Power to outbuilding

29 Dec 2003
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United Kingdom
Can anyone tell me what MCB I should be using for this set-up.

The objective is to bring power to a shed.

Starting at the house end, I have a spare MCB slot on the RCD side of my split load CU.

I have an 8 meter run of 2.5 T&E to a switched FCU. From the FCU the 2.5 SWA exits the house and is buried under a pathway for 15 Meters. On entry to the shed the cable runs to to 2 Double sockets in series. Before the first double socket a 2.5 spur runs to a 5 Amp switched FCU. From the 5 Amp switched FCU a 1.5 cable runs to a light.

It is not a big shed so only standard hand held power tools would be used there (no industrial lathes or anything).

Question is, what sized MCB should be at the CU and what sized fuses should be in the FCUs (I know it should be 5 Amp for the light spur).

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personally i would replace the fcu at the house end with a 20A dp switch and use a 20A breaker at the CU

also you should really have a double pole isolator in the shed
There is going to be a double pole FCU in the house where the cable exits. There is an 8 meter 2.5 T&E run from the CU to the FCU. The SWA will cone out of the FCU and then exit the house.

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have you already put the cable in?

if not then i would seriously advise you to wire for much higher current
you may not need it now but you only want to dig the trench once

done use a fcu at the house and that is just pointlessly limiting your capacity if you must have switching use a double pole switch first
plugwash said:
have you already put the cable in?

if not then i would seriously advise you to wire for much higher current
you may not need it now but you only want to dig the trench once

Yo - that total cable run is approaching the volt-drop limit at 20A
No. The only cable in place at the moment is the 8 Meter 2.5 inside the house. This can easily be replaced.

Do you suggest 4mm rather than 2.5mm.
if i were doing a garage install now after putting in stupidly small setups in the past i would probablly go for something that is overkill for your present requirements but has extra capcity for use later

i would start from a 40A mcb in the non rcd side of the house CU
from here i would run to a metalclad din rail box with a 40A 30ma rcd using 6mm twin and earth
from here i would run to a small metalclad switched CU in the shed
and from here i would run a normal 32A ring and 6A lighting cuircuit
why not go the full hog and get 16mm2 of SWA if you can carry it.

I think it is always best to plan for more things being added as you proabaly will sooner or later add more things onto a circuit. You never know you may want a combi boiler on it in a few years (don't ask me for what though)
Do you mean 16mm2 SWA running both outdoor and the 8 meters in the house up to the CU. I ask because within 2 meters of leaving the CU the cable will be running through a small apature in a wall under the floor boards. I already have a 6mm2 cooker cable and a 2.5 kitchen socket ring running through that small gap.

There is probably a technical name for it, but isn't there an issue about running cables right next to each other - cable warms up, melts plastic, safety and fire thing just to put in extreme terms.

Whilst I have done a few DIY electrical jobs where I have felt completely confident, I am beginning to feel that this is one I should a professional in on.

So is there an issue running these three cable through a small hole.
16mm is way bigger than you are ever likely to need

the plan i gave would easilly support anything you could practically put out there except possiblly a big cooker ;)
16mm is OTT, unless you think you might one day need a 63A sub-main.

But I would go for 10mm - remember, you want to reduce VD on the SWA run as much as you can, to leave as much allowance as possible spare for the parts of the circuit where you can't easily use bigger cables...

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