Wiring to outside workshop - best way?

[Duncan Di Saudelli]

Hello All

25m from our house is to be built an 8' x 10' wooden workshop. I have a spare slot in our consumer unit in the garage which will take a 32A CB - I shall then feed out through the rear wall of the garage, immediately down the outside wall into some underground conduit containing three-core twin+earth 2.5mm sq SWA, thence up the garden (buried down below the soil in some additional piping) to the workshop.

Inside the workshop will be a small two circuit RCD/CU feeding lighting and power (a couple of strip lights, and a pillar drill is about it for typical loads, with the occasional use of other things i.e. circular saw etc.).

Note that I've used the voltage drop calculator at

and realise that 2.5mm sq is larger than the

1.5mm sq needed but I feel that I'd prefer some headroom for the occasional larger load, plus minimum V drop along the cable anway.

The questions are:

1) Do I mount a BS4343 circular 32A wall mounting connector on the outside brick wall at the back of the garage, then run the SWA from a mating connector and then do the same thing (but opposite sex etc. ) at the workshop end , thus making two mating pairs of BS4343 at each end?

or

2) Do I hard wire it at both ends?

I prefer option 2 because it is cheaper and less conspicuous.

I'd appreciate your thoughts and guidance - please let me know what you would do. I might get a local electrician in to do the work too, being unfamiliar as I am with part P and outdoors and 1th ed. and so on.

DDS

[Dave Osborne]

Read:

don't need to use BS4343 connectors as your workshop is not a caravan ;-)

HTH

[Andy Burns]

I'm surprised you think 1.5mm would cope at that distance, I don't know what design current you went for, but more than a 2kW load would need

2.5mm^2 anyway to meet the tighter voltage drop on a lighting circuit.

I'd be tempted to fit 4mm^2 while you're are it, that could cater for a

6A lighting and 16A radial power circuit in the workshop in case you expand your tool arsenal, and keep you under the 3%.

[robgraham]

Be careful about RCD'ing both power and lighting - the power circuit is far more likely to develop faults and take out the rcd - result is no lights either.

I ended up buying a second hand CU off Ebay so that I could do a split system.

As for cable size, the concern is the I x V drop heating power in the cable - there aren't many bits of equipment that are sensitive to voltage drop; they will all work quite happily down to 220v or lower. As the major current demand (and hence voltage drop) is when induction motors start you need to think if that is going to happen. My system is rather marginally cabled but I run a 2.2kw table saw motor off it without any problems - but that will be only load at a time apart from perhaps the heater in the winter.

Rob

[Tim Watts]

A bit of lateral thinking...

I'm planning to take a power feed (20A or 32A) to the shed, RCBO'd at the house CU.

The shed lighting however, will run off a separate feed from the house in lighter SWA also on its own RCBO.

3 reasons I like this:

a) Your point

b) Volt drops - OK, not much of an issue, but will reduce light flickering in the shed when I get run up something large

c) I have other things in the vincinity that require lighting - another garden tool shed, maybe even garden lighting. There's a certain logic to lumping these all together. If those little remote radio switches get cheaper, I could almost run the lighting power all over the place in SWA, using underground joints if necessary to add on - which is a lot easier than messing around taking another cable out of the house.

[John Rumm]

No such thing as twin and earth SWA. You can get three core - but each core is equal in size and fully insulated.

For this application you probably need two core - the earth is exported using the armour of the cable.

I would concur that 2.5mm sounds too light. 4mm is only 30p a metre more.

Decide if you are ok with losing the lights at the same time as the power.

Have a look at:

The questions are:

Yup

A exterior junction box on the outside of the wall is often the easiest way to terminate the SWA.

I prefer option 2 because it is cheaper and less conspicuous.

Indeed. Can't see any need to unplug a shed really!

[Chris Oates]

Undergound you will need min 16mm sep earth.

[Tim Watts]

Can you point to reg for this?

[John Rumm]

I can't think why necessarily. If one were exporting a PME earth and main equipotential zone and hence needs 10mm equivalent CSA for a bonding conductor, then possibly. However a larger SWA using a core and the shield in parallel would be preferable.

If one were using a separate conductor without mechanical protection, then there is a minimum size. This applies to things like connections to earth spikes etc.

However that is not of concern when dealing with armoured cable.

[Andy Wade]

Only if there are any incoming extraneous-conductive-parts in the outbuilding that would require main bonding - e.g. a metal water pipe. If there is none such then the armour only needs to satisfy the requirements for a CPC for the distribution circuit.

Or TT-earth the outbuilding. A safer approach all-round, IMHO, if properly executed.

16 mm^2 if protected against corrosion, 25 mm^2 if bare.

[John Rumm]

and in this case 8 x 10 wooden workshop (aka "shed" by the sounds of it) is probably unlikely to have easy access to a local earth (assuming its got a wood floor).

Indeed, what I usually do out of preference.

It would help if the OP could tell us what earthing the house has.

> If one were using a separate conductor without mechanical protection,

and 50mm^2 if using steel wire IIRC...

[Duncan Di Saudelli]

itself to trying to ascertain whether or not it was recommended to wire via connectors as mentioned. Having said that, it's been a really useful thread so thanks!

DDS