Outbuilding electrics

[Peter Watson]

I've read the FAQ and I think I'm alright but...

I have a garage fed from the house consumer unit a 32A MCB and a 100mA RCD and 10m of 2 core 2.5mm SWA. The house earth is TN-S and the armour of the SWA is used for the earth in the garage. There are no services in the garage.

At the garage end this goes into a split load CU with 16A and 6A MCBs on both the (30mA) RCD and non-RCD side.

The non-RCD circuits feed a freezer and lights - The RCD circuits feed two double sockets , a socket for a tumble drier and 240W of external external lighting.

I now want to put in a 3.5m x 2.5m log cabin approx 2m behind the garage to use as a garden office. The loading in here will be lights, a couple of PCs and monitors and upto 2kW of heating.

Part P aside, is it OK to run a 16A radial from the existing garage circuit using 2.5mm T&E round the inside of the garage and extend out to the cabin with either 2.5mm SWA or 2.5mm Arctic flex clipped to the fence posts?

Thoughts?

Thanks,

Peter

[ARWadsworth]

The 100mA RCD at the house end seems to serve no purpose.

Personally I would not clip to the fence posts, however a 16A or 20A radial would be fine for your suggested load.

[ARWadsworth]

The 100mA RCD at the house end seems to serve no purpose.

Personally I would not clip to the fence posts, however a 16A or 20A radial circuit would be fine for your suggested load.

[John Rumm]

What is the 100mA RCD for?

Unconnected with your question, but based on my experience with a similar sized insulated timber building, you will probably need less heating than that most of the time.

Yup that should be ok. Clipping to fence posts is not "ideal" - but that rather depends a little on the permanence of the fence.

Given your intended usage, I would probably take a 20A radial from the non RCD side of the CU in the garage. Run it on the surface or in visible trunking and then it does not need RCD protection. Then transition to SWA for the outside bit (using weather proof boxes or outside sockets). and slap a dinky split load CU in the office.

[Peter Watson]

A mis-guided attempt to offer some protection for the SWA run - Easy to rmeove!

That's good to hear - The cabin will have insulated floor, roof and walls so I'm hoping it won't need much. 2kW was a guess - i haven't calculated the requirement...

There's a short run of a couple of low concrete posts that I can use (The sort that were used for three wire dividing fences)

Other potential options for the external run? Hi-tuf perhaps, or T&E in flexible conduit?

Thanks for your help...

Peter

[John Rumm]

You would only need that if you had TT at the head end or it was a very long high power circuit and you could not meet the required disconnection times using the TN-S earth alone. So you are getting no real benefit, and do risk a trip on a downstream socket circuit taking this RCD out as well (I am assuming its not a time delayed one here)

I found a wall mounted fan heater on a thermostat - running on its 800W setting was fine for keeping it above the dew point. A quick twiddle of the stat would get it to comfortable within 15 mins. Also in your case you will have the PCs etc contributing a couple of hundred watts into the bargain.

If using the office year round, then a heat pump aircon unit might be a good idea, to keep it nice in the summer.

Hi-Tuff above ground and clipped is ok. Would not bother with T&E in conduit. To be fair SWA is pretty cheap anyway, and most options require glands of some form to terminate.

[Andy Wade]

Well, definitely not arctic flex! Hi-tuf seems to be disappearing from the market(?) so that leaves SWA, NYY-J, T&E in rigid PVC conduit, or pyro. I'd avoid flexible conduit outdoors - or at least check the manufacturer's data to make sure it's suitable.

Don't forget that an outdoor conduit system needs to have drainage hole(s) at the lowest point(s) to avoid trapping water.

[The Other Mike]

What's the problem with arctic?

[Andy Wade]

There's arctic and 'arctic'. Yellow arctic, made to BS 7919 Table 44 is OK for outdoor use, but only on 110 V (55-0-55 V) systems. This can be handled down to -25 deg. C.

The blue 'arctic' stuff is made to BS 6500 and is OK for 230 V, but should not be handled below +5 deg. C. However it's only an 'ordinary duty' cable and for the application here I'd recommend at least a heavy duty (H07) cable. Also for a permanent installation there's a general preference for non-flexible cables so Hi-Tuf or SWA would be much better choices, IMO.

Chapter and verse on Arctic in this /Wiring Matters/ article: