6mm or 10mm SWA, 10mm or 16mm T&E

[Amy_B]

Hi I'm looking have a new installation of an electric shower in the bathroom of my small house.

The new shower unit is 10.5kW so I believe I need a 50A Double Pole pull switch and a 50A MCB fed from a 63A RCD. The current CU has only 4 circuits, Cooker, Upstairs lights, Downstairs lights and a single circuit for all sockets.

I would like to use the new split feed CU I have bought. It comes with 2x80A RCDs so I think I will have to downgrade one of them to 63A. Then, on that side I would put the downstairs lighting circuit together with the Shower. (The bathroom is upstairs.)

The other side can have the sockets, upstairs lights and cooker.

The new cable route for the shower will pass from the CU in the garage through a single brick and block wall to the underfloor space of the bathroom, then immediately up through the floor and battens into the space inside the plasterboard partition wall within 150mm of the corner of the walls. It can be clipped to a batten and not touch the plasterboard. At the top of the wall it will pass through the plasterboard ceiling and 100-150mm of glassfibre insulation Then along a joist for 2 m and duck back throught the insulation to the isolating pull switch. Back alongside the the incoming cable and then back down inside the wall but this time dropping down to the shower unit.

So the question is, given that particular routing and the fact that it can get quite warm in the loft in the summer should I run 6mm or 10mm SWA, or 10mm or 16mm T&E?

Thanks for any other advice you think relevant. And yes I will be notifying LABC but I want to check that my proposal is safe first.

Thanks,

Amy

 

[rebuke]

RCD just has to be rated >= the expected load, the individual MCBs provide the overcurrent protection, so having an 80A RCD is fine, no need to downgrade it or anything...

It's actually better in terms of derating factors for the cable to be touching the plasterboard in the stud wall, rather than being clipped to the battens. A quick play on TLC's cable size calculator ( http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html ) shows that if you have it touching the stud wall, then the largest derating factor is from the >100mm of insulation it's going to be running under (I'm not sure if it is just passing 'through' as to whether that counts or not - anybody know?). I was guessing at the total length, but that shows that 16mm T&E should be fine. Strictly speaking though it's only rated to 46A with this method, so you should probably go for a 45A MCB, rather than a 50A to be safe. This should still be fine for the shower, since the 10.5kW rating will be at 240V, i.e. 43.75A, at 230V showers end up with a lower kW rating...

Does the stud wall have any insulation in it, if not then it's not a problem, if it does, then to use T&E you would have to have the cable touching the plasterboard, otherwise there's no suitable size...

I'd say avoid SWA if possible, as you'd have to terminate it in to some sort of junction box before the shower isolator (can't easily/safely terminate SWA into the back of a standard plastic back box), which just adds extra hassle...

 

[ban-all-sheds]

It comes with 2x80A RCDs so I think I will have to downgrade one of them to 63A.

So if you had a TV stand that could take 50kg, and you replaced your TV with a 20kg LCD, would you go and get a stand that couldn't support 50kg?

At the top of the wall it will pass through the plasterboard ceiling and 100-150mm of glassfibre insulation

Would you be able to clear a bit of it away?

Can you get a 45A MCB for your CU?

So the question is, given that particular routing and the fact that it can get quite warm in the loft in the summer

How warm?

What ambient temperature would bring the capacity of the cable down by the same extent as the insulation?

should I run 6mm or 10mm SWA, or 10mm or 16mm T&E?

Why do you think that you'd need a smaller cable if it were SWA?

What installation methods would keep even 16mm² T/E at over 50A?

 

[Amy_B]

Thanks very much for your reply. It's very helpful.

It comes with 2x80A RCDs so I think I will have to downgrade one of them to 63A

So if you had a TV stand that could take 50kg, and you replaced your TV with a 20kg LCD, would you go and get a stand that couldn't support 50kg?
.

Point taken but I wouldn't need a big stand for a little TV now would I?

At the top of the wall it will pass through the plasterboard ceiling and 100-150mm of glassfibre insulation.
Would you be able to clear a bit of it away?

Yes. How much clearance do I need? I could pass it through conduit til its above the insulation. Or a bit of 32mm plastic waste pipe cut to length as I have some of that.

.

Can you get a 45A MCB for your CU?
.

Don't think so. It's a Contactum CP range and TLC shows 40A or 50A MCB.
And only 40A RCBO or I would have used one of them in preference

So the question is, given that particular routing and the fact that it can get quite warm in the loft in the summer.
How warm?
.

Tricky, until it gets to the height of summer I'm not going to be able to measure how hot it gets in the loft. What do you professionals do if you don't have a measurement for that?

What ambient temperature would bring the capacity of the cable down by the same extent as the insulation?

I see, if I am under the insulation instead the temperature wont rise to that of he loft above it but remain at the temp of the plasterboard with a little heating from the cable? I know the house stays cooler than the loft so I'll do the calculation.

should I run 6mm or 10mm SWA, or 10mm or 16mm T&E?

Why do you think that you'd need a smaller cable if it were SWA?

I thought XLPE SWA had a higher current rating for a smaller cross-sectional area. reference method C gives 62A for 6mm SWA and only 47 for 10mm T&E

What installation methods would keep even 16mm² T/E at over 50A?

Not sure about that one. can you expand on that?

But thanks again for the responses.
Amy

 

[Adam_151]

should I run 6mm or 10mm SWA, or 10mm or 16mm T&E?

Why do you think that you'd need a smaller cable if it were SWA?

I thought XLPE SWA had a higher current rating for a smaller cross-sectional area. reference method C gives 62A for 6mm SWA and only 47 for 10mm T&E


But thanks again for the responses.
Amy

Ah, be carful there, if you read the tops of the tables you'll see it'll say something like 70C PVC/PVC for the twin and 90C for teh XLPE, in essence the rating is higher becuase its allowed to reach a higher temperature in operation... don't ask me the reg number off hand, but if you are going to run cables at 90C you need to make sure whatever it is connected to can handle that.... and while I haven't looked at the specs, I cant imagine anything thats designed to be installed in a house being suitable... so if using SWA, you want to look at the ratings for 70C PVC armoured (even if you are using XLPE)

I wouldn't bother with armoured for a domestic shower if its run internally... its going to make life difficult with the glands, etc

 

[Amy_B]

Ok So I have accepted that I will have to use 16mm cable and change the CU for an MK one so I can use a 45A MCB to feed my 10.5kw shower.

The question now is how do I feed fan required above shower? Or can I use a PIR and a separate circuit to trigger fan when someone's in the bathroom?

Thanks,
Amy