DIY Doctor

Garage circuit - Thanks for the Advice

Postby HeWhoWalksAmongUs » Mon Apr 19, 2010 12:55 pm

Hi All,

Just wanted to say thanks for the advice so far I have been reading a lot of these posts before undertaking my latest project.
I am determined to do this to at least recommendations and overengineered where possible and I think I have prepared everything. but just to clarify in my mind a couple of details.

I have an SWA 6mm running to my garage and plan to use a 32A/6A mcb RCD CU.
For the wiring I will have to protect the circuit in the garage in conduit because it has to be surface mounted. I have decided to use steel conduit just for the extra strength and extra earthing properties. I am also buying a conduit bender for the job. I plan to run the lights and ring main in the same conduit around the top of the garage and come down only for sockets.

Am I allowed to use PVC if i wanted?
Inside the conduit is it ok to use single cables (4mm and 1.5mm) or do I have to use twin core and earth cables? presumably the steel more than makes up for the extra plastic cable coating?

I am also wondering if I can cut down on the number of cables I need to thread. If I want 3 lights each each leading to their own switch in a 3 way switch on the wall. I will need to run 3x cables(9) per light from the switch andx3 cables into the switch. That woul dmeans some stretches of conduit with 12 cables inside. Is there a better way to wire this to reduce cabling?

Finally for running the cable inside the house I am joining the SWA from outside with normal twin and earth 6mm cable. I am using a 60A outdoor junction under the floor boards (the outdoor JB is to allow me to connect the SWA aromour to the earth as well as the earth cable)

Under the floor boards can I run the cable stapled to side of the joists? and rather than running through the horizontal joists can I staple the cable to the underside? Or will I need to drill each joist?
Could I use more steel conduit stapled to the side/underside of the joists? Do I need Joist top metal protectors to prevent future drilling?

I am hoping that everything I have planned so far is sensible and to within or exceeds regulations.

Note: The cable run to the garage is protected by a 60A 30ma RCD (I might change this to a 100ma if it becomes annoying)

I was please another post answered my concern about access to the junction box as I cannot get a 60A wall mounted box. I am hoping I can just screw down the floor boards which will mean the JB is accessible to a "builder or electrician"

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Simply Build It

Postby sparx » Mon Apr 19, 2010 7:08 pm

ok on reading up first, it helps you and cuts down on some of the more basic questions!
It is not necessary to do all wiring in conduit just because its in a garage.
Usual way is to surface clip cables at high level and just use conduit/ trunking down to accessories.
However as you have a bender and want 'best-practice' then it makes for a good job.
Planning of routes so as to avoid crossing over conduits will pay back time spent.
As for number of cables in tubes there are tables showing how many allowed but for your simple installation unlikely to be a problem.
Lights done in single cables will only need four plus earth to switch drop,
these are one feed from consumer unit, linked to each switch, one from each switch to respective light.
Neutral from con. Unit looping to each light.
CPC (earth) from switch box looping to each light.
Power circuit 4mm radial looped to each socket;
max number of cores in any one piece of conduit should then be 8.
You could do the whole thing in plastic conduit if you wish.
If you have clearance under joists then fine, drilling should always be a last resort, then only in the centre 1/3 of the joists depth ie if 6" joist then centre 2" only.
BTW I presume you are aware LABC regs Part P covers your garage work?
Have you pre notified/paid the fee ?
regards Sparx
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Postby HeWhoWalksAmongUs » Tue Apr 20, 2010 12:39 pm

Many thanks, hadn't thought of the neutral cable reduction. Should be able to cut down a few cables.
Part P I have all the books and read up all the details. Enjoy the technical stuff, Beurocracy hummm, nuff said. I may make some changes if and when it requires an inspection.
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Postby jimmy_one_ball » Thu Apr 22, 2010 12:17 am

Just a couple of things

Radial socket outlets in the garage only need to be 2.5mm/1.5mm cpc protected by a 20A MCB if floor space is less than 50sq.m.

Why use 6mm SWA? Why not 4mm? Saves a bit of money, a quick cable calc will decide and why protect the SWA with a 60A breaker? I'd have thought 25A? Don't forget it needs to operate in time under overload conditions, what is the length of run? Stick with 30mA RCD's/RCBO's for domestics.

Finaly when you join two dissimilar cable types (such as SWA and twin and earth) then the joint needs to be fused in keeping with discrimination. It'd be better to run the SWA to the main CU in the house if poss.

And yes, screwing the floorboards is definately best as it allows future access and also highlights the exact cable location. Good luck with that!
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Postby HeWhoWalksAmongUs » Thu Apr 22, 2010 9:35 am

Thank Jimmy,

Yes your ideas make sense, But my aim was to over engineer the wiring to a) ensure upto and exceeding standards b) apply best practice where possible. c) Make the job as hard as possible on myself for fun :-)

but I have used 6mm because the cable total length of house and swa(10M and 12M twin and earth) is 22m long. Also I have calculated that the total capacity is upto 42A.
The reason was that I have spent a week burying this darn thing and digging up 1 foot concrete just to lay. I might aswell spend the extra tenner incase anyone wants to upgrade the capcity in the future.

The 60A fuse is just for RCB (CPC) protection and is not for any load just and not meant for MCB. No load will be taken from the cable before the garage.

Once in the Garage I have another RCB and 6A and 30A MCB.

Thanks for the tip on discrimination on cable types. I shall do some research into in the onsite manuals. I have already changed the fuse box from a normal box to an outdoor box which allowed me to connect the steel cable as well as the earth cable as double CPC protection.

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