Underground cable to shed; how deep?

[drofol]

Could I please have some electrical advice and opinions? In the past , prior to Part P,I have done quite a lot of wiring. I believe I am cautious, and want to comply with regulations. I am currently doing some of the ‘spadework’ (literally) to provide a large shed with a mains elctricity supply. In due course a qualified electrician will inspect, do whatever parts of the work he wishes and certify. I can’t contact him at present, but he advised me that if I ran the cable underground, it should be at least 500mm below the ground, and where digging is possible it should be 600mm. If the electrician was accessible now I would say to him that much of the underground run is below a large layer of concrete, so is there a provision to go less deep, when the cable is covered by concrete? I have broken a channel a spade’s width in the concrete, and will relace it with concrete when the job is done. The concrete is about 3 inch thick. Is what my electrician told me guidance or regulation? Must I really dig 500mm down? It is very tiring! Thanks for any help

 

[oharaf]

He is the electrician taking 'ownership' of the circuit, so if he says 500mm+ then he will probably want 500mm+.

You need to speak to him and him alone.

Either you dig the trench to his spec and save his labour or let him dig the trench and pay him for it!!!!

FWIW, I always insist on 600mm

 

[Chri5]

http://www.eon-uk.com/downloads/m7149_CABLE_DUCTS.pdf


450mm for EON

I've done projects requiring 1000's of m of duct. Across roads, round campus sites etc.

You can easily understand the need for rules in those enviroments, but the rule of 500mm in a back garden run on uncultivated soil is a bit daft- but it is a rule.

There's a very fair arguement that 300mm down with capping stones on top for added protection (in addition to a sand under and around the pipe / cable) is more than enough.

But rules is rules.

 

[BS3036]

But rules is rules.

The cable in question is covered by BS7671 though. The rule is 522.8.10.

'...shall be at a sufficient depth to avoid being damaged by any reasonably forseeable disturbance of the ground..."

I remember that, because it appears in nearly every C&G Wiring Regs paper. Having said that, the electrician responsible gets the final say here, because he is signing for it.

 

[ebee]

If you dig 600mm then add a little soft sand, then lay the cable, then add a little more soft sand then your cable is now completely enclosed in soft sand, the fill with dirt to 450mm below the surface then lay warning tape/electric warning bricks then fill to top.
Your cable is now at least 500mm below ground with a warning before you hit it.

Would you be happy with anything less?

 

[ban-all-sheds]

Also, I'm not convinced by the argument that if it's under concrete it can be shallower, because it's not out of the question that somebody might remove the concrete in the future with a breaker and a digger...

 

[drofol]

Thanks everybody for the answers. They make sense to me, and at least I will know that I am doing the right thing as I toil away!