Shower Capacity for 6mm cable

[1974stephen]

We're having a new bathroom fitted, and the company doing the work have automatically said that the existing 6mm cable will limit us to a 7.5kW shower, before they knew anything about cable length, bunching etc. They're just doing a 7500/230 calculation.

My copy of the reg's is 16th edition, and I'd have to blow the dust of it, but my simple question is, have the tables changed for de-rating, bunching, insulation and length in the current edition, or are they the same? And if anyone could do a quicker calc than me for a 19m run for a cable run through joists, bunched with other cables, and no insulation anywhere near it would be appreciated!

 

[JohnW2]

We're having a new bathroom fitted, and the company doing the work have automatically said that the existing 6mm cable will limit us to a 7.5kW shower, before they knew anything about cable length, bunching etc. They're just doing a 7500/230 calculation. ... My copy of the reg's is 16th edition, and I'd have to blow the dust of it, but my simple question is, have the tables changed for de-rating, bunching, insulation and length in the current edition, or are they the same? And if anyone could do a quicker calc than me for a 19m run for a cable run through joists, bunched with other cables, and no insulation anywhere near it would be appreciated!

With no de-rating factors, 6mm² T+E would have a CCC of 47A, so would need protecting by a 40A or 45A MCB. That would allow for showers of either ~9.2kW or ~10.35kW (at 230V, although powers are usually quoted at 240V, so the cable/MCB would probably be OK for showers 'rated' (at 240V) at 9.6kW or 10.8kW respectively). It sounds as if the only de-rating would be due to the 'grouping' of cables (which, frankly, I think is very often 'overlooked' unless it applies to long lengths of the cable run!) but we would need to know more details of that grouping to give you any figures.

As for cable length, that's only really relevant in relation to voltage drop (EFLI is unlikely to be a problem) and would not be a problem. The VD in 19m of 6mm² would only be about 6.2V (~2.7%) at 45A, well within the 5% 'guide' limit.

Kind Regards, John

 

[EFLImpudence]

the company doing the work have automatically said that the existing 6mm cable will limit us to a 7.5kW shower,

Could that be because you have a 32A MCB on the circuit?

 

[PrenticeBoyofDerry]

As above.
I would suggest you change your bathroom installer or ask someone else to assess on-site the suitability of the cable.
If they are coming up with ill-informed advise as putting a shower in just based on the CSA of the cable and getting that wrong to boot!

 

[1974stephen]

Regarding MCB, they have no idea what it's rated at. They haven't asked, or looked at the CU. The cable is currently terminated in the local isolating switch, as some time ago I removed the old electric shower when we had one connected off the hot water supply. It's being brought back into service for a 'casual use' shower over the bath in what will be a second bathroom.

'More details of grouping'. What details do you need? It's bunched with a couple of 1.5mm T&E lighting circuits and three or four 2.5mm T&E that form part of the up, down and kitchen rings. There's also another 6mm T&E that feeds the oven that runs parallel with it for the first 8 or 9 metres. All these cables run in parallel through sets of three 25mm holes in the joists frilled quite close to each other, probably fitting in the width of one T&G floorboard.

 

[EFLImpudence]

'More details of grouping'. What details do you need?

None. It is all but irrelevant in normal domestic installations.

All these cables run in parallel through sets of three 25mm holes in the joists frilled quite close to each other, probably fitting in the width of one T&G floorboard.

I wouldn't call that grouped.

 

[JohnW2]

'More details of grouping'. What details do you need? It's bunched with a couple of 1.5mm T&E lighting circuits and three or four 2.5mm T&E that form part of the up, down and kitchen rings. There's also another 6mm T&E that feeds the oven that runs parallel with it for the first 8 or 9 metres. All these cables run in parallel through sets of three 25mm holes in the joists frilled quite close to each other, probably fitting in the width of one T&G floorboard.

Formal calculation of de-rating factors for grouping of several cables of mixed sizes is complex, to the extent that much of it is is 'beyond the scope of BS7671' - and, as I said, I strongly suspect is very often 'overlooked' (most cables are 'grouped' somewhere, for at least short distances, but I don't think that many people 'worry about that'!). Others may be able to give you a better idea of how often, and when, they really do de-rate for 'grouping', but I strongly suspect that it is rarely done at all in domestic installations. Furthermore, a lot of what you describe probably doesn't count as 'grouping'

However, if one does attempt the calculations, there are a few things that make the de-rating less 'severe' than it might otherwise be. Cables separated by more than twice their diameter do not count as grouped. Cables likely to carry less than 30% of what would otherwise be their 'grouped CCC' can be ignored, and that probably applies to many/most lighting circuits, particularly if wired in 1.5mm² cable. Also, although I'm not absolutely certain about this, the de-rating factors for grouping in BS7671 appear to relate to the number of circuits, not cables - so, for example, a pair of cables serving the same ring circuit probably only count as 'one'.

Kind Regards, John

 

[1974stephen]

Thank you everyone, and if I've not misunderstood, with a 40A MCB, a 9.5kW shower can be installed safely and compliantly (which will more than suffice for our occasional use requirements).

 

[EFLImpudence]

Yes.

 

[JohnW2]

Thank you everyone, and if I've not misunderstood, with a 40A MCB, a 9.5kW shower can be installed safely and compliantly (which will more than suffice for our occasional use requirements).

Almost certainly, yes ... Indeed, probably even a 10.5kW (at 240V) one with a 45A MCB.

Kind Regards, John

 

[riveralt]

Thank you everyone, and if I've not misunderstood, with a 40A MCB, a 9.5kW shower can be installed safely and compliantly (which will more than suffice for our occasional use requirements).

As others have said -Yes - as long as it complies with the manufacturers instructions - which invariably just dictates the MCB size leaving cable size to the installer, but keep in mind that they will also insist on RCD protection.

 

[1974stephen]

Thank you everyone - RCD is already provided via split board.