Any Electricians On Here? Home Wiring Question.

**mctw** · #1 6th May 2009, 15:09

Heyy guys,

Kind of a odd question i guess, but am hoping maybe theres a few people on here with a bit more electrical Know How than me.

Okay so basically were rebuilding our garage, which is a fair distance away from the house. We have 30amp amoured cable running underground from the house to it. Looking to have a ring main and lights in the garage, so i know i need a fuse box for safety and to split it out properly. The power into the underground cable cant really be wired directly into the main fuse box in the house, but is it safe to wire it directly into the ring main as a spur? and if so, do we need a fuse on the spur, to stop to much power draw. ive wired most the house ring mains, but this isnt something ive ecountered before, so any advice would be greatly appriecated!

MCTW

**serverguy** · #2 6th May 2009, 15:58

I've just done the same thing in a new workshop.

From reading through the big green book of regs in front of me now, I understand you can only have 1 spur off every socket on the ring, and it must be either a single or a double.

So you couldn't run another consumer unit off that spur, at least it's not going to be within the regs, and no sparky is going to certify it (to keep within Part P). At least I don't think they would.

The best option will be to just run a cable from the consumer unit, or failing that, if you have an unused electric cooker socket, you can turn that into your garage feed.

I just ran a cable straight from one of our consumer units, but then again I think we have 9 consumer units in various barns lol.

I suppose your only other option, is to put a 13amp fused spur on the ring you can easily access, then run your consumer unit off this, making sure it has an RCD in it.

However, you're going to be limited to 13A in the shed.

Honestly the best way and safe way to do it is to put a 32A MCB in the house consumer unit, run that out to your shed, then put a consumer unit with a 63A or whatever RCD in the garage, with a 16A MCB for a radial socket circuit and a 6A MCB for the ceiling lights.

**Bubba** · #3 6th May 2009, 21:20

What server said, but also: 30 Amps isn't very much for a garage. That would mean you are running all your lights and outlets on the same feed. Maybe you don't use your garage like I do mine, but thirty amps? my little circular saw draws 15. My little bitty air compressor also draws 15. PLUS the lights. SO if I had only 30 amps and I was cutting a board and the compressor kicked on not to mention the lights...............

I would want a couple of circuits at least for something like that.

Of course once again we get into linguistic differences lol. I have wired tons of stuff here in the US, but I haven't the foggiest idea what a "ring main" is.

**philthomas** · #4 7th May 2009, 04:39

A Ring Main is a term used in domestic house wiring.
Normally it will provide the standard 13amp wall sockets. It's called a ring because the sockets are wired from the consumer unit and a further wire returns to the consumer unit after the last socket. Each ring is rated at 30amps
This allows for the load to be spread both ways back, which eliminates the need for heavy cables. I believe this circuit can handle 7.2kw or therebouts.
Normally 2.5mm cable is used ..... and wired this way, it becomes the equvalent of 5mm cable.

Most houses here have a ring for downstairs, a ring for upstairs and maybe a seperate ring for the kitchen. Bigger properties may have more. Lighting is by seperate circuits.

Not sure about your part of the world, but our domestic mains supply is 220/240v

**philthomas** · #5 7th May 2009, 04:58

@mctw

As serverguy has said, you cant run a ring main via a spur from an existing one .... this would not be legal/safe.
You must come from the main consumer unit, via a spare fuseway, or fit an extra unit, if no spare is available.
Also the rating of the armoured cable is important, as these can come in various sizes. You need to make sure it is beefy enough for the job yo have in mind.

Have a look here for some more info

As Bubba mentioned, it depends what you intend to run.

**Bubba** · #6 7th May 2009, 09:25

Ah, thanks phil. That's right, you Europeans use 220. We use 110. Best guess, what youcall a ring main, we call a circuit breaker from your description, thanks for the clarification.

Sorry to interrupt mctw, but I'm still trying to figure out the Queens English lol.

**philthomas** · #7 7th May 2009, 10:08

Not sure that is correct Bubba ....... here is a diagram of a typical ring main :

The cables at the consumer unit are both connected to the same fuseway .... this may be a simpe fuse or a circuit breaker.

**serverguy** · #8 7th May 2009, 10:10

@Bubba

It is also worth remembering that you're on 110V standard, we're on 230V standard.

We can draw a bit over 3000 Watts from a socket, you can draw roughly 1500 Watts I believe.

So our 30A supply is like your 60A supply at 110V.

And nope, ring main is literally how the sockets are wired up..

If it was just the blue wire, then it would be a radial circuit, but add in the yellow wire as well, joining back up with the blue wire once it gets to the consumer unit, it is a ring circuit.

**mctw** · #9 7th May 2009, 13:35

cheers for the quick feedback guys!
i think ill have to rethink this idea slightly. for now i think ill do what serverguy first suggested, running a 13amp fused spur of an existing ring. 13amp should cover it i hope, but if it blows ill have to wire it directly into the main fuse box, we dont have any spare lanes in it, so thats something trying to avoid if possible!
i see what you mean though, about not being safe to run a ring of a spur, the draw would be far to high!

**serverguy** · #10 7th May 2009, 14:40

Quote:

Originally Posted by mctw

cheers for the quick feedback guys!
i think ill have to rethink this idea slightly. for now i think ill do what serverguy first suggested, running a 13amp fused spur of an existing ring. 13amp should cover it i hope, but if it blows ill have to wire it directly into the main fuse box, we dont have any spare lanes in it, so thats something trying to avoid if possible!
i see what you mean though, about not being safe to run a ring of a spur, the draw would be far to high!

I didn't suggest this

I said it should be safe. I would really recommend getting it all checked out by a proper electrician.

Why not just run the wiring out to the shed, put the consumer unit in etc, but then just put a 13A plug on the end of the cable, and plug that into a socket.

That way a) you stay within the law (it is not termed as permanent as it ends in a plug), b) it will get you exactly the same amount of power to the shed and c) if you decided to upgrade in the future you just have to get a sparky to plumb it in to the fuse box properly, adding in a new consumer unit as well.