Newbie...Garden Workshop Electrics

[W00DY]

Hi all, hope u can help.

Have recently purchased a property with a Timber Workshop approx 25 Meters from house and i wish to run electrics to it.
approx 8 sockets (probably two constant approx 1400w each) and 4x40w tubes.

My basic understanding is i need:
SWA cable (to run under path with warning tape etc)
Garage CU / RCD

Please can you tell me:
a) what size SWA cable and how this connected to both House CU and workshop CU (Glands / earths etc) ?

b) will a 40A -30mA rcd with 6A lights and 16A ring Garage CU be sufficient ? i.e http://www.screwfix.com/app/sfd/cat/pro.jsp?ts=28082&id=63138

c) What size MCB should i add to House CU


Any help would be much appreciated.

Thank you.

 

[ban-all-sheds]

First question back at you - are you aware of Part P of the Building Regulations which make this work notifiable, and therefore require you to either have it done by a qualified electrician who is also registered with a Competent Person's scheme and can self-certify compliance with the Building Regulations, or if you DIY or have it done by a non-registered electrician, require you to notify your council's Building Control dept in advance?

Anyway:

a) For that load 2.5mm² SWA will be OK, but given that most of the cost and effort is in digging the trench, something you only ever want to do the once, my advice is always to put in 10mm² cable, so that when you decide you'd like a welder or a hot-tub or a sauna out there you won't have to dig it up and start again.

You will need to gland the cable, and you won't be able to do that directly to the CUs, so you'll need a couple of adaptable boxes connected to each CU with twin & earth or with the SWA cores continued on through a length of conduit.

CW glands for outside, BW for inside, but you might as well use CW throughout.

3-core SWA, not 2, but the armour should be earthed - the glands will come with a suitable banjo.

b) That will be fine for the load you describe - this is the bit that's easy to change if needed in the future.

c) Right now 20A will be ample - this is the bit that's easy to change if needed in the future. Non-RCD side of the CU, of course.

Second question back at you - do you know if your supply is TT, TN-S or TN-C-S (PME), as this might affect the earthing you need to do.

 

[mapj1]

It all depends what might be put in the shed in the future. If you went for say a 32 MCB in the house (and some would probably make it bigger as future proofing) then you can size the SWA using the tables in 'for reference'.
It will end up being quite heavy stuff, 25m is long enough that voltage drop will be the main consideration, if you are worried about staying within the 9v or so recommened by the regs for loads of unknown voltage tolerance (some fixed loads, like showers for example will be rated 220-240V and can tolerate a greater drop, but as 'anything' might be plugged into a socket, it is as well to assume 9V drop = 4%.)
While you are digging the channel consider dropping in a telephone or data cable and a pipe for water if there is any risk at all this might be useful in the future, even if not needed now.
The CU you have selected will be fine for a lightly loaded power circuit, but if you really do have 2800Watts on full time, this uses up most of it. You probably need to change the breaker for a 32A and do a proper ring main. Don't worry too much about the fact that the lighting breaker is 6A, the four tubes will be less than an amp between them when warmed up.).

Consider if the wiring needs to be protected against damage by conduit or similar, and if it is worth using metal clad sockets - depending on the type of workshop it could be a bad place for anything delicate.

regards M.

Oh, if you didn't start (or contract yourself to start) last year, remember that under part P you need to notify building control of your intentions, and that they will charge to inspect your work.