pull cord would be better in the bathroom. I'm not doubting him on this at all (and have actually got him to quote me for this) but what's the regulations on this, and what's the "norm"? Is it worth scrapping the isolation switch and changing to the pull cord?

Cheers,
Paul.

 

[EFLImpudence]

I've just had an electrician round to quote on moving the shower/installing an extractor fan and he reckons that the MCB is too small for this load. I've worked out that the shower (switched on to the hottest setting) will pull 35amp so he's correct in saying this.

Actually a bit of an underestimation.

He then said that it's dangerous as the cable has the ability to get warmer without the MCB tripping? Surely this is the wrong way round and that the MCB will actually trip sooner and probably won't allow us to use the shower on max? (i.e. without any compromise on safety).

You are correct - depending on the cable run.
If 6mm² and If it does not run through any insulation or conduit it could take up to 47A.

On a side note, we have an isolation switch for shower on the landing. He said that a pull cord would be better in the bathroom.

Rubbish.

I'm not doubting him on this at all (and have actually got him to quote me for this) but what's the regulations on this, and what's the "norm"? Is it worth scrapping the isolation switch and changing to the pull cord?

It's up to you - no regulation as to situation of isolator.

 

[kp7722]

Cheers for the quick response.

The 6mm cable runs in conduit for about 4-5 metres of the run and about 10 metres in uninsulated ceiling space between upstairs and downstairs. Good to know I'm not overloading the cable. (I'd love to upgrade to shower but I'm not going to)

Should the 32amp MCB actually be 40amp then? If I'm not having the MCB trip is it worth getting it changed?

With regards to the isolation switch, he was saying that it's safer to have the pull cord as it'll be closer to the person in the shower if anything does go wrong. This seems a little extreme to me - I'm quite happy with the isolation switch on the landing - how else can I get the missus out of the shower???

 

[EFLImpudence]

The 6mm cable runs in conduit for about 4-5 metres of the run and about 10 metres in uninsulated ceiling space between upstairs and downstairs.

In that case it is rated at 38A

Should the 32amp MCB actually be 40amp then? If I'm not having the MCB trip is it worth getting it changed?

Best not, then, in view of 38A.

With regards to the isolation switch, he was saying that it's safer to have the pull cord as it'll be closer to the person in the shower if anything does go wrong. This seems a little extreme to me

Well, as I said It's up to you. If something did go wrong you'd probably be more concerned with getting out as quickly as possible rather than I'll switch it off.
Then it can be switched off on the landing without having to go near the water to do it.

I'm quite happy with the isolation switch on the landing - how else can I get the missus out of the shower???

 

[Risteard]

He then said that it's dangerous as the cable has the ability to get warmer without the MCB tripping? Surely this is the wrong way round and that the MCB will actually trip sooner and probably won't allow us to use the shower on max?

On the point about the circuit breaker he is correct. A circuit breaker will NOT operate at 1.13 times its rating but must operate within one hour at 1.45 times its rating so he is correct that it may never operate.

 

[JohnW2]

He then said that it's dangerous as the cable has the ability to get warmer without the MCB tripping? Surely this is the wrong way round and that the MCB will actually trip sooner and probably won't allow us to use the shower on max?

On the point about the circuit breaker he is correct. A circuit breaker will NOT operate at 1.13 times its rating but must operate within one hour at 1.45 times its rating so he is correct that it may never operate.

That is very true (for Type B MCBs) - but the important thing to add is that, for that very reason, the tabulated current carrying capacities in BS7671 take that ito account - i.e. it is considered that a current of up to 1.45 times the tabulated CCC for up to one hour is 'safe' for the cable.

However, what the OP's electrician said is incorrect (and the OP's view correct), since there is no way that having a lower-rated MCB can possibly result in increased warming of the cable. Even if the cable were undersized (and there's no suggestion that it is), the lower rated MCB couldn't possibly allow current to flow for longer (hence more warming) than would an MCB of higher rating. As you say, it could be that neither rating of MCB would operate - in which case there would be no difference between the two situations. In all other situations (i.e. if the lower-rated MCB did operate), disconnection will occur earlier (hence less cable warming) with the lower-rated MCB than with a higher-rated MCB.

Kind Regards, John.

 

[janner10]

It sounds like your electrician is try to make a bit of extra work for himself.

 

[kp7722]

My thoughts exactly. Isolation switch is staying where it is. MCB isn't being replaced.

One final question. The bathroom has a flat roof. The electrician said that he needs to cut holes in the ceiling to tap into the lighting circuit for the extractor fan. I know this is standard practice but can he not run a fused spur to a smaller cable off the ring? The switch for the fan is going to be on the wall outside the bathroom anyway so this would save cutting the ceiling up. He also said that any 4" fan will do for a domestic bathroom. I thought the fan had to extract 15-20 times the volume of the bathroom? In which case any old 4"fan won't do the job.

Cheers for all the help guys...

 

[EFLImpudence]

can he not run a fused spur to a smaller cable off the ring?

Could do, anything is possible, do you not want the fan to operate with the light?

The switch for the fan is going to be on the wall outside the bathroom anyway

I take it this is an isolator switch - not an operating switch.

He also said that any 4" fan will do for a domestic bathroom.

That's what is normally fitted. Is it on an outside wall?

I thought the fan had to extract 15-20 times the volume of the bathroom?

Any fan will do that eventually. Look up details on the web.

 

[JohnW2]

My thoughts exactly. Isolation switch is staying where it is. MCB isn't being replaced.

I don't know if it concerns you but if you keep the 32A MCB, that will probably theoretically be non-compliant with the Wiring Regs - 433.1.1.(i) requires that the rating of the MCB must not be less than the 'design current' of the circuit. An 8.5kW shower will draw about 37A at 230V (or about 35.4A at 240V), and I would think that figure really has to be taken as the 'design current'.

However, if you want to become compliant, that could open a can of worms. As has been pointed out, 6mm² cable only has a rated current carrying capacity of 38A if some of it is in conduit - so if you moved up to a 40A MCB you would again be non-compliant (this time because the cable was too small for the MCB rating - 433.1.1.(iii)) - in which case you'd have to go up to 10mm² cable (or remove the 6mm² cable from its conduit!) in order to get back in compliance!

If you don't want to change the wiring, it therefore looks as if you would have to decide which part of 433.1.1 you wanted to be non-compliant with!

Kind Regards, John.

 

[EFLImpudence]

Are you sure the shower is 8.5kW?

Went to a customer last week who said their shower was 9.5 but inside a sticker stated 8.7.
I think manufacturers exaggerate sometimes.

 

[JohnW2]

If you don't want to change the wiring, it therefore looks as if you would have to decide which part of 433.1.1 you wanted to be non-compliant with!

I should perhaps have added that, FWIW, if it were me, and I were happy to be non-compliant with one or other of those parts of 433.1.1, I would probably (from a common sense / engineering viewpoint) probably change the MCB to 40A. I would not be too concerned about the MCB being rated a few percent over the theoretical curernt carrying capacity of the cable, but I'd be less happy with the current flowing through an MCB (when current was flowing) always being above it's 'rated' current.

However, that's all assuming that I were prepared to be non-complaint with some of the regs - which is not something I would ever advice/suggest anyone else should do!

Kind Regards, John.

 

[westie101]

An interesting point, where we have cable laid in ducts we also apply a de-rating factor as we cannot guarantee adequate heat dissipation.
This only apples, however, to duct lines over 10m (e.g anything but a road crossing).
I would suggest that in reality the current capacities quoted are very conservative anyway. I would also suggest that it would also be different for different conduit types as a metal conduit will conduct heat far better than PVC.

All in all I would go for the 40A MCB

Given also the "cyclic" nature of the circuit the load will not be applied continuously over a long period thus giving the cable plenty of opportunity to return to ambient temperature for most of the time.

 

[JohnW2]

I would suggest that in reality the current capacities quoted are very conservative anyway. I would also suggest that it would also be different for different conduit types as a metal conduit will conduct heat far better than PVC. .... All in all I would go for the 40A MCB .... Given also the "cyclic" nature of the circuit the load will not be applied continuously over a long period thus giving the cable plenty of opportunity to return to ambient temperature for most of the time.

Indeed. Not only are the tabulated CCCs undoubtedly conservative, but the duration of individual periods of current flow are going to be limited with a shower circuit. Indeed, we know that BS7671 already accepts that a cable rated at 38A can safely carry 55.1A (38 x 1.45) for up to an hour, by virtue of the characteristics of protective devices.

As I said, I'm with you on this. If it were me, and if I were prepared to accept some technical non-compliance, I'd certainly go with the 40A MCB.

Kind Regards,John.

 

[STI]

Ive not seen any mention of an RCD although i assume it it part of the replacement CU but thought i would ask.