Cable rating

[Handyman]

An electrician has wired a 2kW bathroom heater in my house using 1.0mm cable from a 6A mcb, as part of the lighting circuit. It works fine.

However, my maths suggest that 2kW = about 8A, which exceeds the mcb rating. According to the DIYnot website, 1.0mm cable is suitable for up to 2400 watt loads, so 1.0 cable seems to be adequate (just), but not the mcb.

If I plug a 2kW kettle in my kitchen, I use a circuit made up from 2.5mm cable from a 32A mcb

Shouldn't the bathroom heater be run off a higher rated circuit?

Confused.

 

[Adam_151]

You are correct in your maths, 2kw is over 8 amps, your electrician has broken a fundamental principle of circuit design.

1mm² cable... depending on installation method could be adequate (but most would install at least 1.5mm², and many would feel happier chucking in 2.5mm² and sticking it on 16A breaker)

Just to check, this heater is on its own (albeit badly designed) circuit isn't it? You don't mean to tell me he stuck it on an existing lighting circuit

 

[Handyman]

The house was rewired before we bought it just over 2 years ago. The heater is on the lighting circuit just protected by the lighting circuit's 6A mcb. It's on a length of cable from a junction box on the lighting circuit.

The rewiring was definitely done by a so called "qualified" electrician, but before all these new regs were issued. All the circuits test out OK, but what with this, and the fact that some of the surface-mounted sockets are not securely fixed to the wall (which, of course, would not be identified by any electrical test), it does make me wonder how the latest regs would have been of any benefit to me.

 

[Adam_151]

There haven't been any changes to the BS7671 wiring regulations in the last 2 years, there were some minor admendments in 2004 though (mainly the colour change). The current regs we work to, the 16th edition has been the same since 1992 (we get a new one next year tho)

I suspect the new regs you allude to, was the admendment to the building regs to insert part p to be effective from 1st Jan 2005, this brings all domestic electrical work under the building regs and places one simple requirement on it

P1 Reasonable provision shall be made in the design, installation, inspection and testing of electrical installations in order to protect persons from fire or injury.

Thats it... there was a P2, but that was removed in an admendment in April 2006 (it was about reasonable documentation being left with the install) In addition, some electrical work became notifiable to the LABC (most notably, new circuits and work in kitchens and bathrooms)

So the standard which the electrician should have worked to has not changed (BS7671 is not, and was not a legal requirement, it was always a very sensible thing to work to, and is the best way to meet P1), but the new building regulations do serve to fire a shot across the bows of out of date electricians who upon trying to join a registered body are suddenly made to realise that they need to brush up.


What electrical test are you refering to, did this electrician actually issue an EIC (they are supposed to, but the sort of electrician you seem to have had never tended to bother...), or are you talking about a periodic inspect and test done since... which should really have caught this... wonder what else they missed...

 

[bobapcoed]

If you put all your lights on, and the heater, does the MCB trip?. The heater should really be on it's own circuit, 2.5mm cable and covered by a 15A MCB. The fact that sockets are not securely fixed suggests the "electrician" wasn't too fussy. As above, you could wire ccts in bell wire and they'd test clear. I suggest as a staring point get a reputable company to test. Check whether your local supply company still do it, they'd be reputable and probably don't do contracting anymore so aren't looking for work.

 

[Steve]

ffs. how do they get away with it

 

[Handyman]

I now find that the heater in question has an internal switch to select one of two heat settings. The lower setting has been selected, hence, I suppose, I've got away without the mcb tripping, even with all the lights on.

Anyway, problem now solved because I've disconnected the heater (by removing the cable from the JB) and removed the heater. (It's not required anyway - I just hadn't got round to sorting it out: Now I have) And it has spurred me into moving the electrics to the top of the priority list of improvements.

Thanks for your combined advice on this.