Lighting/cooker/shower circuit questions - please help!

[ThomsoK]

Hi, any advice greatly welcomed on any of the following questions I am stuck with during my re-wire:

1. I'd like to put 2 downlights in my kitchen (off the same switch) and also take a 'spur' from the kitchen light to feed an additional light in my washhouse that is controlled from a separate switch. I think the easiest way is to use a single junction box in the kitchen roofspace to house the kitchen switch cable, the 2 circuit cables, the 2 downlight cables and the feed ('spur') for the washouse light. It's a normal radial circuit. Is this the best/legal way to do this? Can I use 1mm2 cable everywhere on this circuit?

2. When you calculate the allowable loading on a light circuit, do you just add the wattages together and make sure it doesn't exceed the circuit voltagexthe MCB trip current? Or do you apply a margin e.g. total load not to exceed, say, 70% trip value.

3. Probably a really silly question but when you use plastic gangboxes for a lightswitch, what do you do with the earth wire? Just terminate it in the gangbox with a terminal block and connect other end of the earth wire to the earth point in the ceiling rose?

4. For wiring a gas cooker, do you use normal cable of the same rating as the circuit (6mm2) to run from the connection plate to the cooker? I thought you always had to use heat-resistant material in this location but I can only find heat-resisting flex on sale, not cable, and my wiring book says to use cable.

5. My wiring book suggests fitting an RCD to the electric shower circuit separate from the Consumer Unit. Is this strictly necessary? As far as I'm aware it's a standard consumer unit so could be claimed to achieve the relevant cut-off time? I've never seen an RCD in any house I've been in.

The work is due to be checked by a NICEIC spark prior to warrant inspection but I want to get it right first time for obvious reasons. He's generally not available for giving advice hence this post. Apologies for the brain-dump...

Many thanks in advance

[ericmark]

1. Junction boxes must be accessible otherwise looks OK.
2. Other way around you are allowed some diversity i.e. total can be more that trip not sure on amount but I have seen houses with kids will all lights on so I would not use diversity allowance.
3. Basically yes also have wire there in case someone uses metal switches in future.
4. Flex no problem but fused to appropriate size. Within 3 meters you can run 2.5mm² from cooker switch to fused spur.
5. The new regulations warn against doubling up of RCD’s of the same size. Only place it is done is caravans where both the caravan and the pitch both have 30ma RCD’s so coming from RCD in consumer unit is OK.
You are aware that a spark can only self-cert for what he has done, and if you want to do some of the work yourself it will need permission from building controls and fee paying and the three signature forms using where you sign for design and installation and he signs only for inspection and testing. Also from 1st July the new 17th Edition regulations (BS 7671:2008) come into force. This will change much of house wiring with either RCD’s on all circuits and a single one for all circuits is not permitted. And / or different cable like flexishield being used for circuits other than sockets to avoid using RCD’s. All sockets will require RCD protection unless marked as for the use of one piece of equipment i.e. freezer.
I would assume you have already down loaded Part P if not I would follow links in projects and down load as well as do’s and don’t it also gives some valuable info on how to wire. The new 17th Edition test sheets are nearly same as old 16th edition worth seeing what is checked.

All best Eric

[ThomsoK]

Okay Eric. First things first, thanks muchly for the advice so far. Still a bit stuck with the cooker though. So far I've run 6mm2 cable from CU to cooker switch and then 6mm2 cable from cooker switch to outlet plate behind where the cooker will go. Am I okay so far? Are we saying what I need to do now is run 6mm2 heat-resisting flex from outlet plate to cooker? On reflection, using cable might be better for me as I've got some spare 6mm2. Does the full circuit need to be 6mm2?

As you mention, I'll check the MCB rating versus whatever cooker we end up with, plus a bit added for the integral socket in the cooker switch.

P.S. Might be an unfair question but do you know if anything we've discussed so far would be different in Scotland? Forgot to mention the work is in my other wee place, which is in Scotland. I've just had a grim realisation that the version of regs I've been working to actually refers to England and Wales! Doesn't seem to be nearly as much info on t'internet on Scottish regs.

Cheers

[ericmark]

Scotland have their own regulations which I think are tougher but not something I have been interested in. I think to give advice may take you in completely the wrong direction one answered people don't tend to look so may be better to re-do with Scotland in subject.

[ThomsoK]

Okay mate, will do. Thanks for all the help.