10.5kw Shower - Cable and MCB rating?

[FredFlintstone]

Afternoon all,

I'll be changing my shower soon and am planning the cable route (the existing cable runs diagonally up the stairs to the switch outside the bathroom!).

I'd like to take the cable up the wall from the CU through the ceiling and up the wall in the front bedroom into the loft. For this part of the run I was going to either stick the cable in trunking and plaster over or clip it to the wall, cover with capping and then plaster over. Once in the loft I want to lay it on top of the insulation and clip it to the joists where it crosses them. Then out of the loft, doen the wall to the switch, again, plastered in.

Now originally I thought I'd be OK with 10mm cable but my problem is that the route I want to take the cable over means that the run to the switch will be about 12 metres with a further 1 to 2 metres from the switch to the shower. Will I still be OK with 10mm cable or should it be 16mm to be safe?

Also, I was originally going to fit a 9.8 kw shower so had bought a 45amp MCB in preparation. However, with the slightly increased kw I'm wondering if I should be using a 50amp MCb instead?

Any help, comments or advice appreciated.

I'm aware it's notifiable and notification has been made incidentally.

Cheers

Fred.

 

[davy_owen_88]

A 45A MCB is fine for a 10.5kW shower (10500w / 240v = 44A)

Don't put the cable in trunking and then plaster over - it's a waste of trunking/money, covering with capping is fine. Clip the cable to joists for as much of the run as possible when you get to the loft, don't leave it laying over the insulation, it won't shed it's heat as effectively.

10mm² t&e is good for 64A when clipped direct and not subject to any de-rating factors. Voltage drop calcs work out fine for 44A / 10mm² t&e / 15m.

 

[FredFlintstone]

Cheers Davy,

I'd been working on 230v and came up with a figure of just over 45a but then i suppose, even if this was the case, a shower is used for what, maximum 10 minutes at a time so a few tengths of an amp aren't likely to cause much bother to an MCB.

As for the cable run in the loft, it's got to go across the joists rather than with the joists, therefore, the able's going to be resting on top of the insulation, unless I stick it inside a couple of lengths of conduit but would that de-rate it too?

I could see if taking it up to the roof joists and clipping to those would keep it under 15m.

Thanks again

Fred

 

[davy_owen_88]

As for the cable run in the loft, it's got to go across the joists rather than with the joists, therefore, the able's going to be resting on top of the insulation, unless I stick it inside a couple of lengths of conduit but would that de-rate it too?

I could see if taking it up to the roof joists and clipping to those would keep it under 15m.

Running it across insulation isn't a major problem, I just meant that you should clip it to joists for as much as possible to let it shed it's heat more easily. Putting it in conduit would indeed de-rate the cable so leaving it clipped direct is the best option.

The 15m cable length was just a rough figure to do the volt drop calculation based on the cable length you mentioned, you have plenty of capacity for VD so don't worry if you need to go over the 15m mark.

 

[sparky1982]

10mm with a 45amp breaker will be fine

 

[FredFlintstone]

Superb, thanks folks.

 

[RF Lighting]

If you clip the cable to the angle bits of the roof that form the V shape of the roof, rather than the straight bits that hold up the ceiling of the room below, then this will keep the cable out of the insulation altogether



(I'm rubbish with the names of roof bits)

Rafters.......purlins.......joists......etc

 

[Steve]

If you do clip it to the "diagonals" in the loft, dont nail them! You could dislodge roof tiles! Screw the clips in or use plastic zip ties wrapped round the wood.

 

[securespark]

Just a niggling thought:

What's the theoretical difference between clipped direct, in capping plastered over & trunking??

 

[ricicle]

Better ask the suits at the IET

 

[securespark]