Ducting for garage/shed electrical supply

[daiking]

I have a 6mm² cable direct from the consumer unit terminated in a box on the outside wall of the house (approx. 7m run in the house).

 

In the future, I'd like to have a garden building approx. 10m from the house and run this electrical circuit to it. I have no plans of running anything particularly powerful in there. Approx. 15m further along I have a small garden shed that maybe in future a light or electrical supply might be handy.

 

Although I've just missed my easiest chance to do something about it with a digger on site, I can still run a duct down the border without too much bother save a little digging. Then an electrician can run a cable through it in future. There shouldn't be any vehicles going over the duct but there might be in future. Occasional, not daily.

 

I'm sure someone here will tell me what size SWA cable I should use for the total run, please do. I won't be doing anything electrical at the moment, just looking for a guide.

 

What size twinwall duct should I be looking at for that cable? http://www.draindepot.co.uk/plastic-underground-ducting/electric-cable-ducting.html

 

And what general practice should I follow? Depth? marking tape? corner radii? pulling strings? etc.

[Nickfromwales]

Just dig a little deeper and bury the SWA directly into the ground. There's really no need for ducting, and where it may get driven over just sleeve that with an old scaffold pole. Set that into concrete later if needs be but I doubt it would ever need it as the weight can't transfer onto the cable at any one point with the scaffold pole around it ( say 10' long for eg ). You'll prob spend more ducting and fading than just buying the cable TBH   

[daiking]

1 minute ago, Nickfromwales said:

Just dig a little deeper and bury the SWA directly into the ground. There's really no need for ducting, and where it may get driven over just sleeve that with an old scaffold pole. Set that into concrete later if needs be but I doubt it would ever need it as the weight can't transfer onto the cable at any one point with the scaffold pole around it ( say 10' long for eg ). You'll prob spend more ducting and fading than just buying the cable TBH   

But I don't want/need the cable until @Onoff finishes his bathroom. Hence the duct now before things in the garden have settled and I can dig up the border with impunity.

[Nickfromwales]

Just leave it coiled up each end ? 

[ragg987]

A duct gives flexibility in case requirements change - data cables (alarm, ethernet) or electrical vehicle charging point might come into play in the future. My electrician said he would need a sub-panel in our garage for regs and we used an armoured cable from the main panel with 32A capability, plus a spare cable and 6-core alarm cable. I think we used 50/63 twinwall duct, it was buried about 500mm deep and final finish is block paving with vehicular access over. We left a rope in the duct for future pulling requirements.

[Nickfromwales]

Just based on the remit I think it may be overkill / unnecessary cost. Ducting is practical if you need to upgrade but what will be out there that a 6mm SWA can't run?

 

My shed alarm will run off a 12v battery charged by solar and be stand alone. 

[Crofter]

I swapped out the flimsy twine that came with my ducting for a length of fence wire- no chance of breaking that!

[Temp]

I'm an electronics engineer not an Electrician but...

 

I think the exact set up depends on how the house is earthed but the most likely configuration would be something like...

 

MCB in house CU -- Junction box --- SWA to garden building ---- SWA to shed

 

In each outbuildings there would be a "Garage CU" containing an RCD and two MCB (1 lighting, 1 power) and an earth rod.

 

Using the calculator at the TLC web site and an overall length of 32m..

 

2.5mm^2 is ok up to 4kW

4mm^2 is ok up to 6kW

6mm^2 is ok up to 10kW

 

Going up a cable size adds about £20 to the cost of a 50m reel.. 

 

Cable would be 2 core SWA (if the earth isn't exported from the house and earth rods are used). Would be 3 core if the earth can be exported. 

 

I put outside lights on my shed operated by a PIR. If you want a light on the shed that can be switched from the house end you might want an extra core or two?

 

I used 3/8" polyprop rope to pull ours through. If you leave enough rope at the CU end you can pull the cable through AND leave the rope in the duct for future use.

Edited July 13, 2017 by Temp

[daiking]

1 hour ago, Nickfromwales said:

Just based on the remit I think it may be overkill / unnecessary cost. Ducting is practical if you need to upgrade but what will be out there that a 6mm SWA can't run?

 

My shed alarm will run off a 12v battery charged by solar and be stand alone. 

 

God knows. If I can get plumbing to it also, it could be anything

 

It just seems wrong to put a load of cable in the ground now when I have no idea what I have planned for it.

 

where as putting a duct in just seems like sensible planning if you have no idea for the future.

[Nickfromwales]

Granted, 

 

4 hours ago, daiking said:

I have no plans of running anything particularly powerful in there.

So hence my reply . 

 

[Frogeye]

I used some left over MVHR ducting to get to my shed, I have shoved some armoured cable in already in case it collapses with light traffic, but also slightly worry about rodents using it as an expressway!

[Jeremy Harris]

1 minute ago, Frogeye said:

I used some left over MVHR ducting to get to my shed, I have shoved some armoured cable in already in case it collapses with light traffic, but also slightly worry about rodents using it as an expressway!

 

Get some metal mesh, chicken wire or similar.  Tie a bit of wire to the middle of it, then screw up the mesh into a ball and stuff it down the ends of the duct.   Leave the bit of wire accessible so you can pull the anti-rodent plug out when needed.

[Ferdinand]

I will be doing this to my new garden building (which is actually a utility for a T - and will have electric (garage cu, mini ring main with approx 3 doubles - not cut into tiles, light, single room hr ventilator), water, drainage - so I am interested.

 

Is there a reliable way to keep moisture out of the end of a duct for say a decade or two?

 

What is a good price for say 25mm blue hdpe pipe to use as a ducting? I am guesstimating:

 

1 - 25m - £1 per metre

2 - 50m  - 70-75p per metre

3 - 100 or 150 - 50-60p / metre.

 

Seems more unavailable over 150m, so presumably that is forklift territory.

 

All I know is you buy as big a thing as can be handled and will be required for the next 5 years, and have a big shed.

 

Ferdinand

 

 

Edited July 14, 2017 by Ferdinand

[ProDave]

15 hours ago, daiking said:

 

God knows. If I can get plumbing to it also, it could be anything

 

It just seems wrong to put a load of cable in the ground now when I have no idea what I have planned for it.

 

where as putting a duct in just seems like sensible planning if you have no idea for the future.

I can't see the difference. You argued you don't know exactly where it is going, that argument is more true for ducting.

 

Just bury a length of 6mm SWA cable and leave the end coiled up near where you thing it will end up.

[Onoff]

How is that 7m run "in the house", clipped direct, in insulation, around the outside etc. What does it feed at the mo at that "box on the wall" - a socket I'm guessing.

 

Is it 2-core or 3-core SWA?

 

What over current protective device is it currently fed from, an mcb maybe and what's it's rating?

 

Is there currently any RCD protection on that 7m feed? So is the "outside breaker" in a bank of others say on one side of an RCD?

 

Do you know your earth system type in the house?

 

All important to know.

 

But in the interests of doing something now:

 

So we have 7+10+15=32m. Probably going to be a bit longer in reality as you're buried & where it comes up the wall. With a little cu (s)  mounted at a reasonable height in the outbuilding (s) this could easily be 40m total. 

 

I'd go duct(s) every time. Think where in the outbuilding it'll come up. Aim for a corner.

 

You could run two ducts from that box on the wall and let the sparks terminate there so the 6mm splits to feed each building its own SWA. Ducting is cheap. The alternative is to "daisy chain" it - duct to the first cu then from that to the second outbuilding.

 

Let's assume you go down in 6mm and daisy chain. Take the minimum bending radius at x6 the cable dia. which will be about 17mm. Long sweeping duct bends are better than tight ones obviously.

 

As to depth then you say "border" a rule of thumb is "two spade depths" which is about 450mm. I'd say thats a minimum. Dig your trench, lay your duct  (600mm to the top of the duct would be super) and cover with sand or at least sieved soil. (Sand just gives a contrasting colour for the digger to see later). Belt and braces would see interlocking cable tiles over the sand layer. (A line of old roof tiles is a pretty good nod to the idea). Backfill with sieved soil, free from sharps and put yellow warning tape (pence per metre) about 6" under the "grass" so the digger picks it up before the duct!

 

SWA in blue MDPE really isn't a good idea! 

 

You shouldn't but many do, run say alarm wires or Cat5/6 in that same duct. For the mininal cost increase I'd go 50/63 duct. Cap the ends for now with a bit of scrunched up dpc and cable tie and just have them poking up a bit above ground a bit to deter the tunnel bunnies. You might even find an aerosol lid and tape works!

 

As to power, with that length and 6mm you're in the realms of having about 40A to play with over the two buildings. You could have a decent leccy heater in the main building as the main load and just light and socket set up in the little shed. LED lighting of course. Start using shredders etc and nipping inti the shed for a warm up and you'll up the draw but should all be good.

 

 

 

 

 

 

 

 

 

 

[daiking]

Exactly. I'm not SQEP to do the designing of the circuit. Although I will update with a bit of info.

 

But I should be able to run some duct down the garden. I was thinking of 'daisy chaining' 2 lengths of duct rather than 2 separate whole lengths to each spot. The first point will pop out in the border, near a fence and the second will be down the garden in the 'wild' area. If I put a base down for the shed I could bring it up through the concrete into the shed directly.

[Onoff]

Take one duct down and bring it up in the rough corner of the first building. Put the second duct alongside and run down to wherever. My very first shed base circa 1990? I wasn't silly, rope in the FiL and a couple of mates and get behind the camera! IIRC the SWA was buried under the lawn and came up thru the slab in a bit of 3" plastic pipe. You can just see it on the left: