New shower - cable size?

helpimcrap · 5 Oct 2013 at 06:41

Hi.
It is not a new install of cable so does not require part P(is that right?) as it is just the shower.
What shower size can a 6mm twin earth cable support?
Regardless of limiting factors the Triton T80i i have used for last 2 years has been fine.
Guessing that's an 8.5kw so I'd be ok with a direct replacement size - is my logic right?

Schizo · 5 Oct 2013 at 07:01

Copy/paste....sorry

Re: What size cable for 10.5kw shower?
1) the design current is 45A, so the MCB should be 45A.
2) 6mm T&E cable installed covered by 100mm insulation the maximum current carrying capacity is only 34A.

so straight away it is clear the cable is undersized (needs to be at least 10mm).

However, if the cable is clipped direct throughout its entire length then a 6mm may well be ok for current carrying capacity (rated at maximum 47A clipped direct).

As for voltage drop, 6mm cable for a 15m run gives vd = 4.9 V. The maximum allowed is 11.5V.

Kind of somewhat answers your question somehow

So it is only ok provided Zs etc complies and the cable is clipped direct.

helpimcrap · 5 Oct 2013 at 07:16

If i was going 10.5kw i know I'd need to upgrade wiring.

As it's only 8.5kw I'm right in thinking i don't need it certified as like for like swap?

Participant · 5 Oct 2013 at 07:31

Look at the underside of the front cover. Sometimes there's a sticker there with the power rating. 6mm^2 is 8.5kW max depending on other things.

I'd definitely do this job myself. Cba with paying a sparky for stuff like this. When I come to sell I'll just say all I've ever done since I moved in was a like-for-like.

AshD93 · 5 Oct 2013 at 09:22

Just stick 10mm in. Best to future proof it.

Nightglow · 5 Oct 2013 at 10:09

helpimcrap said:
If i was going 10.5kw i know I'd need to upgrade wiring.

As it's only 8.5kw I'm right in thinking i don't need it certified as like for like swap?

Under Part P Fitting and connecting a new electric shower with a different kW rating is notifiable as is upgrading cable from 6mm to 10mm, needs a qualified electrician as it's in a a bathroom.

But, a replacement of a shower is not notifiable, as long as it's of a similar type & power rating.

I agree with AshD93 , get it upgraded to 10mm, gives your some future proofing & gives you a wider range of more powerful showers to choose from.

bitslice · 5 Oct 2013 at 10:27

Nightglow said:
Under Part P Fitting and connecting a new electric shower with a different kW rating is notifiable

Really?
What would be the point if the cable is still suitable :confused:

Surely it's like going from a 60W to a 100W bulb?
Besides that, who is going to know a bigger shower was recently installed?

I've gone from a Triton 50i 7kW deathtrap to an 8.5kW Mira on the original 6mm cable, this appears to be OK.
(Or if it's not OK then I don't care that much)

I did think about putting in 10mm but if I need anything hotter then I'd might as well go for a thermostatic version and move this one downstairs.

paintguy · 5 Oct 2013 at 13:50

bitslice said:
Really?
What would be the point if the cable is still suitable

I would guess it's because it takes somebody experienced / qualified to say for sure that the existing cable actually is suitable.

A previous installation for example may have been right on the limit where cable length & gauge were concerned, so upping the kW rating may tip it over the edge.

Sure it's possible to check for yourself on the internet, but the regs are there to cover for those who can't be bothered to check and just upgrade things with no thought to safety

Nightglow · 5 Oct 2013 at 13:57

bitslice said:
Really?
I did think about putting in 10mm but if I need anything hotter then I'd might as well go for a thermostatic version and move this one downstairs.

Makes a lot of sense having a Thermostatic version, even more so if you have kids, nothing worse than the water going from hot to cold to scalding, if someone turns a tap on elsewhere in the house.

bitslice · 5 Oct 2013 at 18:32

Nightglow said:
nothing worse than the water going from hot to cold to scalding, if someone turns a tap on elsewhere in the house.

The Mira has circuitry to controller the twin heaters in the case of a pressure drop in the incoming.
Although if someone turns on a tap while I'm in the shower then I have bigger problems that cold water, cats can't turn taps on :eek: