help with supplying a garage

[CanIEat]

Hi all, been a while. I'm installing a power supply to my dettached garage. I have installed the cable from consumer unit to garage and my electrician friend will be checking this over and attaching the cable at the CU and garage ends when he gets back from holiday.

Couple of questions to hopefully save some time -

1. I understand that the protective wiring in SWA cable has to be earthed at at least one end. I have put the 'banjo' of the gland inside the plastic junction box, and wired it to the earth wire in the 3 core SWA (black wire). Is this acceptable? If not how should it be earthed?

2. I have a 2 way garage unit for the garage end. What do I need to buy for the CU (house) end? It's an ancient fusebox with no circuit breaker switches. I understand there has to be RCD protection at the source, so do I need another unit for this end?

Thanks

 

[SimonH2]

I have installed the cable from consumer unit to garage and my electrician friend will be checking this over and attaching the cable at the CU and garage ends when he gets back from holiday.

Full marks - that's the way to do it. Also cheapest - it'll probably cost less to employ your friend (and have him self notify through his scheme, assuming he's in one) than to notify yourself to LABC.

2. I have a 2 way garage unit for the garage end. What do I need to buy for the CU (house) end? It's an ancient fusebox with no circuit breaker switches. I understand there has to be RCD protection at the source, so do I need another unit for this end?

Actually no, you are best not having an RCD at the supply end of this cable. As long as the wire armouring is properly earthed, then you don't have to have RCD protection on it - the idea is that either the wire protects the cable from being penetrated, or if it is, a short is created and the fuse blows.

If you do have an RCD at the supply end, then you can be in the garage, the RCD trips, and you then have to find your way out and back to the house without any lights to reset it.

You will need RCD protection on one or more of the circuits in the garage CU - at least the sockets, possibly the lights depending on installation type.

 

[CanIEat]

Ok thanks for your reply. The garage CU is RCD protected so all ok there. I take it I have earthed the SWA correctly?

The part of the cable run inside the house is 6mm squared T+E. This will just go straight into the house CU? It would be useful to be able to easily isolate the circuit from the house, can a MCB be fitted into an old style CU?

 

[mfarrow]

There are a few things to think about first:

- what is the supply type into your house? Is it a TN-S, TN-C-S or TT? There are different ways you should be managing each in terms of earthing.

- how long is the cable run and has voltage drop been considered in the calculations?

- an MCB would probably fit the board, but if your supply type is TT then I suggest to consider an external isolator (external to the board, that is), which may be the way to go anyway.

 

[Taylortwocities]

The approach at the house end has several factors to consider and these should be discussed and agreed with your "vacationing electrician".
These sparkies do seem to have a lot of holidays!

The supply type as outlined by mfarrow is one.
The SWA will need protecting by a protective device, not just an "isolator". Your cable is 6mm² so 40A would be typical.

This can be done by either an additional fuse/breaker in your fuseboard, but if it is

an ancient fusebox

then there may be a limit on the total load that the board can handle. Your electrician needs to review the existing loads.

The other method is to use a switchfuse just fior the garage feed. This will require engineering a branch for the feed out of the DNO's meter and has several implications regarding safety, and earthing that are outside the scope of a DIY/internet discussion.

Again, wait for your sparky to finish rubbing in the aftersun. You've done the donkey work.

 

[CanIEat]

Yeah he never said anything about different supply types, didn't want to confuse me perhaps.

The length of cable from CU to garage is approximately 15m.

Don't worry I have no intention of connecting this up to the house myself! I have 3 weeks to wait so that gives me time to wire up the lights and socket in the garage. I might post a photo or two of the work done for the forum's approval/disapproval, could save having to redo things when the time comes.

Thanks again.

 

[Taylortwocities]

Do send pics

We LOVE pictures.

 

[ban-all-sheds]

Also cheapest - it'll probably cost less to employ your friend (and have him self notify through his scheme, assuming he's in one) than to notify yourself to LABC.

CanIEat

Joined: 26 Feb 2008
Posts: 23
Location: Stirlingshire,
United Kingdom


Although even so I would still say that it should be the friend who makes the design and construction decisions.

 

[CanIEat]

Ok looks like it will be next weekend when electrician is coming to connect this up, so here are some pics of what I have done for your scrutiny...

The house CU. Looks like there are no spare slots, although two are unlabelled, so I don't know what will need to be done to add in my garage supply.









The big loop of 6mm T+E cable going into white conduit down to the ground is the cable I have put in - from here it runs under the floorboards to the basement, along the basement wall and then out through the exterior wall to the box on the left of this pic -



There is then 3 core SWA going underground and back up to the box on the garage wall. This is how I have wired this box -



Is this ok? Is the earth connection correct? I have taken the black wire as earth, I think I have to sleeve it green/yellow?

The next section of cable goes through the garage wall and runs for about a metre before going into the garage CU -



Here is a bit I am unsure about. I didn't have any 6mm T+E cable left so I used a bit of the SWA coming from the junction box to garage CU. It obviously comes from the box without using a gland - is this acceptable? As you can see there is a gland at the CU end so the armour cable can be earthed there.

Finally for now here is a picture of the lights I have put up. There will be a switch at each end of the garage, I also have a double socket to wire up.




I was also thinking a security light at the garage back door would be a good idea. The breaker in the CU for the lighting is 6A - will the 2 strip lights and security light be ok with this? The cable I have for doing the lighting circuit is 1.5mm squared 3 core + earth - if someone can link me to a diagram of how to wire up 2 lights to 2 switches it would be much appreciated!

Thanks.

 

[Aragorn84]

Wouldnt it have made more sense to run the SWA all the way from CU to CU?

Save using those huge surface mount boxes and having to worry about earth continuity throughout the cable run.

At the very least the run from the garage CU all the way to the house could have been continuous and you could have had the SWA to T&E joint inside in the basement where its less likely to get wet...


As for connecting it, I suspect your going to have to have a henley block installed on the meter tails and a seperate switchfuse for the garage supply.

 

[Aragorn84]

also, wiring diagrams for your 2 way light switch are in the wiki.

 

[bernardgreen]

Those push fit plumbing joints above the CU would worry me a lot.

I have seen them start leaking too often to trust them above electrical equipment.

 

[dhutch]

6amps * 240-320 volts = 1.4kw

So assuming your not running blackpool iluminations as an outdoor light you should be fine. A 120w halogen should be more than enough, less if you used a florecent floodlight.


Daniel

 

[CanIEat]

Wouldnt it have made more sense to run the SWA all the way from CU to CU?

Save using those huge surface mount boxes and having to worry about earth continuity throughout the cable run.

At the very least the run from the garage CU all the way to the house could have been continuous and you could have had the SWA to T&E joint inside in the basement where its less likely to get wet...

Ok thanks for that, I didn't think the SWA would bend and go down the wall neatly after coming out of the hole, so went for the boxes.

I wasn't too worried about the outside joins getting wet with them being inside sealed boxes but I think you are right it would be better from the earth point of view if I had just one section of SWA going into the basement.

Is there an easy wasy of doing a sharp 90 degree bend in SWA cable?

 

[Scoby_Beasley]

Sharp bends in any cables are a no-no.

For armoured cable, there's a formula for the minimum internal bend radius of the SWA to avoid damaging the internal conductors.

Formula: Dia of SWA x 6 = Min internal rad.

In your case I'd be chiselling a tapered channel, from the hole, in the wall so at the end of the bend it is flat to the internal wall.