8.5 kW Shower

[TopTrumper]

TT... your attitude is highly irresponsible.

As the other SS says, quite rightly, 8kW is already exceeding the capacity of the MCB and quite possibly (from what has been written) the cable too. It may only be 500W, but that is an increase on what is already an overload situation.

You are increasing the load on an existing circuit & should be making every effort to ensure that every aspect of that circuit complies with the regulations, which means providing a new circuit & consequently, an EIC.

We shall have to find out who built and designed these new homes on this 40 house estate and maybe haul the electrical contractors over the coals.

 

[sparkyspike]

Ok. The OP is being advised to change the cable to comply with BS7671. If he chooses to ignore our advice and fit his new shower and breaker with an underrated cable, he will not be providing a new circuit and therefore will not be required to issue an EIC. However, he will then need to provide a Minor Works Certificate at the very least, which is issued to confirm that electrical installation work has been designed, constructed and tested in accordance with BS7671. So we're back to square one again. Personally I am of the opinion that changing a breaker requires an EIC to be issued as MWCs do not allow the 'replacement of distribution boards or similar items'. This is an argument for another day, as in any event, BS7671 needs to be complied with, therefore the cable will have to be changed, therefore an EIC will have to be issued.

 

[TopTrumper]

http://www.psidea.org/images/BangHeadHere.gif

 

[sparkyspike]

My thoughts exactly...

 

[treacletown08]

Guys,

Just to clarify, I need to know if I need to change the cable from 6mm to 10mm ? From your messages I think that you advise that it is.

Thanks for the advice.

 

[sparkyspike]

No problem.

 

[GaryMo]

How thick is your loft insulation?
If it's over 100mm then your 6mm cable will carry a maximum of 27A. This means the conductors will be at their maximum operating temperature for the insulation which surrounds them (70 degrees). Now, imagine what happens when a load above 27A is consumed by the cable, more heat, up and beyond the capabilities of the 70 degree insulation.
Installing cables which aren't capable of carrying the circuits design current contravenes BS7671:2008 (current edition of wiring regulations) and the manufacturers installation instructions, not to mention it's unsafe!

Trumper, wouldn't you agree with this?

 

[pompeygit]

What about laying the cable on top of the insulation and clearing it around the entry exit point so the cable does not need derating?

 

[treacletown08]

From what you have told me, I am concerned about my present circuit design for the 8 kW shower. The only reason that we havent had problems is that we run the shower mid range on its thermostat and therefore it only draws half its max current (17 Amps ish).

I will call an electrician and get him to install it.

 

[GaryMo]

The thermostat as you call it is actually a variable water flow value. Slowing the water down gives more heat, speeding it up gives less. There are sometimes switches that go from cold to heat 1 to heat 2. If its in the heat 2 position then it will always pull full load.

 

[ericmark]

At 37 amp on a 32 amp MCB looking at table it should run for around 15 minutes or longer before it trips.
If it was me I would inspect the loft and check the cable for temp with shower running and I would consider some form of junction box maybe an isolator if not already fitted and either re-routing or up grading the cable in the loft space where it should be easy to access anyway but at £70 to DIY plus materials then even with my skill I think to satisfy insurance I would employ a registered electrician. Only on large Part P work is it worth going through LABC small Part P I back heal.

 

[GaryMo]

At 37 amp on a 32 amp MCB looking at table it should run for around 15 minutes or longer before it trips.

I don't know where you're looking but the graph in BS7671:2008 doesn't have a B32 crossing the 37A point so you're looking at way way above the maximum figure of 10000 seconds (166.6 minutes). Basically it will not trip under normal usage but that doesn't make it right!

 

[treacletown08]

Thanks for all the advice and posts. I understand a lot more what is going on now with my shower circuit. I will get an electrician in to upgrade me to 10mm cable.

How much should it cost for the labour ? 10mm cable is normally about £3 per metre.