Mike Barnard wrote:

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

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

2 x 150w pir floods 300w

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

1 x small freezer 60w?

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

4 x wall sockets (occasional battery charger, power tools etc)1000w?

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.

Say a potential max load of approx 1500w.
1500 / 240 = 6.25a.

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

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

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.

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.

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