Here's a good electric shower wiring question

[Bas]

I'm replacing an electric shower for a customer and as usual my quote said I would only connect it if the wiring was suitable.

The old shower was 8.5kw and had been working well for years (but the on/off switch was faulty so it had to be replaced).

When I come to do the job I find the supply cable is just 4mm. Fuse is 32A

New shower is 7.5kw and draws exactly 32A

The cable is about 10m to the fuse box and where I can see it it is clipped direct.

I'm obviously unhappy to connect to a 4mm cable (and I won't) and tell the customer that the wiring is inadequate and will have to be changed.

They understand completely (apparently the original sparky didn't have a wonderful reputation).

But. The regs (7671 table 4D5A) say a 4mm clipped direct cable is ok for 37A.

So what exactly are my grounds for replacing the cable (apart from the fact i can't see the whole run)?

 

[pdcelec]

So what exactly are my grounds for replacing the cable (apart from the fact i can't see the whole run)?

I would say that these could be the grounds (not being able to see the whole run), since the cable could be running through insulation and de rated by nearly half (taking the worst possible scenario). Could more exploratory work be undertaken?

 

[smike]

I haven’t done any maths, but would it be something to do with the cpc condutor in the 4mm tw+e not adequate for reasons of protective device and earthing arrangement

 

[ban-all-sheds]

That argument means that the whole concept of 4mm radials is flawed and unsafe.

pdcelec is right - the cable route needs to be fully discovered, but if ATEOTD it is Method 1 throughout then there are no grounds for replacing the cable apart from superstition or avarice.

HOWEVER - 7.5kW showers are pathetically feeble - are you sure the customer knows what he is doing? He may not notice it now, but I guarantee that come winter he will be very disappointed with its performance.

 

[Bas]

Good points. Its not averice - the thought of pulling up carpets and floorboards is not one that lights up my eyes, and I'm stacked with work. I did connect and try the shower as i was curious as to the current actually being drawn and even taking summer water temps into consideration i was pleasantly surprised as to the flow and temp. It's a Mira zest shower which seems to manage the flow through the rose very efficiently.

 

[smike]

Well done pcdelec. I think I was looking for trouble that wasn’t there.
Let’s have some more BAS all good practice for a those like me

 

[Bas]

another thought... opinions as to whether the whole route should be surveyed whenever a high power appliance is changed? how do you know the 10mm cable hasn't been connected to a 1.5mm cable somehwere along the route?

 

[Qedelec]

Now then Bas, you are being silly. Who would dream of connecting 1.5mm to a 10mm cable?