Different size load and supply for shower.

[macbac]

A builder recently asked me for a length of 10.0mm2 t+e cable so that he could put in the load for a 9.7kw shower so that he could carry on working in the bathroom as i couldn't get there for another week.

So now that the load for the shower is in and 'enclosed in capping inside a wall' which can have anything up to 52 amps running through it according to table 7.2 in the OSG, i've gone to the job to look at getting the supply in. The easiest way of doing this is by using the old 6mm2 as a draw wire but the only problem with this is that it is 'surrounded by thermal installation in a stud wall not touching a wall' at least i can't tell if it is touching a wall. So table 7.2 in the OSG says that a 10mm in this situation can only take 32 amps where as a 16mm can take 42.5 amps, which is what i'll need as i'll need a 40 amp mcb for the shower.

So finally would it be ok to have a 16mm2 supply for the shower switch and a 10mm2 load as putting in a 10mm2 as the supply would be a lot more time consuming and costly?

 

[ericmark]

If you read BS7671:2008 it says different current carrying capacity not different cross sectional area so using 16mm through insulation and smaller size where no insulation would be OK.

However it's the joining of the cables which becomes a problem as every join is a place for future fault.

Personally changing from 6mm to 16mm and back to 6mm is unlikely to cause an overload problem in future. But if an electrician looks in the consumer unit and finds a 16mm cables he is likely to assume it will continue all the way to shower or mini-consumer unit in that size and with general lack of plans with domestic and even when there unlikely to be shown to future electrician I would not feel happy down grading cable size.

433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.
NOTE: A reduction in current-carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.

Your signing installation cert it's your call. No one can do that for you.

 

[macbac]

If you read BS7671:2008 it says different current carrying capacity not different cross sectional area so using 16mm through insulation and smaller size where no insulation would be OK.

However it's the joining of the cables which becomes a problem as every join is a place for future fault.

Personally changing from 6mm to 16mm and back to 6mm is unlikely to cause an overload problem in future. But if an electrician looks in the consumer unit and finds a 16mm cables he is likely to assume it will continue all the way to shower or mini-consumer unit in that size and with general lack of plans with domestic and even when there unlikely to be shown to future electrician I would not feel happy down grading cable size.

433.2.1 Except where Regulation 433.2.2 or 433.3 applies, a device for protection against overload shall be installed at the point where a reduction occurs in the value of the current-carrying capacity of the conductors of the installation.
NOTE: A reduction in current-carrying capacity may be due to a change in cross-sectional area, method of installation, type of cable or conductor, or in environmental conditions.

Your signing installation cert it's your call. No one can do that for you.

Thanks for that, even though i'd be using a 10mm not a 6mm and i would only be changing the cable sizes once (at the shower pull cord) but i still get what your saying so what you suggest is to carry on with the 10mm even though it'll be more costly and time effective?

Or would it be acceptable if i clearly marked this on the CU cover?

 

[RF Lighting]

As the cable size changes at the shower switch, I don't see any problem at all with it.

 

[Lectrician]

Yep, perfectly acceptable.

You don't see many who do install 16mm when required.........

And you still get many (sparks, plumbers or just numpties) that try to justify 6mm.

 

[macbac]

hypothetically speaking as i fully intend to wire the shower in 16mm t+e but what could be the consequences of running it in 10mm2 bearing in mind the cable will obviously not be used that often and then it will only be used for 5-10 mins at a time?