Conduit OK or tile removal !

[Do_it_later]

Hi Folks

I have partly installed a 9.5kw shower. I've run 10mm T&E (via 45A DP Switch) to the cupboard where the consumer unit is. I need to upgrade to a new CU anyway so will be replacing my 8 way - wired fuse type box with a 12 way split load. ( I'll be getting a spark to do this bit).

I've buried the cable in plastic conduit in the bathroom wall, then through the ceiling and down into the cupboard. (Switch is positioned through the cupboard wall into the hallway outside the bathroom door). I was intending to surface mount the cable from the back of the switch, over the cupboard ceiling to the new CU.

From reading a few posts here, I'm concerned about the sizing of conduit for the 10mm cable, I'm using conduit that is a snug fit around the cable, and I've read a few posts suggesting that there may be a minimum "free space" required inside the conduit.

Q1 - Is there a specified conduit suitable for the section which I have buried in the wall ? (I've already tiled over it !!)
Q2 - Conduit size suitable for surface mounting in the cupboard ?

Q3 - The 12way CU kit I'm looking at comes with a 40A MCB, will I get away with this or need to buy a 45A. I know the arithmetic says a 40A will keep tripping but in reality, how often does the shower draw its full 41.3 A.

Many thanks for any advice available.

Andy
PS. Shower has been like this for 6 years - is this a record ?

 

[securespark]

It depends upon whether your shower has heat settings or not. If not, then the shower will draw full load current all the time. However, if you have a half heat setting which you use to avoid overloading the circuit, you will end up with a pitiful flow rate to get a decent temperature: in any case, this is bad design. Some may say that a type B's characteristics are such that this overload can be withstood, but for my money, the only course of action is to design the circuit taking full load current into account. There is a 45% spacing factor in conduit & trunking.

 

[Do_it_later]

It depends upon whether your shower has heat settings or not.

Many thanks,

On reflection I should have guessed the response. Upgrading the MCB will only cost me £4 odds. I'm more concerned about having to rip out my tiling to get at the conduit. I'll do some measuring tonight and get back to you. How critical is this 45% ? Total cable run will be around 6m.

Andy

 

[securespark]

Well, space is fairly critical, and the bigger the cable gets, the more current that flows, and the more heat is generated. Therefore, it is important that any heat generated is dissipated quickly and easily.

 

[ban-all-sheds]

How long is the part of the run that's in conduit?

Are there any bends in the conduit?

When you say "snug fit" - what are the internal dimensions of the conduit?

 

[Do_it_later]

Thanks guys,

I've done my homework now and tracked down a table which suggests I'll be OK (just). I now realise I should have been referring to trunking rather than conduit. I've used 25x16mm.

Wall route is 1.5m vertical with no bends.

Route in cupboard will involve 2 x 90º bends, 3m length (switch to CU).

Still OK ?

Andy

http://www.tlc-direct.co.uk/Technical/DataSheets/Trunking/TrunkingCable.htm

 

[securespark]

25 x 16 for 10mm T & E?

Hmm. Basically, you need something that is half as big again.

 

[ban-all-sheds]

If that table on TLC's website is accurate, 25 x 16 has a 45% area of 120mm². The total csa of 10mm² T/E is approx 150mm², so it's not too bad...

 

[securespark]

I'd go for MT3.

What size is the conduit buried in the wall.

 

[Do_it_later]

Thanks,

Buried trunking is the same stuff 25x16mm.

The csa for 10mm given in the table is 121mm2 so you can appreciate why I thought I was probably OK ? B-A-S reckons 150mm2, - that's quite a big difference.

Following your advice, I'm happy to upgrade the (surface) trunking that I have still to fit (what size is MT3 ?), less happy about digging the 25 x 16mm out of the wall unless a real hazard.

Regards.

Andy

 

[Do_it_later]

Sorry - forgot to mention that when checking everything over last night, realised the shower is actually 8.5Kw not 9.5Kw. I can now go with a 40A mcb, unfortunately measured up and can't fit the MK unit in so will have to get the Volex or Wylex CU. Any comments on the quality of either of these, or is that a big no-no on this forum ?

Andy

 

[securespark]

Well, of the two, I'd go for Wylex.

 

[ban-all-sheds]

The csa for 10mm given in the table is 121mm2 so you can appreciate why I thought I was probably OK ? B-A-S reckons 150mm2, - that's quite a big difference.

Yo - the table does say that - I didn't notice. I just calculated the approx csa of an oval cable, 17mm x 10mm..........