MCB too small?

Ive not seen any mention of an RCD although i assume it it part of the replacement CU but thought i would ask.
No, there has not been any mention of an RCD. That's another potnetial issue, but not the one which we have (yet!) been discussing in this thread. However, given that the OP tells us that the CU was installed by a 'reputable electrician/compnay' only 2 months ago, it seems pretty unlikely that the shower circuit would not have been provided with RCD protection.

Kind Regards, John.
 
Thanks for all your responses on this guys...I only thought it'd be a yes/no answer!

I can't believe I pay to have a consumer unit replaced which was effectively compliant many many years ago (I presume), only to be non-compliant now! Jeez...

You are correct in assuming that the shower is RCD protected. At least this is right (hopefully) - I'll think I'll go with with 40A MCB. I'll contact the company who did the replacement and see what they say about it all. To be fair, the hall and landing are both stripped bare at the minute so I *might* consider whacking in some 10mm to be on the safe side. I'll have a look at how much work would be involved. I hate the thought of lifting up floorboards!

As far as the shower being 8.5kW. It's a Triton Caselona (first model) - it reports 8.5kW on the label but I have no idea what's inside. I'll flip the isolation switch tonight and have a look. I've also had a look at the installation and operating instructions for the later model and it says 40A MCB is required.
 
I can't believe I pay to have a consumer unit replaced which was effectively compliant many many years ago (I presume), only to be non-compliant now! Jeez...
I'm not quite sure what you meant by that. In terms of what we know (hopefully there are not other problems we don't know about!), the only thing non-compliant about your CU is having a 32A MCB for an 8.5kW shower- and that would not have been compliant with old regs any more than it is with current ones.

Kind Regards, John.
 
No, sorry. I knew that could be (and probably would be) taken two ways. I meant that the old Wylex board was (hopefully) compliant when installed many many years ago with the regulations then - I didn't mean the current installation was compliant with old regulations! Sorry.

Either way, the shower cable has never been compliant by the sounds of it (it used to run off a 30A fuse).

I'm just trying to think of the easiest (and least messy) way to run 10mm to the bathroom now! I'm probably going to need around 20m of the stuff...
 
No, sorry. I knew that could be (and probably would be) taken two ways. I meant that the old Wylex board was (hopefully) compliant when installed many many years ago with the regulations then - I didn't mean the current installation was compliant with old regulations! Sorry.
Regulations obviously always evolve. A car which was compliant with all the rules and regulations 10 or 20 years ago could not be lawfully manufactured and sold today. However, just as with cars which complied with old requirements but not current ones, it is usually accepted that the old ones don't necessarily have to be updated simply because the rules have changed.

Either way, the shower cable has never been compliant by the sounds of it (it used to run off a 30A fuse).
Possibly. However, as has been explained, it is the way in which the cable is installed (partly in conduit) which produces the (marginal) current non-compliance (6mm² would be fine if just clipped on, or buried in, a wall - even today). It's quite possible that older regs were less demanding.

I'm just trying to think of the easiest (and least messy) way to run 10mm to the bathroom now! I'm probably going to need around 20m of the stuff...
10mm² cable is not the most pleasant thing tio deal with - it has a habit of knowing where it wants to go, which is not necessarily the same as where you want it to go. As I mentioned before, is there no possibility of simply getting the 6mm² cable out of conduit (and, say, burying it in a wall) - you might want to discuss that with your electrician. The advantage of moving up to 10mm² is 'future proofing', since that cable would be OK would the largest showers around.

Kind Regards, John.
 
Hi John,

Actually it's very easy to get the 6mm² out of the conduit. About 2 metres is taking the cable up to ceiling level from the CU (which we're going to box in anyway behind plasterboard or plywood) and the other 2/3 metres is taking the cable from the floorboards up to the isolation switch and back down again - this can be clipped. That solves the issue, doesn't it? 40A MCB and I'm compliant again?

Just as a matter of interest what will the 6mm² cable be rated at if I do this? (don't worry - I'm not thinking of upgrading the shower)

Much appreciated...
 
Actually it's very easy to get the 6mm² out of the conduit. About 2 metres is taking the cable up to ceiling level from the CU (which we're going to box in anyway behind plasterboard or plywood) and the other 2/3 metres is taking the cable from the floorboards up to the isolation switch and back down again - this can be clipped. That solves the issue, doesn't it? 40A MCB and I'm compliant again? Just as a matter of interest what will the 6mm² cable be rated at if I do this? (don't worry - I'm not thinking of upgrading the shower).
'Clipped direct' to a wall or buried in masonry/plaster, 6mm² cable would have a current-carrying capacity of about 47A.

Kind Regards, John.
 
However, as has been explained, it is the way in which the cable is installed (partly in conduit) which produces the (marginal) current non-compliance (6mm² would be fine if just clipped on, or buried in, a wall - even today). It's quite possible that older regs were less demanding.

The original tables for PVC 6 sq. mm. T&E (in the 14th revised metric edition) specify a maximum of 30A enclosed or 35A clipped direct to surface, using coarse excess-current protection. For close protection, as would be the case with a modern MCB, the ratings are multiplied by 1.33, so not substantially different from the 47A figure today for clipped direct.

Edited for typo.
 
The original tables for PVC 6 sq. mm. T&E (in the 14th revised metric edition) specify a maximum of 30A enclosed or 35A clipped direct to surface, using course excess-current protection. For close protection, as would be the case with a modern MCB, the ratings are multiplied by 1.33, so not substantially different from the 47A figure today for clipped direct.
Thanks - that's interesting. As you say, not a lot of change. The OP's interest would, of course, relate to the rating with 'close protection' but partially enclosed in conduit (i.e. to tell him whether his 6mm² cable would 'ever have been compliant' for an 8.5 kW load) - do you know what that was? Also, I suspect that the shower was probably installed post 14th edition (I would guess most likely 16th) - what was the situation then?

Kind Regards, John.
 
The OP's interest would, of course, relate to the rating with 'close protection' but partially enclosed in conduit (i.e. to tell him whether his 6mm² cable would 'ever have been compliant' for an 8.5 kW load) - do you know what that was?

That would mean that the enclosed rating of 30A was applicable, or 39.9A after applying the 1.33 factor. So yes, with close excess-current protection it would have been compliant under the 14th edition for an 8.5kW load, assuming that no other derating was needed due to high ambient temperature or grouped cables.

I no longer have a copy of the 15th edition for reference, but the applicable 16th-edition table is here:

http://www.tlc-direct.co.uk/Figures/Tab4.7.htm

Note the 32A rating if the conduit is in thermal insulation or 38A otherwise. The 16th edition figures are already corrected for close protection.
 
That would mean that the enclosed rating of 30A was applicable, or 39.9A after applying the 1.33 factor. So yes, with close excess-current protection it would have been compliant under the 14th edition for an 8.5kW load, assuming that no other derating was needed due to high ambient temperature or grouped cables.
Thanks.
Note the 32A rating if the conduit is in thermal insulation or 38A otherwise. The 16th edition figures are already corrected for close protection.
I doubt that the conduit would have been in thermal insulation, so it sounds as if the OP's 6mm² cable in conduit would have been OK under 16th ed. - which, as I said, I suspect are the regs under which it was installed.

Kind Regards, John.
 

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