Just removed a plastic / glass fibre bath, and the edge of the bath is slightly melted / warped in front of the electric shower supply cable that is embedded in the plaster. Does this mean the cable is underrated ? How hot should the cable get ? The bath has probably been there years, so I cannot tell how long its taken to warp. It would be a pain to replace the cable, but the run is not too long. If I removed the bath, would the room no longer be a "bathroom" for part-P purposes ?!
Simon.
What's the power of the shower? Also, roughly measure the dimensions of the cable and post them here. It sounds like you are absolutely correct: the cable is undersized for the load and hence is a fire hazard.
Mungo
P.S. A friend of my wife kept complaining that her electric hob took ages to heat things. When I went round to take a quick look I found that the man of the house had wired the new hob via a piece of "flex" to the consumer unit that was about three feet away from the hob on the other side of a partition wall. All attempts to volunteer to replace the wire free of charge (five years ago) met with a vacant smile as the lady said she would just persevere with it... even after I ranted about the fire hazard! Some jobs you just gotta walk away from...
Well, the shower is 9.5 kW. I measured the (presumed twin-and-earth) grey cable, approx dimensions are 6 x 13 mm. I ran the shower for about 10 mins - felt surprising bad about the hot water pouring down the drain, and filled up a washing-up bowl ! The cable got noticeably warm in 10 mins, but it would take an awful long shower to melt the bath I would have thought. Finally, I melted the bath back into shape with a blow-torch, so it would fit snuggly against the wall. When rating cable sizes, to the regs include length of time passing the current, as this is surely relevant.
Simon.
On 31 May 2005 12:07:46 -0700, sm snipped-for-privacy@hotmail.com strung together this:
Sounds like 6mm cable, which is too small for a 9.5kw shower in most circumstances.
Over time, it can melt the bath. Lots of wshort showers, heating and cooling the bath, little bit at a time etc...
The cable has probably been installed for a 7.5kW shower in the past, and then never upgraded to accept the heavier load of the new 9.5kW one.
The cable should be rated to accept current, at the least, half again what the full load rating would be, for sure safety. So, for a full load rating of an appliance at 30 Amps, then you'd rate the cable for 45 Amps to be sure it doesn't do what your cable is doing. This also allows for a small upgrade in the appliance in the future.
Of course, this also depends on the length and the routing plan of the cable, but going by a pretty standard install, then this is the safe option.
I wonder if it's not thermal problems at all, but due to the plasticiser leaching from the PVC. Does the cable appear discouloured along its length, or just where it youches the bath?
Not the best of signs. When 'fully loaded', conductors of PVC cables are allowed to reach 70 degrees C (that's the point above which PVC gets too soft and can start to creep at stress points, reducing its thickness perhaps all the way to zero). If it's 'noticeably warm', it's probably up to 50 degrees, at the surface of the insulation, in an area where at least one side is open to the air. That, and Lurch's '6x13, hmm, sounds like 6mmsq' diagnosis, suggests to me you're on, or over, the edge for that shower and that cable. Upping to 10mmsq if you're at 6mmsq would be worth doing sooner rather than later, I'd-a-thunk.
Not really - it doesn't take long at all for cable to reach its 'final' temperature. The reasoning behind diversity allows for 'short-term' overloads, but fixed appliances like showers are treated, as is only reaonsable, as drawing their full load indefinitely.
And it's not only the 'will it melt' factor which determines cable thickness. We also worry about whether the resistance will be low enough to blow the fuse/trip the MCB quickly enough in the event of a L-to-N or L-to-E fault (and we calculate that resistance assuming the cable's at full temp, when the resistance is higher), and that the poor little earth conductor won't rise in temperature to the point of irreversible cable damage during those few seconds. (Yes, we're sad enough to work this stuff out - or at least, either use software wot works it out, or (for the ordinary mortal electrician and consciencious d-i-y'er) to look up the Tabulated Values and plug them into the Sacred Adiabatic Formula. As someone pointed out to a 'can someone recommend a single simple book to make me an expert on wiring matters' query, there's a bit more to electrical installation planning than being able to do a bulb-switch-battery circuit...)
HTH - Stefek
The cable does not actually touch the bath, it is embedded in plaster at that point.
I was amazed at the thermal capacity of the bath plastic. Having heated it with the blowtorch to reshape, it was very hot for nearly 10 minutes ! So several showers in a row, and the heat could build in the bath, which would not help the cable, etc. Simon.
Cables have large overload margins, and 7.5kW of cable running 9.5kW of shower should not normally be a problem. Bear in mind you can just about run a 7kW shower on 2.5T&E.* What the problem is, I dont know.
NT
On 1 Jun 2005 10:20:04 -0700, snipped-for-privacy@meeow.co.uk strung together this:
*note to clueless tit who posted it, don't post ridiculous ideas like that, it only gives other n*****ts ideas.
Note to unthinker: censorship does not reduce stupid actions, it in fact increases them by leaving people uninformed. The best way to reduce stupid actions is to be honest about what does go on, many a time, and explain why its a bad idea.
Running a shower on 2.5T&E is a bad idea because under some situations it will cause a fire, even though under many it wont. It is a real risk to life, the fact that it often works doesnt change that.
NT
Sorry to butt in on somebody else's topic here, but it's a similar question...
When I took out my electric oven to install a new (gas) hob, I noticed some scorch marks in the plaster of the wall behind. This is where the cooker cable exits the wall en route from the consumer unit to the cooker switch. (For some strange reason, the cooker is not connected directly to the cooker switch, but via an unswitched 13A socket.)
Is this common, or should I be worried about it?
TIA
It seems to me that you know the cable is inadequate, if it was in my house I would change it (or the shower). In your case the cable is buried in the wall which means it should be derated (as it cannot disipate the heat) this should help
What gauge is the cabling, and what load rating does the oven have?
Yep, I do know. Its just that I only take the odd 10 minute shower. In the long run I want to put in a combi/thermostatic shower, so I either leave the shower till then, or do it now. I may leave in an electric shower and combi (I'm sure its been done!) Simon.
NT
Looks like 6mm sq. There's no rating given anywhere on the oven, but it's been happily plugged in via a 13A fuse for as long as I've lived here (3 years).
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