Are my calculations correct?

[WDMDL]

My ex-Next-Door-Neighbour has asked me about changing a shower at his new house.
Now I can't do it, (I'm not a scheme member) as it's not a like for like, he wants an uprated one.
But I told him previously he would be looking at replacing the 6mmsq Twin+cpc with 10mmsq if he wanted to uprate because the existing is only just compliant.

He rang someone out of the local paper and they said the 6mmsq is perfectly adequate.
I think not, please could someone check my calculation and reasoning:

MCB is 40A, RCD protection provided (16th Ed style board)
Firstly, it's not a recognised combination in table 7.1 of OSG.

But calculating each element separately:

Existing shower is 240V 8.5kW, 230V 7.8kW.
Supply voltage at the new house is 230V so that's 33.91A

Between CU and loft, cable run is a mix of buried in plaster and enclosed in trunking.
So, Reference method B brings that down to 38A. (Why no mention of B in OSG?)

In the loft, the cable runs over the top of the insulation to the pull switch, then is clipped to the side of the joists over to the shower.
That's reference method 100 which brings it down again to 34A.

And the 40A breaker is OK because of 433.3.1 (ii)

So I can demonstrate that the existing arrangement is compliant (Just!) but there's no way a shower of higher rating should be fitted to this circuit.

They trust my engineering judgement so will not use that guy out of the paper...
They said they take quite a few mins in the shower and they're all in there one after the other so don't want something overheating.
And as they've just decorated everywhere except the bathroom, I've discussed a few options and it looks like the solution will be to fit a thermostatic mixer shower - far more powerful and no part P nonsense.

Simon.

 

[telectrix]

you don't say what the rating of the new shower is. 9.5kw is 39.6A ( manufacturers use 240v to make the showers seem more powerful, not 230v). even then i would be surprised to find a 9.5 taking more than 35A. suggest fitting a 9.5 on 6mm and 40A MCB, measure the current with a clamp meter, then , if you find it does need 10mm, then that's the time to start re-cabling.

 

[nickblake]

Whats not been mentioned is the length of the cable , i must admit if the cable is installation method 100 and the current carrying capacity is then 34 amps so the protective device should be 32 amps , any thing above 8.7 kw should realy be in 10 mm , i know 6mm will do but i always work on the worst case scenario you cant go wrong then

 

[IQ Electrical]

He rang someone out of the local paper and they said the 6mmsq is perfectly adequate.

Another 'Google Earth' design calculation....

 

[ezzzekiel]

depending on all the installation factors a 6mm could be adequate, although you would find most sparks would run a 10mm in, however you are not the spark doing the job, maybe take a better look and check all the installation factors before questioning the sparks opinion just incase you cause problems and then turn out to be wrong.

 

[WDMDL]

That's what I was working on, the worst case... If it was a single person household it might be OK but this is a 2+4 family.
When they moved in, they asked me to give things a once over, basically a PIR without a cert.
I was happy to do this to gain experience and had checked everything.
Aside from 17th-related changes, I found:
The main protective bonding conductor to the water supply just coiled up in the cupboard under the sink(!)
17Megohms N-E on the downstairs ring (tile adhesive squished into socket back box in kitchen, touching terminals)
Broken plastic earth pin in bedroom socket holding shutters open
Shower circuit didn't look right so did a few calcs as above and found it was OK.

Rings were complete and all Zs's were fine.

They've just phoned me to tell me they've picked up a thermostatic mixer so will be fitting that sometime next week.

Nick, I'd thought about should it be a 32A (and the breaker will take a mighty long time to trip at 33.9A), but seen as it's a circuit feeding a single fixed appliance that you can't actually overload, reg 433.3.1 (ii) means a 40A is fine.

Simon.

 

[ezzzekiel]

well you appear to know what your talking about so why not take your findings to the spark who quoted and get his opinion

 

[WDMDL]

Thanks guys,
Just wanted to be sure in case I'd overlooked something.
Because of what I'd told them a few months ago, they wanted to make sure before actually buying a new shower. But they asked this guy about fitting a 9.5kW model.
True, a "9.5kW" shower would be 8.725kW at 230V, that would be just short of 38A.

IQ's right, this guy didn't come round to look. They did mention to him on the phone about the existing setup being borderline and because of his "no problem at all" type response they asked me again.

I will find out who it was though...

Simon.