Choosing cable size. Formula?

[Mike Barnard]

Hi.

I want to run an armoured cable from the house to my new shed. It's a total of 15m distance and it will power:

1 x 4' daylight tube 36w 2 x 150w pir floods 300w 1 x small freezer 60w? 4 x wall sockets (occasional battery charger, power tools etc)1000w?

Say a potential max load of approx 1500w.

w/v = a?

1500 / 240 = 6.25a.

Local supplier wants to sell me 4.0mm cable. Seems a bit OTT to me for this. Surely 2.5 would be fine? Opinions?

Also, would you use a consumer unit in the shed to split the lighting and sockets or be happy with a junctionbox? There will be a dedicated fuse at the house end after all.

Thanks.

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[Bob Minchin]

[Alex]

But... if the fuse to the lights blows, you'd be operating said machinery in the dark at full speed.

Alex

[Andy Wade]

x 1.8 if not power factor corrected, so say 65 VA.

300 VA, near enough, so 365 VA for lighting, say 2 amps, rounding up.

Optimistic, 150 W is more typical. Allow one amp (230 VA).

Several 13 A sockets on a radial circuit? The smallest I'd allow for that is 16 amps. You might want to plug a fan heater in in the winter, for example.

Min. acceptable design current ~19 amps - call it 20 and use a 20 A fuse or MCB in the house.

Voltage drop for 2.5 mm^2 at 70 deg. is 18 mV/A/m, i.e. 5.4 V (2.3%) for

20 A & 15 m - that seems OK and gives you 3.8 V to play with for the shed wiring. Current rating won't be a problem - 2.5 XLPE SWA to BS 5467 derated for 70 deg. conductor temp is 28 A clipped direct (more when buried).

So yes, 2.5 would be OK, provided your length figure is OK (cables lengths often end up being more than you expect) and it's fed directly from the house consumer unit.

4 mm^2 cable costs very little more and will give you some capacity in reserve for the future.

The simplest satisfactory method is to take the SWA cable straight to a metalclad box (could be one of the socket boxes), and use one or more switched fused connection units as lightswitches. Fuse your lights at 5 amps, either all on one circuit, or one for the fluorescent and another for the floods. Sockets are protected by the fuse/MCB at the house. This gives no fault discrimination, so the light goes out if there's a fault - not recommended if you intend to use serious machine tools or woodworking machinery.

Supply arrangements at the house end? Earthing arrangements? RCD protection of the sockets is essential, where is that going?

[Andy Wade]

Far more likely is that the socket circuit trips for some reason, and takes supply for lights with it, so you are in the dark as machinery is slowing down. The design will get more elaborate to achieve full discrimination. I got the impression this a just a small shed, not a serious workshop.

[Owain]

Stick in 4mm. The cost of the cable isn't much compared to the work of doing it.

It'll be much more comfortable in the shed with a fan heater, coffee maker, and a few other little luxuries :-)

Owain

[Dave Plowman (News)]

If 15 metres is the total run you're looking at 16 quid for 2.5mm 2 core,

21 for 4mm at TLC prices including VAT. The larger size can do no harm. ;-)

[David Hansen]

On Thu, 30 Nov 2006 16:58:37 +0000 someone who may be Mike Barnard wrote this:-

A main switch to switch off the installation in the shed is advisable and can also be used to terminate the incoming cable. A small consumer unit does this neatly and then all there will be are light switches and sockets.

[Mike Barnard]

Thanks to all. I'll use 4.0mm and an rcd consumer unit as reccomended.

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