Electric shower

[martinh89]

Hi guys it's been a while since I've been on here, but I got called to a job today for an electric shower tripping the mcb.
Upon investigation the shower is 9.8kw on a 32amp type b mcb. My first thought was it's being wrongly rated but I decided to carry on with a few checks. whilst on it heats up perfectly fine and stays on for around 5 minutes before tripping the mcb not the rcd. I checked all terminations for tightness, bypassed the neon in the dp pull switch and did an ir test, and it came back fine.
My question is with the shower being a purely resistive load could it take time for the shower to reach its operating current or would it be instantaneous? The consumer unit is Eaton, and they only do 40amp so I've ordered a 50amp type b for the shower circuit.

The cable route from what I've traced is around 17meters and it's clipped direct.
Thanks for any advice.

 

[Darkwood]

Are you really questioning why the mcb doesn't trip straight away ?

If you check your time/current curves for a 32amp mcb with a load of approx 40amps in the BS7671 all will be revealed

The element will heat up instantly and draw the rated power.

 

[The Ghost]

Sounds like the circuit was built for a 7.5 KW shower and they replaced the shower not realising the circuit needs upgrading including the cable

 

[martinh89]

Yes that was my question and now I feel kind of stupid for asking haha, cheers for the reply

 

[telectrix]

what's the cable size and method if installation.if it's 6mm and method C a 40A MCB would be appropriate.however, as it's a fixed load, overload protection could be omitted and a 50A would provide short=circuit protection. tin hat on.

 

[martinh89]

Yes the cable size is 6mm, and it's clipped direct and grouped with 1 lighting circuit. ive just had a look through my 17th, I'm going to give cef a call tomorrow and order a 40amp type b mcb. Cheers

 

[martinh89]

An update on this job. I've since been back and pulled a 10mm cable from c/u to the pull cord and I've changed the circuit braker to a 40amp type b, however the load side of the pull cord to the shower is still in 6mm, it would have been extremely difficult to change the cable due to tiling and no access to the false ceiling. Based on engineering judgment I've decided to leave the 6mm and my reasons for this is, that the 6mm cable is about 3m in length, in a false ceiling with no insulation at all and comes down to the shower behind the tiles again with no insulation. Just wondering on what you guys would have done and any thoughts. Cheers
Martin

 

[timbobelfast]

What cable size did you record on your EIC?

 

[martinh89]

Both, with details of length of both cables, to be honest I'm pretty confident with the installation and the way I left it, I was just wondering on what your thoughts were towards this, in any other circumstance I would have installed 10mm to the shower the run wasn't accessible at all

 

[Wilko]

Hi - here's my 20p worth - 10KW means 42A in the 6mm cable which will have a voltage drop of about 0.31 V per m. and a heating effect of about 13W per metre (I think, but some red has been consumed). So the question is, will this 13W heat each metre of cable beyond 70deg in that ceiling in the time the shower is on? As a shower is only used for minutes rather than hours and usually not at max temp will also help. Unless they've been blessed with teenagers of course, then all bets are off. Cheers, David.

 

[David M]

As above ^^^^

I wouldn't worry to much about that short run of 6mm, most problems I've encountered tend to be poor connections degrading the cable at the terminals causing arcing and heat damage. If you are confident the cable is fit for use and all connections are sound and tight then I would be surprised if you have any problems with the cable over loading.

 

[martinh89]

Cheers for the replys, yes I am confident and going off the installation method and time/current characteristics in bs7671 I can't see there being a problem, but there's always that little problem where you second guess yourself especially on this occasion, it's the first time I've every installed a shower circuit in 2 different sizes

 

[westward10]

Would only need to use 10.0 on EIC as the 6.0 is existing, still not impressed though, job is incomplete.

 

[richy3333]

Would only need to use 10.0 on EIC as the 6.0 is existing, still not impressed though, job is incomplete.

Life's not perfect and as the OP said he used his engineering judgement and was happy to certify his work. I too would have listed both cable sizes even though only 1 was installed. The clearer the circuit details are for future potential numpties that might come along the better.

 

[westward10]

He asked for opinions and that was mine. Are you an Engineer Martin to use engineering judgement?

 

[martinh89]

Yes I did ask for your opinions good or bad I would like to hear them, it's only going to make me better at what I do, I would like to consider my self as an engineer yes, 4 year apprenticeship and 5 years as an electrician and hnc under my belt so I'd like to think so at least

 

[westward10]

Okay engineering judgement made. Me personally I would not be happy, would the cable overload, probably not, would conductors rise to a temperature which may effect the insulation, maybe. And it is the latter which would concern me because ratings of cables are judged on the conductor not rising to a temperature which could cause harmful effects to the insulation.