Garage/Workshop circuit extension

22 Nov 2006
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United Kingdom
First of all hello - this is my first post afterall.

Now my question:

I need to install sockets and lights in my new garage - it is timber with a real tile roof (no change of water ingress) and there is no water supply to the garage. I will need a couple of lighting circuits so I don't need to light the whole thing all the time. I need loads of sockets - but will probably use a MIG welder, heater, radio all at once (substitute MIG welder for any typical hand held power tool for most of the time).

However, my house CU is full.

There is a MCB (40A) in the house CU feeding my shed which has its own CU with a 6A light MCB and 32A socket MCB. The shed power comes from the house CU via an old shower cable then SWA (3 core 4mm) underground. There is a RCD at the end of the SWA before it goes into the shed CU.

What is best for getting power to my garage?

Adding a MCB to the shed CU? and running SWA to a garage CU?

As the existing SWA to the shed is only 4mm is that big enough?

I'm concerned that the old shower cable is too small - how can I tell? It's the same size as my cooker cable.

By the way the shower cable setup to the shed was done by previous owner or an electrician they employed not me.

Any advice greatly appreciated.
Mike :)
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You dont mention distances. We need these.

All work must be carried out in accordance with BS7671 and notified to the local building control for inspection and testing. I doubt it would pass due to the twin and earth bodge. You need to take power directly from the house, and renew the shed's power cable while you're at it.

But your electrician will advise on the best course of action to provide power from the house.
The distance from the house cu to the shed cu is about 15m via the current cable route.
I was considering changing the current cable to the shed and therefore rerouting it which could take it down to 8m.

Distance from shed cu to garage cu would probably be 7 or 8m
You're right about the shower cable bodge - I take no responsibility for that. There are plenty more socket/light switch bodges that need sorting out too.

So is it basically ok to string consumer units together in the way I'm thinking:

house cu -----------------------> shed cu -------------------> garage cu
40A MCB____6mm SWA______40A MCB___6mm SWA_____split load

I'm just not sure what rating MCB should be in the shed for the cable to the garage.

If I have to run a cable from the house cu to the garage bypassing the shed then can that go into the same house cu MCB as the shed?

I'm not saying I will do all this myself, I just want to know what's likely to be involved (comments on inspection duly noted).

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The problem with the circuit you have drawn there is that you have two 40 amp MCBs in series. They will not discriminate. In the event of a fault that would trip a 40 amp MCB, either could trip. Its not dangerous, but bad design.

For the appliances you describe in the garage, I would want to put in a 50 amp supply. This would require a 63 amp supply from the house to the shed, and a 50 amp supply from shed to garage. The whole run must be robust enough for the garage load ("loads of sockets" and a mig welder and heater, and no doubt, lights). This is why I suggested supplying the garage straight from the house, bypassing the shed, since no doubt the electrics in the shed would have to be upgraded to be capable of handling the 63 amp supply (not a rubbish "garage CU")

You'd need 10-16mm² from house to shed, and 10mm² from shed to garage. You'd also need two 4-way metal consumer units, and a handful of RCBOs and MCBs. The feed would best be sourced by splitting the meter tails in the house and taking them into a switchfuse to feed the shed. This will get expensive.

Ideally, you will need an electrician to come and assess what you have and see what he recommends. What I have described above might be seen by some as overkill, but if it were my house/shed/garage, this is how I would do it.

You will save money by digging yourself ;)
crafty1289 said:
You will save money by digging yourself ;)


I'm all for saving money !

Thanks very much for the comprehensive answer, it's very useful and backs up a lot of what I was thinking myself but wasn't quite sure about. I'll have a think about what is realistic for the load in the garage as most of the time it will only be me in there and I'm unlikely to be welding and using other hand tools at the same time :LOL:

I'm off to do some thinking.


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