10kW electric shower

[tomba]

Hi,

I just moved into a house, where the previous owner has installed an electric shower (which obviously replaced the one that was there originally when the house was built).

The shower is rated at 10.5kW (Triton Ivory 3) but the MCB on the consumer unit is only rated at 32A, so I also suspect that the T&E cable is probably also too small (I have read that for showers >8kW you should use 10mm sq. T&E with a 45A MCB).

Q. Is there an easy way of identifying the gauge (cross sectional area) of T&E cable - i.e. should this type of cable have any external markings?


I just resorted to examining the cable's live/neutral wires, to see that each one contains 7 conductor strands each of 1.06mm diameter (measured with a vernier caliper). I calculate that this equates to a cross section of just over 6mm sq. - i.e. I appear to have 6mm sq. T&E.

I have seen some photos of 10mm sq. T&E cable, and the earth conductor seems to be multi-stranded. Perhaps that is a clue?...

Cheers,

Tomba

 

[BigJon]

yep, 6mm earth is a solid core and 10mm is stranded.
10mm is what you will need, with a 45 or 50 amp MCB.
I'd also ensure you have 30ma RCD protection.

 

[plugwash]

yep, 6mm earth is a solid core and 10mm is stranded.

I wouldn't rely on whether the earth is solid or stranded as an indication of the cables size for three reasons.

1: while the most common 6mm T&E is 6/2.5 which has a solid earth other combinations such as 6/4 also exist.
2: over the years the boundry between solid core sizes and stranded core sizes has changed.
3: Even if you assume modern cable with the most common main-earth ratios it won't tell you the difference between say 4mm and 6mm.

I just resorted to examining the cable's live/neutral wires, to see that each one contains 7 conductor strands each of 1.06mm diameter (measured with a vernier caliper). I calculate that this equates to a cross section of just over 6mm sq. - i.e. I appear to have 6mm sq. T&E.

That is probablly the most reliable method of determining cable size.

 

[tomba]

Many thanks for all your replies - much appreciated!

The shower circuit (along with a couple of other circuits) is RCD protected (63A, 30mA)...so my next project is to run a new 10mm cable and install a 45A MCB.

As a DIYer, would I need to get any such work inspected (by a pro), and do I need to notify (or is it only NICEIC installers that can do this)?

Cheers,

Tomba

 

[Adam_151]

If you are in england or wales, then technical requirement P1 directly applies, in addition as its pretty major work its notifiable to the local building department.

There are two ways you can notify this job, a guy who is registered to self certify domestic electrical works with a club such as NICEIC, NAPIT, BRE (ECA), BSU or ELECSA, can do the work and submit it through the website of whoever he is reg'd with and is declaring that he has done the work and it complies with all relevant building regs (may not be just be part P, there are other relevant parts as well. He can't self cert work done by you, only himself and those working for him (thats not to say that somepeople won't find a guy who doesn't know this, or doesn't care for it who will fraudently submit it though!)


Or you can submit a building notice and 48 hours later start work, and they should send a building inspector to ...ah inspect , I say should, but becuase they've only had nearly 3 years to get used to it, a lot are as I understand it, woefully unprepared to deal with it...

If you go this route... speak to building control and post what they say and we'll tell you if they are trying to shirk their responsibilities

 

[briwire]

How has it been working if it has a 32A MCB, surely it would have been tripping regularly.

 

[securespark]

I saw a 9.5kW on a 30A type 1 breaker, been like that 12m+ & it hasn't tripped yet.

 

[tomba]

How has it been working if it has a 32A MCB, surely it would have been tripping regularly.

The shower in question - a Triton "Ivory 3" has two power settings on the main control switch. We've been using it at its lower power setting, which doesn't trip the 32A MCB. If we try and use the 'full monty' the MCB usually trips in 10-15 seconds (although as soon as we moved into this house and discovered said problem, we are refraining from doing this).

I am glad that the MCB has been working properly, as otherwise we would probably have continued using the shower on an underrated (and possibly warm) cable!

I spoke to a tech at Triton who confirmed some of these details to me and emailed me a copy of the user manual (which also covers installation details). I couldn't find the manual on their website - the reason being that the "Ivory 3" is a unit which they only market through Homebase (which confirms that this was a botched DIY installation).

Cheers,

Tom

 

[tomba]

Many thanks for the explanation on bldg regs and control process Adam_151!

If I decide to tackle this job myself I will post my experiences!

Cheers,

Tomba