10mm Cable Capacity.

[Biffco]

Hello all, just a quick question.

I am planning to upgrade my shower to a 10.8 kw unit and upgrade my wiring to 10mm twin and earth. (usual run in a standard terraced house).

I'll need to replace my old 30 amp MK mcb with an uprated unit but the only ones I can get from my local retailers are 40 or 50 amps.

My question is, will my cable be up to carrying 50 amps if required? I've had a quick look in my on site guide and it seems close and will depend upon the installation method used. Which method applies to this installation? (usual plaster walls/under floorboards etc)

Any help is gratefully recieved.

Biffco

 

[Steve]

10mm can carry about 60 amps optimum (depending on the length and route (derating factors) of the cable)

10800watts/230volts=46amps. You need a 50 amp MCB.

 

[Albert]

It depends how you run the cable, if it is clipped and not in a thermal insulated wall, you can use the 50A. I would recommend that you use the 40A unit, as there are many other parameters that will influence the capacity of the cable. if it is not too late I would consider to get a lower capacity shower or use a different type of cable that can carry more than the T&E.
BTW a job in a bathroom is a notifyable job.

 

[Softus]

10800watts/230volts=46amps. You need a 50 amp MCB.

Steve, electric showers sold in the UK are commonly rated at 240V, so the current drawn by 10.8kW would be 45.0A. Doesn't change the advice of course.

 

[megawatt]

Always best to do load calculations assuming 230V Softus, errs on the side of caution.

 

[Softus]

I disagree - it's best to look on the shower label or in the documentation.

If a shower rated at 240V is then run at 230V, then the current will be less, not more, than the design loading.

 

[megawatt]

You are, of course, entitled to your opinion.

 

[Softus]

This isn't about opinions; it's about accuracy and safety.

 

[megawatt]

I'm not really sure what you're getting at here Softus but I'm sure you'll enlighten me

If you work out loading for cable size based on 230V rather than 240V you will always end up overspecifying capacity ... Which is safer, is it not?

[email protected]

 

[Softus]

If you blithely overspecify the design current for a known constant load then it isn't safer.

 

[megawatt]

Stick to plumbing.

 

[Biffco]

I'm thinking that the 40 amp breaker may suffer from nuisance tripping, so I would prefer to use the 50 amp unit safely if possible.

As for the cable run, which installation method number correctly refers to running in a house? Most state run in conduit in a wall or clipped direct among the usual examples. Could someone point me towards a table whereI can see the cable ratings for myself please?

 

[ricicle]

BS7671
Table 4D2A

 

[Biffco]

Thanks, i'll go and dig out my n site guide before I go to bed. I did look earlier and I thin that table states 40-45 amps if we assume method no 4 (if that is the correct method)

I may have to opt for a 40 MCB

 

[Spark123]

4D5A
You base the MCB size on the design current of the circuit. You base the size of the cable on the MCB size including de-rating factors.