Electricians! 8.5kw shower on 6mm cable help

razord

Hey guys,

We've got a mira sprint electric shower, its rated 8.5kw at 240v. (something like 7.8kw at 230v), and the voltage at the shower is about 243v (i assume this fluctuates a little, but that was last night)

Towards the end of 2008, the shower switched stopped working, as either the switch, or the cable had melted at various points - this could have been an incorrectly rated shower switch though.

We're covered by british gas homecare, so they came out and replaced the switch with one i know was rated for 45A.

On saturday, the shower stopped working again. 243v was between live and earth, but only about 40v between live and neutral. So i opened the switch again to find the shower-side neutral cable had melted once again to the switch.

This doesn't make sense to me, as i'd of expected the live to melt if anything, and more so, if 6mm cable should be able to handle 40A ish, the circuit break for the shower circuit is a B32, so why isn't that tripping out if the shower is drawing over 32A.

I believe i'm correct in saying its the amps across the cable which would cause it to melt?

Anyway. British gas came back out this morning, but refused to change the switch again, saying we need 10mm cable and they'd quote me for it.

Does anyone have any advice on if it is the 6mm cable causing problems, or if theres something else i'm missing?

Thanks.

-Liam

zax47

90% of the shower switches I change have a melted neutral - usually because of a loose (or more accurately, incorrectly tightened) connection. The other 10% are as a result of the cable being bent through too tight a radius to fit a) the connector correctly and b) bent inside the back-box when the switch is pushed home.

The 6mm cable is, at best, decidedly borderline for an 8.5kW shower - depending on the other de-rating influences affecting the cable and it's routing (overall length, bunching, installation method, insulation etc.), I would never fit less than 10mm for a shower of more than 7kW. Have a play with the figures here and you will see what I mean!

Whoever fits the switch needs to be more skilled as well (IMO). Be sure that the hole the cables come through the ceiling (and that in the back box) is large enough (or make 2!) to allow the cables to come straight down to the connections without being bent through a stupid angle when the switch is forced back up to the back-box and that all connections are suitably tight. Of course this will be made even harder when you change to the correctly sized 10mm cable!

razord

Thanks for the reply, zax.

As the circuit breaker is a B32 - does this mean the shower isn't drawing more than 32A?

Also, now the cable has melted, i assume it needs to be stripped back to clean copper - but this would mean there isn't enough cable to connect to a new switch, and a quick look on wickes & co says they only do 30A termination blocks, no higher.

Does this mean theres no temporary measure we can take until we can get the cable replaced to get the shower working?

zax47

32A breaker is too small - 8500W at 230V draws 37A. But the breaker doesn't trip at 32A exactly - it will pass a substantially higher current for a period of time. Feasibly could allow 40A for X mins, or 45A for Y(<X) mins. Should really be a 40A breaker and 10mm cable for anything beyond 7kW.

The DIY sheds don't generally do anything beyond 30A connectors (thank God!). I'd be wary of any DIY jointing method for those sort of currents - get a second opinion from a local electrician with respect to cost for replacing the cable. BG are famously expensive!!

razord

zax47 wrote: »

32A breaker is too small - 8500W at 230V draws 37A. But the breaker doesn't trip at 32A exactly - it will pass a substantially higher current for a period of time. Feasibly could allow 40A for X mins, or 45A for Y(<X) mins. Should really be a 40A breaker and 10mm cable for anything beyond 7kW.

The DIY sheds don't generally do anything beyond 30A connectors (thank God!). I'd be wary of any DIY jointing method for those sort of currents - get a second opinion from a local electrician with respect to cost for replacing the cable. BG are famously expensive!!

I will do! Thanks so much for your help

DVardysShadow

zax47 wrote: »

90% of the shower switches I change have a melted neutral - usually because of a loose (or more accurately, incorrectly tightened) connection.

razord wrote: »

...
On saturday, the shower stopped working again. 243v was between live and earth, but only about 40v between live and neutral. So i opened the switch again to find the shower-side neutral cable had melted once again to the switch.

This doesn't make sense to me, as i'd of expected the live to melt if anything, and more so, if 6mm cable should be able to handle 40A ish, the circuit break for the shower circuit is a B32, so why isn't that tripping out if the shower is drawing over 32A.

I believe i'm correct in saying its the amps across the cable which would cause it to melt?

A little bit off topic, but zax experience is that the neutral is the one which has the loose connections. Razord's expectation is that the live would be the one to melt if the cable was overloaded.

Is there a common misconception here?

It is the current which causes the overheating - the heat is proportional to the square of the current [so double the current is 4 times the heating effect] for any given size of cable. And current is flow of electricity. So what goes into the shower must come out again. So the live and the neutral will carry exactly the same current for any appliance without a fault. And the heating effect will therefore be the same on the neutral as on the live.

People, if the explanation went over your head, remember the conclusion. It is just as important to ensure that the neutral is firmly connected as it is to ensure the live is firmly connected. And for other reasons, it is actually MORE important.

razord

I uploaded a photo of the switch so you can clearly see exactly whats happened:

flickr.com /photos/lgladdy/4284156447/sizes/l/

(i'm new, so can't post links - so sorry to make you work - you'll need to remove the space between .com and /photos!)

Does that change the diagnosis at all, or match what you'd expect?

zax47

Doesn't change the diagnosis one bit!

With all due respect, is that how you found it or your attempt to do some sort of repair? Either way, the words "deadly" and "bodge" spring to mind! What's with the black insulation tape?

razord

zax47 wrote: »

Doesn't change the diagnosis one bit!

With all due respect, is that how you found it or your attempt to do some sort of repair? Either way, the words "deadly" and "bodge" spring to mind! What's with the black insulation tape?

Most definitely how i found it! I'd never dream of anything using anything like that - my knowledge of electrical stuff is enough to get out a multimeter, wire plugs and do the maths for ratings, etc - but i wouldn't dream of doing anything more than a straight replacement of the switch.

The British Gas guy did the insulation tape to try and repair the cable a little the last time it melted - clearly not that smart.

I think i'll call them and maybe ask for a different guy.

vaio

My guess…..

Original switch developed loose connection (or overheated due to being too small) which caused local overheating at the ends of the cable.

BG (bless them) couldn’t trim the overheated section back because the cable was too short so they “repaired” the damaged insulation with tape and connected the previously overheated (and oxidised) cables to the new switch.

Any connection with oxidised cable is always going to be a high resistance contact and is going to overheat again which it has.

razord

Thanks for all your help guys, i've contacted british gas complaints about the job who say they will get back to me within 10 working days (which is crazy, because we're without the shower for now) and they refused to send a different engineer until that was done.

As i understand it, while a 10mm cable would be ideal, a 6mm cable powering a 8.5kw shower should be fine, and this is almost certainly caused by their previous repair being a bodge job?