Garage power supply - Recap [Longish]

[Roger Mills]

Hi again Andy,

Thanks for the above information. I'm still a bit confused!

From earlier discussions, it looks likely that my main supply is TN-C-S or

*possibly* TN-S.

I've looked at the MK stuff on TLC's website, and can't quite work out what I need - not for a reasonable price anyway! Their split load CUs are 70-odd quid +VAT before putting any innards in. Can you suggest a shopping list of bits which would give me MCB protection for lights and RCBO for power - and be compatible with my mains supply - without costing an arm and a leg? [The price reference is the Screwfix garage enclosure for £37 - so I'm hoping to get something which *doesn't* take out the lights when the RCD trips for not too much more than this].

Incidentally, my current thinking is to join the T&E to the SWA using an adaptable box in the roofspace. This means that there won't be any joints exposed to the weather, and the cable won't need chasing so deep into the wall of the en-suite. Hopefully, I can strip the end of the SWA and fit the gland in the en-suite, and then poke it up through the hole in the ceiling, and then grovel in the attic to make the actual connection, which shouldn't be too bad. Does this sound reasonable?

[John Rumm]

If you go the split load route, then you don't need the RCBOs and can use ordinary MCBs for all the circuits. However as you have found; split load CUs tend to be big and expensive.

Does it have to be MK? If not, how about (For a TN supply):

price reference is the Screwfix garage enclosure for ?37 - so I'm hoping to

Your difficulty there is the price of RCBOs - one of them costs about that!

Cheapest (but not smallest) option may be:

that still tops £50

Goveling never sounds reasonable, bit if is what you have got to do.... ;-)

[Roger Mills]

Not necessarily. I quoted MK because someone said you could buy their loose bits, and do a mix & match. I'm sure you can do that with other makes.

If not, how about (For a TN supply):

Ouch - that's expensive. I wasn't sure whether you could use an RCBO with that board - 'cos it isn't listed among the compatible MCBs.

According to my calculations, it's about £48 - including VAT and a load of blanking bits. It would probably do - but it does seem a bit of an overkill to have a bigger CU in the garage than in the house!

Just to stick another spanner in the works . . I was told today by one supplier that a detached garage is considered to be 'outside', and that everything in it has to be protected by an RCD - hence the design od the standard garage CU. Is there any truth in this? If so, the above solutions wouldn't comply anyway - so I'm back to losing the lights when I saw through my power cable?!

Actually, it's not *that* bad. It's a difficult bit of roofspace to get into because it's part of an extension without proper access - so I have to grovel under a purlin of the main house to get in. Once in, I can pull the cables through to a place where there's a reasonable amount of headroom, and make the joint there. I just have to make sure that I take everything I need with me, to avoid multiple grovellings!

[<me9]

Screwfix do a nice MK split load one (10-12 way) complete with MCBs for a trifle over 70 squid.

--

[John Rumm]

The contactum ones are pretty tall[1], and ought to take the single module RCBOs - but obviously TLC would be able to confirm if you askem them (with a four way board you could afford the extra width of the normal RCBOs).

[1] Here is one I dismantled before:

>Cheapest (but not smallest) option may be:

OK just thought of a cheaper scabbier solution[2]:

Two of:

a plain incomer switch (which I am sure they must sell on its own, but can't seem to construct the right search phrase to find it!)

Take your submain into one encosure and then to the second. Fit an incomer switch and 6A MCB for the lights on one, and a 30mA trip RCD and

20A MCB for power on the other. [2] Although to be totally correct you opught to use metal clad CUs for a TN supply)

Depends on your garrage I guess. If it has a bare soil floor, then to all intents it is outside - you would be hard pushed to extend an equipotential zone into it. A nice concrete floored building on the other hand is probably as "inside" as your front room!

[David Hansen]

On Wed, 30 Aug 2006 22:50:55 +0100 someone who may be "Roger Mills" wrote this:-

Of course a supplier will say this, they want to sell you things.

However, there are garages and garages. Some garages are essentially built to much the same standard as houses, some are a few sheets of rusty corrugated iron or asbestos. It is up to the designer to assess the possible risks and proceed from that.

That will give you an incentive to triple check you have everything before going in the first time:-)

[Roger Mills]

bits have just arrived.

A couple of questions - surprise, surprise!

The live busbar is supplied in one piece. I'm sure I've read somewhere - though it ain't in the instructions supplied! - that I need to cut this into two in a place appropriate to how many ways I want protected by the RCD. Is this correct.

There is already a live cross connection between the output of the main switch and the input of the RCD. I assume that the part of the busbar serving the non-RCD-protected ways needs to connect into the same output from the main switch?

As supplied, neither the main switch nor the RCD will turn on. Is this because they need a mains supply before they will operate, or are they duff?

Finally, bearing in mind that this is an insulated rather than metalclad CU, what's the best way of terminating the incoming 6mm SWA cable? There's no obvious place on the casing to fit an SWA gland. Do I need a junction box next to the CU for this purpose?

Thanks.

[Ben Blaukopf]

Roger Mills wrote: ice will be much appreciated.

a) It's covered by part P b) I'd google anyone I was buying a house from, and find this thread. c) It probably won't get lost in the noise, because it's the noise that is precisely the problem when buying a house. Well, that and solicitors covering their backs. d) But, if I found it by google, I'd then chat to you, and provided I was convinced that you knew what you were doing and did it properly, I'd tell you to forget about disclosing anything about it, and the solicitors would never be the wiser. Much the best way.

Anyway, just pointing out that posting plans to ignore buildings control isn't a good idea when done under your real name!

Cheers,

Ben

[John Rumm]

This is often the case. IIRC it was with the last contactum split load CU I fitted... (thinking about it I remember it because I forgot to take a hacksaw!)

Yup, check the wiring carefully though, making sure it is correct and tight! (I had a contactum CU where the wiring as supplied was seriously wrong!)

[Next bit I am doing this from memory - so if it does not make sense when you look at your one, ignore me!]

Assuming you are going with the main switch on the right, and the RCD in the centre, then the phase and neutral from your SWA run into the top terminals of the main switch (phase on left). The bottom right of the main switch ought to have a neutral connection to the non RCD neutral busbar - there may also be a connection to the top neutral on the RCD (or that will more likely come from a wire on the end of the first nutral bus bar). The phase (bottom left) of the main switch ought to connect to the main phase busbar AND the flylead to the (top left) phase input of the RCD. The phase output of the RCD (bottom left) should then feed the other bit of phase busbar, while the bottom right of the RCD should have a lead to the second neutral busbar.

Any of that seem plausable?

It is normal for a RCD, but not a switch (assuming it is just a bog standard two module wide switch). The Switch should snap from one side to the other, with a bias toward "off".

Personally I would drill a suitable sized hole to take the SWA gland to make it neat and tidy. Then use a ring terminal on the gland to pick up the earth from the shield, and take that to the earth busbar (along with the third core if you have one!).

[John Rumm]

Yup, agreed.

I bet few would though.... still its certainly possible.

(I hope you like reading - there are getting on for 10,000 posts that mention Rogers's name!)

I expect a few years into the future they wil be showing as much interest as they do in FENSA certs now (i.e. getting progressively less)

Yup, I would much rather find evidence of someone asking sensible questions before doing a job!

Hmmm, doubt there are many on this group who haven't wrt part P!

[Roger Mills]

Yes, the way it's pre-wired corresponds with the leaflet which came with it.

Yes, that's exactly right.

On trying again, they *do* in fact turn on. It's just that they take more force than I was expecting - and I didn't want to break anything!

There isn't really anywhere suitable in the body of the thing to drill holes - apart from some knockouts in the back. But I'm not planning in coming straight through the wall into the back for several reasons. My current thoughts are that I will drill a 20mm hole in a piece of aluminium angle to take the gland, and screw the angle to the wall near the top RH corner of the CU, and just take the conductors in through a cutout in the cover. If I fix the angle and gland *first*, and then position the CU next to it, there won't be any exposed cables.

Thanks again for your help and support.