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Type of cable for power to brick garage electrical unit with 40A RCD

Postby sebpinski » Thu Apr 23, 2020 3:42 pm

Firstly, I'm not planning to do the wiring myself, I simply want to run the cable which is a long length to reduce the cost of the electrician (and its just something to do during lockdown).

We've had a brick garage installed, it is separate from the house, but only a matter of metres away (and the garage and house are connected by a double leaf brick wall. The garden is on a slope, with a holding wall so burying a cable between the buildings would not be possible. I'm looking to run conduit out of the house along the wall, into the garage.

The other thing to note is that our consumer unit is at the front of the house, so the total cable run (from consumer unit to garage) would be something around 40m. We have another unit towards the back of the house, but judging by its age, I'd rather ignore it with the view to remove it altogether at some point later down the line with other planned renovation.

I've purchased a garage unit:

It comes with a 40A RCD and 20mm knockouts. So I assumed I should just buy 20mm conduit. I then looked for legislation on running cable outdoors and information was quite mixed. There is little information on which conduit is appropriate to use. Most answers to running cable to out buildings suggested to bury SDS cable (I guess as most peoples gardens aren't walled and you probably shouldn't affix conduit to fencing).

I'm wondering the following:
- is it safe/legal to run conduit between house/garage for this application?
- what kind of cable I should run to the garage (40m distance) that is suitable for the 40A RCD, that doesn't need to be buried. Ideally a high enough gauge, but would also fit into the 20mm conduit I've already bought.

Thanks for your help.
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Postby Mr White » Thu Apr 23, 2020 9:34 pm

The cable size depends on the distance and the load.
But often, when doing such a job as you intend it is better to install a bigger cable as you will always buy another "gadget" and then find out the supply is too small.

It is also best to consult the electrician who will be doing the job, often they will not sign it off as they have no knowledge of the job.

I would suggest that a 6mm SWA cable will be your best option, SWA cable does not need to go into conduit, as not only is it not required, it will not fit, and if it did, it would use too much space in the conduit. (When using conduit, no more than 40% of a conduit can have cables.) So that is another reason you can not use your conduit. Not to mention the colour of your conduit.
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Postby ericmark » Fri Apr 24, 2020 6:58 am

I agree with @Mr White in general 6mm steel wire armoured cable (SWA) is used, to thread and mount steel conduit and bend it is a big job, and plastic conduit does not really give enough protection, and both cases water ingress can be a problem, as the conduit warms up air escapes, and then when it cools it can suck water in, common to find conduit outside full of water.

There are a range of cables which don't need RCD protection, and SWA is one of them, so the normal method is to take the supply from a consumer unit with either 3 neutral bars, or all RCBO's but the garage supply is from a MCB normally no more that 32A and the RCD is mounted in the garage. This means if some thing goes wrong in garage house supply is unaffected, and it is easy to local reset.

There are two types of installation certificate, one with one signature and one with three, design, installation, and inspecting and testing, but most scheme member electricians are not permitted to inspect other peoples work, so only have the single signature forms, if you want to part DIY then in England there are some electricians who can do it, but not in Wales, other wise it is the LABC route often with a £100 plus fee, and in all cases the inspector has to be in at the start, he may allow you to lay cable, but he has to decide when to inspect in essence you become his labourer, and he tells you what to do.

Many electrician when they first start think getting the home owner to do the digging etc is good, but as they get more experienced they find so often they arrive on site and have to re-do the work, it is really not worth the hassle. So finding an electrician who will allow you to do some of the work may be hard. Once bitten twice shy.

With the lock down you may be lucky, but do engage an electrician first, once cable is in, it's too late. New metal CU not really a problem with SWA cable, but old plastic they can warp, so seems easy, but in practice not as easy as it looks.
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