4mm shower circuit 8.5kw shower

timhoward · Mar 4, 2023

At a bread and butter job, loose socket, socket not working, change two lights.
Sticking out rather like a sore thumb was this (I removed the wire to check size):

[ElectriciansForums.net] 4mm shower circuit 8.5kw shower

I finished the other work, then said could I have a look at the shower as I've got a concern.
It was 8.5kw shower. 6 sq mm cable at shower (new colours). 6 sq mm cable at isolator, feed was old colours.
Customer added that the isolator had been moved a long while ago. I stopped short of probing around the loft for a joint box.

Current shower was fitted 3 years ago, they never got a MWC in spite of wanting one.
My assumption is it was once a B32 and a lower rated shower.

It's been there 3 years, no signs of thermal damage. CU is in a former garage so possibility cable is clipped direct.
What would your comment to customer have been? (I may show the customer this thread so I won't say what happened yet)

davesparks · Mar 4, 2023

Is that definately 4mm? Solid core 4mm is pretty unusual in a domestic installation.

At 8.5kW you're looking at around 36A so possibly at the upper end of the tabulated value for 4mm. Obviously rating factors can increase or decrease the capacity of the cable.

It's a fixed load so cable size need only be calculated for the design current.

timhoward · Mar 4, 2023

davesparks said:
Is that definitely 4mm? Solid core 4mm is pretty unusual in a domestic installation.

I did have some doubts. Access was tricky - at the time it looked and felt thicker than the 2.5mm. Now looking at it on a big screen I'm really not certain I got it right.

Believing it was 4mm I arrived at it probably being just about ok and yes I mentioned it was a fixed load to them.

It's definitely 6mm by the time it gets to the isolator. I'm now wondering if when the garage was converted the CU was bought forwards and this circuit wasn't long enough and it's actually a 6mm circuit with a short length of 2.5mm or 4mm on the end.
I think I'd better see if I can have another look.

Lucien Nunes · Mar 4, 2023

Not convinced by that conductor size either. The only solid 4mm² T+E I've ever heard of was aluminium, which was equivalent to 2.5 copper. I'm more inclined to believe it's 2.5. If so, this is a good demo for all the people who say '32A rings are dangerous because a break puts the full load onto one leg of 2.5 which will overheat and catch fire.' Of course 2.5 works fine at 32A, and will take more clipped direct, even 40, 50A albeit going a bit floppy and with reduced lifespan.

Nonetheless, whoever deliberately re-purposed 2.5 for an 8.5kW shower probably wasn't the most competent, so there might be other quality issues at the point where it meets the 6mm².

brianmoooore · Mar 4, 2023

First time I've looked at this thread on my phone, where l can magnify the pic, and that disconnected wire matches the size of the three in the RFC (?) to the left. The next to the left has a stranded wire, which is much bigger overall

timhoward · Mar 4, 2023

brianmoooore said:
First time I've looked at this thread on my phone, where l can magnify the pic, and that disconnected wire matches the size of the three in the RFC (?) to the left. The next to the left has a stranded wire, which is much bigger overall

Yep, left of the group is a 6mm cooker circuit, then RFC. I'm confident I messed up now.
It was in a dark high kitchen cupboard and the photo I took is actually much clearer than it was to the naked eye...if only I'd looked at it more or walked back to the van to get a head torch!

Anyway, I'm going back to investigate further. It wasn't the job I was there for so I can't push it that hard, but they were wanting to know it was safe when I queried it and I've misled them. I think we can safely say that while 2.5 sq mm might manage the load no design calculation would ever condone 2.5 sq mm for this.
Thanks everyone.