Under Rated MCB?

[spongobongo]

I've recently had my consumer unit changed and I've only just noticed that my 10.5kW shower is now on a 40A MCB whereas it was previously on a 45A MCB.

Even with my grade C in A Level physics I can work out that a 10.5 KW shower pulls a tad more than 45 Amps. But when I spoke with the sparky who installed the new consumer unit, he argued that a 40A MCB is fine because it wouldn't trip under overload conditions unless it was pulling around 80A.

I'm not particularly worried (new MCBs are inexpensive enough) but it seems a bit daft to me to use an underrated MCB.

 

[1john]

What size cable is the shower on?

 

[spongobongo]

10mm2

 

[1john]

He didn't have a 45 Amp breaker available or cannot get one for the consumer unit he has installed.

 

[ban-all-sheds]

Get him back and ask him to do the job properly.

If he insists that it's OK, ask him to explain how his design fits in with the principles outlined in section 3 of Appendix 4 of BS 7671:2008.

Then when he fails, ask him to do the job properly.

 

[BSBS]

Well the MCB should carry 1.13 * its rating for 1 hour without tripping. So 40*1.13 = 45.2A. Unless you take extremely long showers I shouldnt worry and at least its wired in 10mm..

 

[spongobongo]

He didn't have a 45 Amp breaker available or cannot get one for the consumer unit he has installed.

It's a 17th edition 10 way MK consumer unit. The day MK fails to supply 45 Amp breakers is the day we all give up hope.

As for Ban's suggestion of getting him back to sort it out, that's exactly what I'm doing. I could change it myself, but that's not what I stumped up £400 for.

Before you ask, he is NICEIC registered, he did a full PIR, the bonding has all been replaced in 10mm and I have no problem with his work. And to give him his due, he hasn't made an issue of coming back to replace the MCB. I just wondered if I was being paranoid.

 

[ban-all-sheds]

Well the MCB should carry 1.13 * its rating for 1 hour without tripping. So 40*1.13 = 45.2A. Unless you take extremely long showers I shouldnt worry and at least its wired in 10mm..

433.1.1 (i)

134.1.1

 

[BSBS]

Well the MCB should carry 1.13 * its rating for 1 hour without tripping. So 40*1.13 = 45.2A. Unless you take extremely long showers I shouldnt worry and at least its wired in 10mm..

433.1.1 (i)

134.1.1

The design of the circuit may not on the face of it meet 433.1.1 (i) its does not mean that the design of the circuit does not meet the requirements of BS7671:2008
The circuit does meet 433.1 in as much as you are unlikley to take a shower for more then an hour. you could also read all of appendix 4.

we could always start throwing requlations around becuase of cable support distances and bending radius. At the end of the day the circuit is not instantly unsafe becuase its been design to have a small duration overload.

 

[Spark123]

The circuit protective device rating needs to be higher than the design current of the circuit. The rating of the cable as installed needs to be higher than that of the protective device.
You cannot apply diversity to a fixed load such as a shower circuit hence it does not comply with the regs to the letter.

 

[ban-all-sheds]

1) The electrician cannot predict how people will use the shower.

2) Do you really think that a 40A breaker for a 10.5kW load counts as good workmanship and proper materials, or that it's likely to comply with what the shower manufacturer specifies?

The guy was slapdash and lazy.

 

[BSBS]

Your correct the electrician could not know how people will the shower and if you want to say that then you could never apply diversity to an installation. You have to account for what is likley or unlikley and it is unlikely for someone to be in the shower for more then an hour.

It would be poor workmanship / design if he had used a 32a breaker since that would be right on the limit of its 45% tripping current.

But i can see were not going to agree on this anytime soon.

 

[BSBS]

1)

The guy was slapdash and lazy.

Becuse a circuit was designed with a small overload and doesnt instantly meet every single requirement of the regs? There is more to designing circuits then sitting in an Ivory Tower with a regs booked ramed up your a**

 

[Spark123]

Come on, it isn't rocket science that the OCPD needs to be bigger than the load!

 

[BSBS]

Come on, it isn't rocket science that the OCPD needs to be bigger than the load!

so you would wire a 8kw shower on a 40A for the sake of a 2A overload.