Electric Showers - maximum cable lengths

[coachwheeler]

Hi guys,I'm new to this forum so please be kind.

I have no doubt these questions have been asked before but I have spent the last hour seaching the threads and can't find the answers. Can you please help

We have a bungalow with a family bathroom which contains an electric shower rated at 7.5Kw fed by 6mm2 cable and protected by a 40A mcb.
All is well.

We are planning an extension which will allow us to increase bedroom sizes and hope to install a further 2 ensuite shower rooms (1 to each) with electric showers rated at 10.5 Kw which will need 50A mcbs. These will be at the other end of the bungalow to the kitchen which houses the consumer unit. Obviously each will need its own independant supply, which will be 10mm2 cable.

Q1. Is there a maximum length of cable between shower and consumer unit.

Q2. It is most unlikely that all 3 showers will ever be used at the same time; can all three be put through the consumer unit on separate mcbs or would you recommend separate CUs for each of the new showers. The CU RCD is rated at 100A

Your suggestions, comments and/or recommendations would be welcome.

 

[Lectrician]

51m is the maximum length for 10mm cable with a 45amp load. At that length you drop the full 4%(9.2v) allowed by the regs.

3 showers on a standard domestic supply is not a good idea. If not used at once they would be ok, but who is to say they NEVER will? What happens when you move?

Can't you fit standard plumbed showers, possibly with a pump to give you a decent shower?







And I don't want to see people posting the crap about having a changeover switch fitted so you can only use one at a time - that has to be the crappiest solution ever. Seriously.

 

[coachwheeler]

Thank you ... 51m will be more than sufficient, and separate CU's it is then.

I would never consider a changeover; can't see any reason to complicate the simple!

As for standard plumbed showers with a pump, will probably have to install instananeous water heaters anyway as the hot water tank is relatively small, the head and thus hot water pressure is not high and the run will probably be too long - about 25 m from tank to new shower-rooms

 

[Lectrician]

Even having seperate consumer units for the showers will still put the same load on your supply arrangements from the DNO (the supply cable, cutout and meter).

You should seriously consider other methods of providing showers, as 3 on a standard domestic supply is not good at all.

I would speak with a plumber and see what he suggests.

 

[Steve]

An "interlocking changeover relay" would be a neat solution here. If they exist. Some kind of device that allows one at a time, and automatically returns to neutral when no demand is there.

The only correct way to supply 3 showers is with a 3 phase supply, like what small shops have. You have, in effect, 3 x 100 amps to play with, which is plenty for your showers, BUT it costs an absolute bomb to get the upgrade, probably looking at thousands.

I will echo what others have said, putting 3 showers on a single 100 amp supply is not a good idea, you cannot guarantee that they wont all be used at once (unless you use a fancy relay gadget). Even 2 showers is frowned upon.

 

[coachwheeler]

Thanks for your advice; will have a rethink and look at other alternatives.

Bit surprised noithing was said by the planning department before they passed our application - gave a full spec on what we had in mind and and they made a few 'suggestions' on the drains, but not the electrics!

 

[dingbat]

Bit surprised noithing was said by the planning department before they passed our application - gave a full spec on what we had in mind and and they made a few 'suggestions' on the drains, but not the electrics!

I'm not at all surprised. And neither, I'd imagine, are any other sparks on here.

 

[Spark123]

I wouldn't be surprised if LABC or the architect stipulated something like all electrical work to comply with BS7671.