there's a garage at the bottom of the garden...

[Deeman]

Hi guys,

I'm new to the group, but very impressed with all your advice...and quick wit...

For my first enquiry, I would appreciate your advice in regards to a hardware spec. for electrifying my garage.

I'm after about 32A at the garage (6A lighting, 6A extra lighting/spare, and 20A radial power - welder, etc).

The distance from the domestic mains supply is 150ft / 50m, and there's already an (RDL) 100A 2x5 way junction box after the mains meter.

This junction box currently serves 32A to main household consumer unit, and 32A switch to a loft conversion.

...So I'm thinking there's just about enough room to fit a 63 Amp 300mA RCD 2 Pole in it's own consumer unit, which would then feed a 50m length of 16mm 3-core SWA to the garage. (I'll use the earth on the core supply cable).

I have every intention of securing the SWA over 3.2m high on the exterior house wall, and then dropping it to waist height, and running it through black plastic elec duct pipe with a wrap around warning tape - this will be attached to concrete fence posts. but hidden inbetween exisitng fence panels.

At the garage end, I'm thinking of a 63 Amp 100mA RCD 2 Pole running to an insulated consumer unit, with separate trip switches for each of the stipulted circuits.

I'd like to run a spur from the garage consumer unit to a plug about 20m's from the garage - to keep it separate from the house.

This pretty lot is going to cost in region of £400-450 from tlc-direct.co.uk - is this the cheapest? and have I missed anything? have I set myself up for ridicule?

Needless to say, all of this will be Part B checked, tested, and certificated - albeit not by me ! ..I'm just footing the bill, and laying the SWA cable.

cheers

Deeman

 

[rebuke]

Firstly, you can use diversity on your usage - you say you want 32A, but are you really going to use all 12A for lighting (hint: unless your garage is enormous, or you are floodlighting your garden you won't).

Are you sure your household consumer unit only uses 32A, that seems unlikely - normally you'd not put any fusing between the meter and the main CU...

Depending on your supply type (TN vs TT), and whether the garage has any conductive parts, as this has an effect on whether you can export the earth or not, you probably don't need the RCD on the SWA itself. If it were me, I'd put an output from the henley block (name for the junction box after your meter) into a mini CU, with e.g. a 32A MCB in, run 6mm SWA (assuming voltage drop is OK, I haven't done the calculation) to the garage, and then put a CU in there with a 30mA RCD main switch.

Your plan for two RCDs doesn't take discrimation in to account, if you get a big enough fault, it will trip both RCDs, and also, all general purpose socket outlets must now be protected by a 30mA RCD. If you do need to RCD protect the cable, then put a 100mA time delayed on at the house end, and a 30mA at the garage end.

Also note, that it's Part P, not Part B that covers electrical work. If you are getting a spark in to do this, then I would suggest getting him/her to spec the cable size etc, as otherwise he/she may not be prepared to sign off on it. Also, he/she might have preferred manufacturers for CU etc...

 

[Taylortwocities]

20A radial power - welder, etc

I'd have thought that a welder might take a tad more than that??

 

[Deeman]

Thanks guys.

Just two points to clarify, I've just checked again, and you're right, there is no fuse between the meter and the main CU - just the henley block.

I seem to remember being told 6mm was too small, 10mm about right (but with no margin), and goldilocks favourite was 16mm. Would this not dictate a 62A MCB in the house mini CU? (I don't speak from experience, so feel free to comment).

So, assuming I did use the 16mm SWA (not that I want to throw money away), to follow the progression of hardware from the meter to the garage:

Meter
henley block
Mini CU (with 62A MCB and 100mA time delayed RCD)
50m of 16mm 3 core SWA
Garage CU with 30mA RCD main switch

Ta in advance !

 

[ban-all-sheds]

run 6mm SWA (assuming voltage drop is OK, I haven't done the calculation)

But Deeman has...

50m length of 16mm 3-core SWA

 

[^neo^]

If you're welding that distance from the supply, i would strongly advise putting a separate earth rod outside the garage.
Know next to sod all about electrics, but very experienced as a welder!

 

[Deeman]

Sounds like a good idea.

I assume the earth they're a big copper rod slammed in the earth?
[/url]http://upload.wikimedia.org/wikipedia/commons/7/7d/HomeEarthRodAustralia1.jpg

..soudns easy enough, although I've never installed one before.

Would this change the spec of hardware as stated?


...I haven't bought the 16mm yet, but am awaiting confirmation from my Part P (!) qualified associate - or if anyone can confirm? please?

 

[rebuke]

I don't know enough about welders to know what requirements they have. For the circuits you stated (2 x 6A lighting, 1 x 20A radial), at 50m, then 10mm would be needed (not taking diversity into account).

16mm can carry 94A at 50m, so that would never need upgrading, but will cost you roughly another £40, and be tougher to install...

However, from what people have said about the welder's power requirements, I'd now say put in a mini CU with a 63A MCB in (to provide overcurrent protection) in the house, run that out to the garage in 16mm, to a CU with a 30mA RCD main switch. Then probably put in a 32A ring, rather than a 20A radial (people who know - will that be enough for the welder?).

You could put a time delayed RCD in to protect the cable (note you need overcurrent protection as well if the RCD / cable is rated at less than your cut-out (which will probably be 100A, and given RCDs are normally 63 or 80A, you will therefore need it), but given it's SWA, an RCD is not essential for the cable (even if buried in a wall anywhere), as far as i know (note I'm not a qualified spark etc).

The final thing to say, is I believe (haven't got regs in front of me to check), that you should have one point of isolation for everything, so you may need to get your spark to fit an isolator between the meter and your henley block (otherwise someone might get confused when they turn off the main switch in the CU and the garage still has power etc)...

 

[^neo^]

I used a 1" diameter bit of stainless about 4ft long.
We have some rather big TIG sets here that seem to run better with an earth rod close to the machine.
I'm sure one of the clued up electricians will fill you in with more detail

 

[flameport]

You probably don't need 3 core SWA - 2 core is very likely to be suitable.

 

[Deeman]

Guys

Your advice is much appreciated.

FLameport - I was starting to think the same about 2 core, but I'm going to play it safe and go for 3 core 16mm AND have an earthing rod at the garage.

I think I'll go for MCB and RCDs and both ends. Total overkill. (The garage is all wood, and I can't afford to build it again - although saying that, these parts are going to cost enough).

Just need a cheap source of quality parts - anyone just see than hen chewing a piece of steak?

 

[ban-all-sheds]

these parts are going to cost enough

Maybe you should reconsider fitting a very expensive RCD at the source end of the armoured cable...