Supply for 10.5kW shower

[duncanT]

Hello experts

Would you give me advice on the following project?

I'm installing a new electric shower in my bathroom - a Triton "Opal" unit, rated as 10.5kW at 240V.

There was no shower in the bathroom before, so a new supply is needed. The plumber has run a new cold water pipe in copper, and he says that I have to notify my local council because I'll be carrying out the electrical work, with the possible exception of the final connection to the consumer unit.

I've read a lot of your posts on the electrical forum, and I've examined the existing installation, which is as follows:

The service fuse is 100A.
There's some kind of isolator switch between the meter (in a cupboard outside the front door) - it's marked as "63A type 2".
The tails between that switch and the CU are of an unknown (to me) gauge.
The CU is an MK Sentry unit, with two spare channels in it.
The CU contains an RCD rated at 80A, which serves all of the MCBs.
The distance between CU and shower position, via the loft, is 8 metres.
The distance between CU and shower will be within studwork walls for about 5 metres, and then within the loft for 3 metres.
There is supplementary bonding in the bathroom, from bath to hot tap supply to cold tap supply. I believe that the cable size is 4mm².

If I've understand correctly what I've on the forum, the main points about the installation are as follows:

The expected maximum current consumed by the shower will be 45A.
The shower should be supplied by a dedicated circuit, run in unbroken cable.
The new circuit should be protected by a new MCB at the CU, rated at 45A.
The minimum cable size for the shower circuit is 10mm².

My questions are:

(a) Is the MK Sentry a good unit?
(b) What's the minimum size of the cable between the isolator and the CU?
(c) What's the external diameter of 10mm², 16mm² and 25mm² cable, so that I can check the size of cable currently installed.
(d) Is 10mm² cable adequate for the new shower, or should it be 16mm²?
(e) If so, please could you help me understand why?
(f) What "type" of MCB should be used for the new shower circuit?
(g) What are the bonding requirements for the new shower?
(h) Do you have any other advice to give me on this project?


Sorry for my post being so long - I worry about the detail cos I want to make this as safe as houses.

Thanks
Dunc

 

[Adam_151]

(a) Is the MK Sentry a good unit?

There is probably better, but MK is pretty good

(b) What's the minimum size of the cable between the isolator and the CU?

10mm

(c) What's the external diameter of 10mm², 16mm² and 25mm² cable, so that I can check the size of cable currently installed.

Google for tlc direct, and look at their cable section, they give you the o.d. of cables

(d) Is 10mm² cable adequate for the new shower, or should it be 16mm²?

Quite unlikely 16 is needed, but possible depending on how the cable is to be installed

(e) If so, please could you help me understand why?

Look up derating factors in the onsite guide (£17 from IEE)

(f) What "type" of MCB should be used for the new shower circuit?

MK B45 (possibly with retrofit kit, depending on age of your CU)

(g) What are the bonding requirements for the new shower?

Just the usual main and sup bonding, as per the OSG

(h) Do you have any other advice to give me on this project?

A picture of your cu and surroundings would be usful, your CU sounds like it is served by a 63A submain... oh and check with LABC how their notification works, all areas are different, and if they want you to issue an EIC, I doubt you will be able to (no offence intended)

Sorry for shortness, its late, any questions on that, then come back and I'll try and go into more detail

 

[duncanT]

Thanks for your help Adam, and so fast!

I have a question though, maybe more than 1.,

(b) What's the minimum size of the cable between the isolator and the CU?

10mm

Will that cope with 80A then?

(c) What's the external diameter of 10mm², 16mm² and 25mm² cable, so that I can check the size of cable currently installed.

Google for tlc direct, and look at their cable section, they give you the o.d. of cables

Any chance of a link. I have found the tlc site but can't find the o.d. section.

(e) If so, please could you help me understand why?

Look up derating factors in the onsite guide (£17 from IEE)

OK. Is there a short tehcnical answer to this, just as a hint?

(h) Do you have any other advice to give me on this project?

A picture of your cu and surroundings would be usful, your CU sounds like it is served by a 63A submain... oh and check with LABC how their notification works, all areas are different, and if they want you to issue an EIC, I doubt you will be able to (no offence intended)

I'll try and put to a picture - not very good at that..

What is a submain btw? And why would the submain be at 63A if the CU RCD is at 80A? Is that a wrong thing?

 

[JohnD]

Like Adam was saying, a photo will be a tremendous help - your installation will be easy to recognise and save a lot of describing/questioning.

Can you lay your hands on a digital camera?

on question (e) the things that make you need a big cable are:

(1) insulation. the current makes the cable get warm; if it is packed in insulation, or an insulated wall, or passes through e.g. a section of insulation in the loft, this heat can't get out, so the temperature can run away. a bigger cable generates less heat at the same amps.

(2) if it passes through a hot area, like an airing cupboard, again, the heat loss from the cable is slowed

(3) the length of the cable makes the voltage loss greater due to cable resistance over a longer length (this is not usually a prob in an ordinary house). a bigger cable has less resistance.

p.s. you can also borrow the On-Site Guide through your local library - has lots of useful info, tables of data, things you might not think of. the 2002 version was ISBN 0852969872 but they should have a later one. Probably under 621.31924