Changing an electric shower Ask questions and find answers to many subjects relating to electrics and electrical work

[Ophid]

Hi to all. Apologies for the plumbing style start, but my inquiry IS electrical.

Our 8.5KW shower (replacing an older 7KW one) was judged by her indoors not to be up to standard. I tried swapping the old shower head onto it which was fine for one use, then the pressure relief device blew on the second go - I should have known.. plus no point in replacing that as she'd still be unhappy!
So I've got a good price on a Mira Advance shower which would take account of our combi boiler and provide a good, strong jet of water. This shower is rated at 8.7KW.

My Wylex CU has a dedicated "B32" MCB (30A??) for the shower on the side protected by an 80A RCCB. The main switch is rated at 100A.


I suspect that extra 0.2KW will tip the balance - would a 40A MCB be necessary?
I also have 6mm wire to the shower - will that be enough?

[ericmark]

Have a look at post "Having an electric shower installed" not sure how it is compatible with combi boiler? With showers connected to Combi boilers there are no electrics involved.

[Ophid]

I mentioned the combi boiler instead of going on about it being a high pressure system, not feeding from a header tank - too much plumbing!

The combi was fitted recently, the existing shower was electric so that's why I'm re-using the set up.



I had a look at "Having an electric shower installed" and I take the point about a 45 amp fuse. It looks like Wylex only do 40 or 50 amp, not 45. Is 50 too tolerant?

From that post it looks like 6mm wiring will only take about 39 amps?? (the post quotes 65 amps through 10mm, I simply worked out 60% of 65 but I'm sure it doesn't work like that !). What would be a more realistic figure?

[ericmark]

On 6mm cable we start at a maximum for each type of cable for example a Multicore pvc-insulated two core single-phase at 32 amps but thermosetting insulation is 43 amps and rubber 48 amps this is before we de-rate cable because it goes through an insulating wall or loft space and does not take into account volt drop etc. So unless we have details we would consider 6mm as 30 amp. Its all to do with temperature according to insulation it can run at different temperatures. Even as electricians it is difficult to identify cables. What you need to consider is if you use a very low power shower on existing cable or go to expense of upgrading. And if your going to upgrade will you use electric with a max of about 10.5Kw or plumb into combi boiler and have 25Kw plus available at the shower? I used latter best thing I ever did but with combi can't use power shower or side jets. My daughter was the same saw a shower being sold cheap and expected us to just fit it. I hate these just jobs!
Oh and by the way 8.5Kw is 37amps and 7Kw is 30.5 and 10.5Kw is 45.5amps so you are already over the top. With a fuse since one is not in the shower long it will most likely hold but a MCB with most likely trip out. Especially with type B but changing for type C may cause more problems than up-rating amps so don't even think about it. Sorry to be bringer of bad new. If an electrician visits he may be able to identify exactly what cable is fitted and maybe there is an easy way out but not with this remote mode.

[Ophid]

I have thought long and hard about a mixer shower instead of electric but the pipes have to travel a long way and wter pressure at the bathroom hot tap is noticably less than in the kitchen so we're not convinced it can deliver hot enough water at a good rate of flow. I'll probably throw the idea at the plumbing board though as I can definitely fit one of those.

The cabling is probably PVC - adding on a bit for safety, it's about 35 feet from CU to shower, enclosed in floor and wall space for about half the way. It does look like an upgrade job, doesn't it. Ah well!

Thanks for the help though - it's not bad news, I'm just trying to see what the limit was and it sounds like I've been pushing it anyway!


I also dread hearing the words, "While you're here.."