Garage Wiring- The Plan

[FreddieLIVES]

Hi all,

Just a bit of advice to see if I am on the right track.

New external garage (not attached to the house) and I would like to wire it up for lights and a couple of applicances(fridge+spin dryer) + usual garden stuff.

The plan is to take a feed from the non-RCD side of the CU in the house to the garage using 6mm T&E at high above ground level (in conduit) with it's own MCB in the House CU.

In the garage, feed into a smaller CU with an RCD and the necessary MCB's required for the various circuits.

The supply earthing system in the house is TN-C-S (PME) so I assume I will have to put an earthing rod in garage somewhere.

Does that sound ok?

Any advice is much appreciated.

TIA

Steve

[Dave Plowman (News)]

Easier to use SWA cable. 6mm 2 core is 1.50 + vat per metre from TLC.

[BigWallop]

Then split the tails from the meter to the consumer unit with Henley blocks and install a completely separate isolator switch for the out-house / garage. Using Steel Wire Armoured cable will let you connect everything securely at both ends, and also allow for a completely separate earth system at the out-house end of the installation to protect the whole lot.

This is done by only supplying the new isolator switch with a Live and Neutral feed, and leave out the earth bonding totally so it is protected by the new spike at the out-house. This prevents any interaction from the new out-house supply interfering on your house installation.

The new isolator switch should only have some kind of safety breaker device to protect the supply cable in the event of damage or short circuit along its length. The new CU in the out-house will take care of everything else connected to the new supply.

[FreddieLIVES]

Would the isolator switch be in the garage end of the SWA or the House End?

[BigWallop]

The house end. All you're doing is supplying directly from the house meter to a new separate isolator switch that controls the out-house. The new isolator switch will also house the safety breaker device that protects the new SWA cable run and also keeps it separated from any protective device used for the house. This way, any fault occurring in the SWA or the out-house will not interfere with the house at all.

[Doctor Drivel]

Sounds fine.

[Doctor Drivel]

This sounds like overkill for someone who just wants power to run the odd light and socket in the garage.

If the garage is run as a workshop then what you say is the way. But I would presume overkill in this case. The original proposal sound fine.

In some new properties they run the garage off the downstairs ring, which is off the main CU RCD, and then having a garage CU in the garage, with a fuse as main protection for the garage.

[Dave Plowman (News)]

You'd use TW&E in conduit?

Figures.

[Doctor Drivel]

If it is cheap enough yes. If it is internal cable then putting it in conduit will protect it, and make the job look neater. Conduit looks far neater than external cable.

You could use single cores.

[Martin Angove]

The only issue with T&E in conduit -v- SWA (note that the proposed route is above ground level) is the possible loop impedance of the earth path if you intend to export the earth.

Hwyl!

M.

[John]

Exactly what do you think will happen if the length of SWA from the house to the out-house suffers an earth fault? The chances are an outside earth electrode will struggle to meet a low enough earth impedance to give a rapid disconnection from the supply since you have conveniently glossed over the overcurrent protection provided at the supply end.

If your technical advice is limited to stating "some kind of safety breaker device" I suggest you should refrain from giving it. Do you have any electrical qualifications at all?

To the OP I would suggest you obtain a copy of the On Site Guide to the wiring regulations, study it and armed with a little more knowledge be in a position to ask again. I missed your original post so I can't make definitive answers.

[BigWallop]

This is a copy of the original post again, just for you.

Hi all,

Just a bit of advice to see if I am on the right track.

New external garage (not attached to the house) and I would like to wire it up for lights and a couple of applicances(fridge+spin dryer) + usual garden stuff.

The plan is to take a feed from the non-RCD side of the CU in the house to the garage using 6mm T&E at high above ground level (in conduit) with it's own MCB in the House CU.

In the garage, feed into a smaller CU with an RCD and the necessary MCB's required for the various circuits.

The supply earthing system in the house is TN-C-S (PME) so I assume I will have to put an earthing rod in garage somewhere.

Does that sound ok?

Any advice is much appreciated.

TIA

Steve

[Dave Plowman (News)]

Few would have a drum of 6mm lying around, unlike 2.5mm.

Neater than SWA? I don't think so. SWA can be paged to look as neat as your skills. Plastic conduit not. Of course I could make a decent job using steel conduit since I have a bender, but knowing your pipe working skills I doubt you could even with the correct tools. Which you wouldn't have.

TLC prices give 6mm singles at 40p per metre. Add in the cost of the conduit - and the hassle - and 6mm twin SWA at 1.50 a metre is the cost effective answer as well as being the correct way to do the job.

[Doctor Drivel]

As long as the conduit is for external use, and fitted properly so any water that happens to enter, is naturally drained out, and the MCBs are sized properly, it is sound. Personally I would use separate conductors (single wires for Live, Neutral and Earth). The more air around them the better.

[Doctor Drivel]

SWA looks crap.

[Dave Plowman (News)]

[snip dangerous and misleading advice}

[Dave Plowman (News)]

What, round plastic covered black cable looks crap compared to conduit?

Do you know what SWA is?

Stick to giving misleading advice about boilers. It's what you're best at.

[John Rumm]

Might be worth pointing out that this ought to be a switch fuse unit rather than just a switch - otherwise you have no overcurrent protection for the SWA.

Not only that, but with a PME system in the house you would have to ensure that the equipotential zone is maintained in all the garrage (i.e. all metalwork bonded etc). Hence why it is usually simpler to make the garrage a TT system.

I would suggets a slight modification here....

Do the earthing the other way arround. i.e. connect the earth to the armour at the house end, and then isolate it by using a plastic clad CU in the garrage. Otherwise you will be relying on the earth spike to to try and sink enough fault current to clear any phase (live) to earth fault on the cable. There is very little chance that the earth spike fault loop impedance will be low enough to do this, and you will end up needing a RCD at the head end of the cable.

[John Rumm]

The original proposal does not deal with the issues of extending the equipotential zone into the garage - this may or may not be easy / desired.

Don't do this as it is very poor practice, is unlikely to comply with current regulations.

[John Rumm]

T&E is a right pain to feed through conduit - more so when it is a thick cable like 6mm^2. I would be tempted to use HiTuf or SWA - probably cheaper as well.

You will need to take care here with sizing the MCBs so that you still achieve discrimination between the MCBs in the garage, and the one in the house. Otherwise a large overload on a socket circuit in the garage could trip the MCB at the head end rather than the one in the local CU and hence plunge you into darkness.

Using a HRC fuse carrier in place of the MCB at the head end would likely solve this issue.

Unless you want to get into extending the equipotential zone in the house to the garage as well this would be wise.

Mostly ;-)