Running Power to my shed

4 Feb 2004
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United Kingdom
Looking for a bit of advice on running power to my newly erected shed.

The shed is approximately 30m from my house consumer unit. But their power run to an outisde light (approximately 5m from the shed), but this is on 6A MCB.

I have no RCD on my house CU, but do have a spare way.

I was planning on fitting a 40A 2 way 30mA RCD CU in the shed to power 2 double sockets and a couple of lights on 16A and 6A MCB respectivley.

The question is how I run the cable to the shed, the portion of the cable that will be run outiside I plan to fit into a recess in my wall filled in with mortar (approximately 2m in length), and then tacked along the outside a wall (3m in length). The remaining cable will be run through my loft.

My questions are this:

1. Is their a way of using the existing outside light to power my shed?

2. If I was to run the cable from the house CU, what size of cable would be required for the 30m length

3. Can I use standard PVC cable for the inside portion then SWA for the outside portion.

4. Is their a problem with burying SWA cable inside a mortared recess, as Id rather the cable wasnt running down the outside of my door frame.

If anyone requires more information to help me out just ask.

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you cant use your light to power it, as that is only 6A max, depending on what else feeds it.

I wouldnt run it in mortar either as it will look a mess.. you could just run it up the side of a drain pipe which should be inconspicous..

If you cant run all the way to CU with it, you could terminate it outside, under the eaves in an adaptable box, then bring the inner cores through into the loft and then in another adaptable box, directly behind the other bring say 6mm cable to the consumer unit to a dedicated 32a Mcb.

The house should really be protected by a split load CU, and it may be worth looking into sorting this at the same time.

in the shed, you could terminate the swa direct into a metal clad CU, plastic are pretty useless for this, and using boxes etc looks messy.

Ps do you not have floorboards downstairs with access under these, rather than running up to loft, across and down again?


My current house CU is in my bedroom, where the cables exit and go directly into the loft where the are distributed accordingly. I have very old T+G flooring so lifting them is way more hassle than its worth.

I like the idea of running it through the loft directly to the wall then down the outside near a drainage pipe, much more discreet, and alot less hassle than raking out the existing mortar and re-pointing.

What are the adaptable boxes you are talking about? Id rather not runt he SWA all the way to the CU, as no need for SWA inside, and it costs more. SO I will probably run the SWA up the outside through the wall where I will temrinate it and use standard cable to run through the loft to the CU.

Should I run 6mm cable both inside and out, or will 4mm suffice. I presume I only require 3 core.

I have a victorian house with pretty thick walls, so I'll need to go looking at drill bits so I can drill a hole big enough through the wall to run the cable. It is also going to be tricky as it will be about 14m from ground level where it will exit the wall, so Ill need to to politely ask if I can borrow my neighbours extremely large step ladder.
To be honest.. lifting T&G flooring isnt a problem, if you know how to do it.. ie dont just try and pry it up..

Theres not much 'that' difference in cost between SWA and T&E.. not once you factor in T&E, clips, 2x Adaptable boxes etc... Adaptable boxes are like patress boxes with lids.. available in metal or plastic perfect for terminating into.

You wont be able to terminate swa into the CU properly unless it is metal clad really.. I have done before (used a steped cone cutter and cut a hole in the bottom / side, but you need to check on the consruction and lid fitting first..

Cable size depends on loading, cable location, length etc. But when using amoured I allways go at lest one size bigger than needed and never touch less than 4.0mm2 (also use a three core, and use the third in addition to the amouring as and earth, unless you can calculate whether the CPC is adequate, you need cable specs and breaker specs for this)

6.00mm T&E should see you right for circa 35A any more and its into 10mm which is a pain in the arse...

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Also dont forget depending on which way the joists run, you only lift 2 boards..

Make sure if running through joists you check where about you can drill (50mm iirc) - but check first.

Can anyone provide a link to a recommended adaptable box that I can terminate the 6mm SWA 3 core wire.

I will connect it to 6mm T+E which I will run to both consumer units in the house and shed.

What is the best method of joining the tails.

I gues as both termination points will be internal it will reduce the specs of the termination. Or is it best to terminate the SWA on the outside of the shed. Anyone had any pesronal experience of which is best.

I was also just thinking of having some garden lights and a pump for my pond which is a further 10m from my shed. Could I run these off the shed 16A mcb. I wouldnt expect total power consumption in the shed to exceed 3kw. And no more than maybe 1.5kw for the garden.

Does this effect the size of cable I need to run to the shed.
Take the cable into the shed and terminate in a metal CU inside. Always better to be out of the rain.

How big are these lights and pump? 1500W seems quite a lot. Floodlights?
That was just a rough guess but I have been havin a look at previous posts, and I think I will run a pump and a double socket into the garden and run low voltage lighting off one of the ext sockets.

Is it possible to have a 15m radial circuit off the shed CU to power the following, where the lights will come off the exterior dbl socket

CU--16Amcb--Dbl Sckt--Dbl Sckt--------Pond Pump---Dbl Socket

What size of cable will I need from the shed to the garden approx 14m. After looking I would say the maximum load on the circuit will be no more than 3.5kW.
go for 6mm it allows flexibility and you could then use a 32a breaker inside. (IMHO)

I also would run the pump and lighting wires to a socket inside then you can use timers etc, out of the rain.

Radial isnt a problem. use 2.5 - 4mm cable. - depending on breaker size.

Thanks for the info.

The breaker the pump and lights would run from would be 16A, so can I use 2.5mm SWA? It would be a 15m run.

You can terminate SWA into an insulated CU easy - here's how:

Make off the end of the SWA in the usual fashion, leaving enough length of tails to reach the terminals in the CU.

Attach the made off cable to a galv through box mounted so the outlet is right up against the CU wall.

Screw the galv box down. Take the tails to the relevant terminals, making sure you take a suitably sized cpc (earth) from the earth terminal and terminate it to the galv box threaded hole with a brass 4mm screw and washer. Fit a lid to finish.
Will a gland bolt into a galv. through box?

I presume you mean the metal conduit (round) one in one out?

Are you asking if you can run 2.5swa from the shed to the pond? if so It will be fine, its just a case of what you terminate into.
I dont like the idea of joining cables outside, regardless of what its in..

PS dont seal with silicone.

Yes and Yes.

BTW if you have to join outside, Pratley boxes or resin joints are excellent. See TLC.

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