Hi,
I want to wire up my garage! I am reasonably competent with electrics and wouldn't do it if I wasn't ok with it, but can someone confirm the following points for me - by the way I will get it tested and certified by a part p mate!

The distance is about 25m from CU to the garage. I know there will be voltage drop but for the life of me can't remember how to work it out! So to make it easy what size cable would be best? I will add one or two double sockets then 2 flourescent tube lamps. The sockets will be used for power tools. One day I may fit a door opener!

Also what size rcd switch in the CU?

Finally, anyone know of a good supplier of armoured cable?

Many thanks
Paul

You would usually put the RCD at the garage end, a 30mA should suffice, but dependng on your earthing system you may need to install a TT system to the garage, which will require an earth rod also.
Check whether it's a TN-C-S, a TNS, or a TT system at the house.
Your 'part p mate' should be able to help you there.

If you have a TN-C-S system at your house you should put in a TT system to the outbuilding. This will require a 2 core SWA and an earth rod at the outbuilding.

If you have a TN-S system at your house then you can install a TN-S system to your outbuilding. Basically a 3 core SWA incorporating both live cables and earth.

Voltage drop is worked out by calculating nominal current, then using the appropriate mV/A/m figure found in the regs for your chosen cable.

Working off a 4kw load and 25 meters of cable, you're looking at 2.5mm, but I'd put in a 6mm if it were my house. "What'll do a lot will do a little" I always say. and a 32A MCB at your house consumer unit will be plenty.

Now the rest of the stuff you need to do will depend on the type of system you're using, so I'd just get the cable in, and let your mate do the rest. He should know the score, and really shouldn't be testing other peoples work anyway.

SWA can be sourced from any local wholesaler.
Try Edmunson electrical, Newy and Eyre, or CEF in your local directory.

Other things to consider:
Do you have a spare way in your consumer unit?
If not, you'll require a larger fuse board installing, or a 13A supply off a spur on the downstairs ring main. In which case your ring main will have to be in an adequate condition to pass testing.
Are all of your incoming services bonded correctly?
If not, then you can't do any of the work until it's rectified. It will fail a test without it.

thanks John. I know my consumer unit is big enough and I was thinking 4mm cable. 6 is fine by me tho!

Hi

If you are installing a buried SWA cable then you might want to consider additional refinements. It's a lot of work digging a 20 inch deep trench! I put in a separate 4 core SWA cable for two way switching the outside lights. On or off from garage and house, all the outside lights on the same circuit. I would have the RCD in the house to protect the cable and an isolating switch at the CU. Voltage drop for 2.5mm SWA cable would be OK for 4KW on a length of 25m i.e. with a 15 or 16 amp overcurrent protection. It is quite a bit cheaper than 6mm and easier to handle too.

Also, could be worth a water supply for an outside tap. I also put in a telephone cable (might not need it with cordless phones these days but they had not been invented 30 years ago). And a front door bell cable too. Needed a relay and transformer in the garage as the voltage drop was too much for it to work otherwise.

I ended up with 6 double sockets in the garage and 5 double fluorescents. You can't have too much light and although you won't use more than 1 power tool at a time, its a pain to unplug them. Some of them at the work bench end and others at the far end - battery charger, inspection lamp etc. Much better than having to use an extension lead. Also, one waterproof outdoor socket on the garage wall.

I've never regretted the extra work - it was well worth it.

Cheers Allan

I'll make it simple for you mate 40A 30mA RCD at the CU this will protect the cable if a fault develops and will also give you protection at the sockets, 6mm 3 core SWA gives you a dedicated earth as opposed to using the armour and will allow for any future expansion.

Just tie wrap a bit of T&E along your neighbour's trellis and be done with it!

Bloody regs, they make a simple job tedious! :mad:

:lol1: I've seen that done before. In fact, it wasn't just a piece of T+E, it was several pieces, and some flex, with connector blocks joining them up:wonder:

Getting back to what scotthldr suggested.
The 3 core 6mm SWA route is ok so long as the house has a TN-S system. It's far quicker and easier as you say. However, If it has a TN-C-S system then that isn't really an option.
It's generally accepted that you should install a TT system to the garage in this case (2 core SWA and an earth rod at the garage). Giving the garage its own equipotential zone.
If you don't do this then there's a possibility that the earth could become live in certain situations (Failure of the PME for instance).
On my last NIC EIC inspection the inspector actually checked that I'd fed an outbuilding in this way for that very reason.:)

Aaaagh! Don't mention old bits of cable strung along fences and hedges!

Just replaced the cabling to my garage by hanging it from an earthed overhead catenary wire.

Since this is the first chance to start working on the garden since we moved in I found that the previous owner had run T&E over the top of the carport, down into the next door's leylandi hedge, then looped behind the fence to the garage. The cable changed colour part way - that's where I found the large choc block connector, securely wrapped in two plastic bags with duct tape wound round as waterproofing! :eek: :eek:

Since the old boy who did it has been dead over 15 years (natural causes - not electric shock AFAIK!), it must have been darn good duct tape! :D

^ I remember it well, one of my first foreigners :D

:lol1:
Reference method 55 and two thirds if I remember correctly?:thumb:

catenary wire with some 6mm (and a 4 mm earth if you love the regs):D and remember you started this job on the 31st dec 04

Rather than asking how on here, shouldn't you first be asking your "part p, mate" how he wants it done, seeing as he will be signing the job off?

Look on ebay for the cable - put as much CSA in as you can afford digging the trench is the difficult bit! or try screwfix!

I ran my shed power up the inside of a scaffold pole for extra protection!

Make sure you get it certified to part p or you may have "problems" in the future with paperwork when you sell up and move!