Garage / workshop electrics

[JimM]

Hi all

I could do with some advice on the electrics in my garage. At present the garage is on a radial circuit with armoured cable running from a

20A mcb in the house to a single socket and a light at the garage end (cable run is approx 10m). As I want to use the garage as a hobby workshop I'd like to add half a dozen double sockets and a few more lights - equipment will include a lathe, milling machine, welder etc but there will never be more than one thing running at one time.

Based on the above what would be the advantages to adding a further CU in the garage & how would you suggest I split the circuits - I was thinking radial circuit for the lights, ring for the power sockets, separate radial for the welder (this needs a 16A socket) and another circuit for the door opener (garage only has entry via the up and over door so don't want this tripping accidentally)

Thanks

Jim

[David Hansen]

On Tue, 23 Mar 2010 01:13:59 -0700 (PDT) someone who may be JimM wrote this:-

Presumably the MCB is correctly sized, so cannot be increased.

Given that the incoming supply is protected by a 20A MCB anything more than a 20A radial for sockets is no use.

Do you intend to plug the lathe and milling machine into sockets?

You have a big problem then. The welder circuit and sockets (even if fused at 16A) will not discriminate with the 20A MCB feeding the garage. A fault on either circuit is likely to cause the whole garage to be disconnected.

[Roger Mills]

What size is the armoured cable? Will it stand a higher rated MCB at the house end? If not, replacing the MCB with a wire fuse may make it less likely that you'll lose the lot in the event of a fault in the garage.

You could really do with a personnel door into the garage, which doesn't rely on the electrics. Is this feasible?

Presumably, if you do lose the electrics while *inside* the garage, you can still open the door manually - apart from not being able to see what you're doing?!

[John Rumm]

OK, first things to look at: What size SWA (i.e. armoured cable), and also what size cable feeding the SWA if it starts out using T&E for example?

Is the MCB in the house RCD protected?

There are a few of things to look for here...

Can you get some more capacity to the head end? How you are going to provide RCD protection for the sockets? How you are going to protect the lighting and garage door opener.

The full range of options are covered in more detail here:

ideal situation may be say 30A fuse protected submain with no RCD at the head end. Local Split load CU with RCD for sockets etc, and non RCD protection for the door opener and the lights.

Alternative options may be a UPS to protect the door opener and lights, and/or some non maintained emergency lights to cope with loss of power.

[deano]

Hi Jim.

I did a similar thing, but at the end of my garden... built a workshop

300ft away from the house. I had to put very large diameter armoured in the ground, to cope with the voltage drop.

At the house end, what you should do is add a couple of meter tails, from the meter, using a Henley block if necessary. Run these into a new box with a 100A RCD to protect the armoured to the garage (check the armoured can take this). At the garage end, install a new distribution board with enough expansion to install the circuits you need. You might want to use RCBOs at the garage end to stop any trips taking out the whole shebang and you definitely need to look at another means of escape from the garage, should the power to the electric door fail.

Obviously this kind of work requires the use of a competent person, so get speaking to a couple of sparcs (better if they're mates). Have you also thought about dust extraction? You might need extra power for that. need to be careful how you set this up as workshops come with all sorts of dangers.

HTH

deano.

[george [dicegeorge]]

YES.

[Andy Dingley]

C time curve for all MCBs involved in feeding the welder (and potentially a compressor). They don't take as much current as you might think, but they do take it with one hell of an inrush spike.

Do sensible diversity calculations for the real load in the garage, then rate the supply to match. If that involves uprating the cable itself, then you're going to have to do it (but chances are you won't, as diversity with only a single person working in there can be fairly generous).

My worst case isn't welding, it's plasma cutting. The plasma cutter takes a moderate current, then the compressor kicks in too and bang goes the fuse to the workshop feed...

[David Hansen]

On Tue, 23 Mar 2010 03:36:24 -0700 (PDT) someone who may be deano wrote this:-

I have never come across one of these:-) It wouldn't be particularly effective. I assume you meant a 100A MCB.

[dennis]

I would think twice about that.. around here they are swapping all the main fuses for 80A when you get your new meter.

[js.b1]

Upgrade the SWA as necessary.

- 10mm 6943X (3 core SWA) for a 10m length and workshop.

- Run the SWA right to each CU, terminating into adaptable box

- Piranha earthing nut if box is plastic (TLC Direct)

- 45-50A MCB in CU

Replace the garage CU

- Dual RCD system

- RCD-1 Lights & Garage Opener, Non-Maintained EmLight

- RCD-2 Sockets & fixed appliances

RCD-1 has MCB 6A for Lights & EmLight, 6A for Garage Opener. RCD-2 has MCB 20A for 1-2-3 etc dedicated 16A sockets, 20A radial or

32A ring final for sockets.

The only other alternative is another run of SWA if the existing is concreted in and another CU way used.

An issue is that the garage may be damp, have incoming services etc. In which case you would not export a PME/TN-C-S house earth from the house, you would make the garage a TT install with earth rod. That is achieved by exporting the house earth ONLY as far as the end SWA adaptable box which would be plastic with the SWA cable terminated by a plastic SWA gland (CCG off Ebay or Sitebox) or plastic IP68 gland. Then the garage earth is provided by a local 2x 4ft Furse earth rod driven into the ground, earth rod pit cover & 6491X run to the garage earth busbar, EFLI tested to be

[John Rumm]

Can't see that adding much functionality what with it being in circuit with the 60 or 100A main supply cutout...

100A may be over egging a workshop supply little as well given the stated requirements!

[David Hansen]

On Tue, 23 Mar 2010 12:26:31 +0000 someone who may be John Rumm wrote this:-

True.

It would certainly make for a larger than average cable being run to the garage. However, it is always worth planning ahead.

[Dave H.]

"js.b1" wrote A Very Comprehensive Reply... which prompts me (in the design and planning stage of a New Big Shed) to ask:

I believe that if the OP uses XLPE insulated 2-core or 3-core 10mm SWA he can go up to a 63A MCB at a push - assuming he needs that much current?

I suspect I may - 10HP lathe with a VFD may be in the plan... plus a Serious Welder (up to 45A on 230 volts), both as fixed installations with lockable isolating switches, not plugged in! I like bright halogen lights, too...

Is it necessary to have a separate RCD for the lighting? Not completely up to date with the regs myself, but if he has emergency lighting would there need to be a maintained supply in the event that a fault disconnected the primary lighting ?

Again, not familiar with the current regs, is a belt-and-braces install permitted with a local earth supplementing the TN-C-S earth, or does that cause issues with earth currents in the SWA? I was anticipating bonding the local earth rods to the local CU and hence the SWA - happy to make it off unterminated at the workshop end, though!

Cheers, Dave H.

[js.b1]

#1 - Cable Current Carrying Capacity being limited (derated) to 70oC PVC BS6004 unless wiring accessories can handle the higher temperature of 90oC XLPE BS7211. #2 - CU support for 63A MCB - some ranges were limited to 50A.

Wiring must be RCD protected unless... #1 - Cable compliant with 526-06-06 eg, SWA, MICC, BS8436 or buried at

#2 - Cable is surface run

So you could surface run the lighting cable and avoid RCD protection.

No, maintained refers to light output - not the supply.

Maintained EmLight =3D Fire Exit Sign.

- Light is always "on" and is maintained "on" during power loss for

3hrs.

Non-Maintained EmLight =3D Emergency Lighting.

- Light is always "off", but is turned on during power loss for 3hrs.

#1 - DNO would not like it #2 - DNO network loses its earth your rod just became its replacement :-)

[John Rumm]

More to the point, maintained lighting can be used as ordinary switched lighting in addition to its backup purpose, non maintained can't.

[John Rumm]

You could with 2 core PVC sheathed as well (or more to the point using XLPE sheathed SWA but within the 70 deg C temperature budget of the accessories that you are likely to terminate it in)

Table here:

> Replace the garage CU

The RCD requirement comes from the 17th edition regs on cable protection. If the cable is otherwise protected, or surface run for example then it sidesteps the need for RCD protection.

The RCD requirement is motivated by shock protection (via contact with a damaged cable - say when fixing things to the wall etc). Its separate (and in some ways obstructive to) the desire to keep the lights on in the event of another fault.

Non split CUs and individual RCBOs for each circuit are often worthwhile in these situations.

You can have a local earth rod connected with PME - it just becomes another of the multiple earths. However I would be reluctant to export a PME (TN-C-S) earth to an outbuilding anyway, since you would also need to ensure the equipotential zone that goes with it is also exported and maintained. Depending on the nature of the outbuilding this can be difficult.

More info here:

cause issues with earth currents in the SWA? I was anticipating bonding the

If you make the workshop a TT install in its own right then the supply earth need only protect the SWA. You could terminate at a plastic box for example and use the local earth from there on.

[ARWadsworth]

I would suggest that by the time a garage/workshop needs a 100A supply then it is time to look at 3 phase.

Adam

[js.b1]

Most crop growers do :-))

[ARWadsworth]

True, however when I rewired my parents garage some years ago I used a 4mm

32A radial for the sockets.

When we got around to replacing the supply cable to the garage (exactly the same time as the new driveway was laid) a couple of years later the circuit became a true 32A circuit.

Adam

[JimM]

Hi all

Thanks for all the replies they're very informative. Just got a bit more info to clarify things

1. The 20A MCB is on the RCD side of a split load CU, but I could switch it to the non-RCD side if needed
2. The SWA is 15mm in diameter if that's any help. If this is of a size that would allow for a higher rated MCB, are all MCBs the same or would I need to get one of the same make?
3. This is strictly for hobby use, lathe and milling machine use less than

1hp motors (albeit 3 phase running through inverters) I think the biggest current draw would actually be the 3hp compressor on start up

1. Ignore my comment about the door opener. It wasn't that I was worried about getting trapped inside more that I didn't want to be locked out, but I can get an 'emergency release' for the opener so will be fitting that.
2. I'll be surface running all the wiring in the garage cum workshop
3. Garage is dry so presumably no need for separate earth

Any further comments or suggestions appreciated

Cheers

Jim