Electric to shed. What capacity of the circuit?

Speak to an electrician?

Might be prudent to get something reasonable for now, but see if you can get larger cables for future upgrades. Probably only a slight extra cost.

(i've got 16a 3 phase in mine, so no further upgrades not really possible without getting my mains supply changed...)

16a breaker for mine, but I do have a Kiln and a tumble dryer in mine
I have 16mm armoured cable going out to mine, I just went for the biggest size that was still manageable to fit

32a with proper armoured cable, fitted by a proper electrican and signed off

Posted by: Simon Griffiths

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fitted by a proper electrican and signed off

Mine's 16A, which I regret on the odd occasion that I need to wind the welder up a bit.

Lights, sockets and smaller welding jobs are fine

Depends what you need/want to run in there?

Any heating at all then I'd suggest 32A (or at least 20A or 25A depending on what breakers are available for your CU) otherwise it'll become a question of heating or .....

32a with proper armoured cable, fitted by a proper electrican and signed off

Would be the correct advice.  

However after getting quotes, and taking some smelling salts to recover. I decided to run a couple of oversized extension leads in some conduit.  

It obviously doesn't comply with part P because part P says you can't permanently use an extension lead.  But anyone who want's to argue has to post a photo of the rat's nest behind their TV and / or Hi-fi to prove they're not also permanently using an extension lead  

the previous owner of my house was not very diligent when it came to wiring up the garage as he buried some extension cable a few inches under the ground which I ended up putting a spade through

The length of run has an impact on circuit size, particularly if you want to use disturbing loads like welders and motors (compressor for example).  

10A socket for tools etc. 20A for the leccy car. Both with separate 30 milliamp differential breakers. both with 2.5mm2 cables. If you follow nwgiles and thisisnotaspoon's advice above of 32A you'll need 6mm2.

10A socket for tools etc. 20A for the leccy car. Both with separate 30 milliamp differential breakers. both with 2.5mm2 cables. If you follow nwgiles and thisisnotaspoon's advice above of 32A you'll need 6mm2.

I was assuming a 32A RCBO at the CU in the house, then IIRC you need another main switch and 5/10/20A MCB's in the shed for lighting/workbench/big tools or heating  circuits as required. 

n.b. the sum of the individual MCB's can be higher than the RCBO, so you can have 3x 20A circuits for heaters, compressors, welders, car charging, etc, it'll just trip the RCBO if you go over the allowable current for the cable to the shed rather than the MCB's that trip to protect each individual circuit.

We've just had an electrician around, who said the 12 metre buried 4mm2 SWA we have out to the garage can only handle 20A. From what I've read, it can handle a lot more (42a) and a 32A RCBO would be fine, since SWA has better heat dissiptation that normal 4mm2 which I think the electrician is getting muddled up with. Anyone know?  Honestly think I'd be better/cheaper to just get trained up myself after seeing the quotes.. 

You need a new electrician. 37 amps capacity in a duct underground for 3 core 70 degree 4mm SWA cable. You disregard the 3rd cable if that is used as a CPC, so it is NOT rated as 3 or 4 core cable, but as 2 core. I suspect he has looked up the wrong column,and used the 3 / 4 core cable column, which has a current carrying capacity of 30 amps. Volt drop will be insignifcant at 12 metres length, so that doesnt come into play for cable calculations.

My shed had an electrical supply installed in 2010. It has a 32a RCBO supply  in the house  and armoured cable to a distribution box in the shed. I don’t know the rating of the armoured cable but it’s about 19mm external diameter 

I'm looking at doing this myself as prices are silly - how about just running a fused spur from the nearest socket into some over sized armoured cable and then into a shed consumer unit to split into lighting and sockets?  Wouldn't the fuse protect from overload?  Maybe using the welder would be a problem as it would generate too much load?

Then once you have won the lottery / sold a kidney you could get a sparky in to remove the FCU and run directly to the main consumer unit to upgrade the supply?

Alanl has it above. 4mm SWA will be fine for a 32A supply, and you will want 32A tbf. To do it properly you'll need to have a non-rcd supply in the house, run the SWA to the shed, and then have a 30mA rcbo CU. You also should not export the PME earth (assuming you're a TNC-S earthing system), so you'll need to gland the cable into an insulated enclosure at the house end so the armouring doesn't export the earth. At the shed end you'll need to create a mini TT system with earth electrode etc, and earth the armouring back from there. There should be no connection between the house and shed earthing systems.

Unless you're fully au-fait with the requirements it is really a job for an electrician, if I'm honest.

And don't eff about with extension leads, etc. Do it properly, do it once.

IAAE

Why wouldn’t you use the pme for a shed? Assuming it’s made of wood and  there’s no extraneous conductive parts in there that need bonding then putting an earth rod in seems pointless

Just looking at this for my son's shed power. SWA can take a lot of current, more than house twin n earth. Its 90C rather than 70C rated, and the armour makes for a larger diameter to dissipate the heat.

It then becomes important to look at the voltage drop at the circuit current rating(i.e. feed end breaker). Limit is 3% for lighting, 5% for other circuits. So really that's 3% to your shed CU, as lighting final will be negligible with modern low power lighting. (See appendix 4 in the regs, table 4Ab). You've then 2% left for final power circuits. In my son's case I've 25m to the shed, and then another 25m to sockets at the bottom of garden. He might balk when he's sees the cable cost!

Regarding separating earth systems, usual practice is to earth the cable armour at the feed end, and use a plastic box at the shed end to isolate the house earth from the local TT (earth rod) you will create in your shed. You can use 2 core SWA, as the earth core would be doing nowt. Suitable warning label to stop some amateur numpty joining the earths in future.

I've some sums to go and do....

Posted by: steveb

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Just looking at this for my son's shed power. SWA can take a lot of current, more than house twin n earth. Its 90C rather than 70C rated, and the armour makes for a larger diameter to dissipate the heat.

It is only 90 degree rated when the terminals can handle 90 degrees. Even large panel boards are not rated at 90 degrees now. The only things that are rated 90 deg. are large metal switch fuses and other such switchgear, all domestic style accessories and consumer units are 70 degree rated.

Thanks gents. Total cable run will be about 10m so shouldn't be outrageous expensive. I'm happy putting the ring main into the shed myself, with sensible conduit arrangement and of course all the lighting is LED so very low power. Maybe 2 double sockets in the shed and an outside waterproof one to start with. As long as it's properly, professionally installed to a small consumer unit for me to work off, I'll be happy with that. Think I'll target 32A, seems a sensible precaution against future expansion. 

Caveat: I'm out of date

Why wouldn’t you use the pme for a shed? Assuming it’s made of wood and there’s no extraneous conductive parts in there that need bonding then putting an earth rod in seems pointless

I suppose it depends on what you think is best, quickest and easiest on a particular site. Hard-paved yard, PME. Lawned, TT. The main point is not to mix the two, i.e. PME and TT, together to avoid nuisance tripping and worse

Regarding separating earth systems, usual practice is to earth the cable armour at the feed end, and use a plastic box at the shed end to isolate the house earth from the local TT (earth rod) you will create in your shed.

I'm interested in the comments on earthing. I'm not in the UK so regs and practices differ slightly but earth is earth is earth right? Or not? I've got a couple of earth rods daisy-chained 7m apart which serve both the "shed" (garage) and the house with connections to the all copper plumbing in both at the points required by French regs. It never suffers "nuisance tripping" and I'm trying to imagine what "and worse" implies. Happy to learn.

Happy to learn.

I'm out of touch by a few years and I'm not an electrical engineer, so hopefully others will do that

You're describing what is possibly a TT (Terra Terra) system in the literal earth. Fault currents flow to earth, but this depends on the earth and can vary with ground conditions, water content, etc.

A PME (Protective Multiple Earthing) system uses the supply neutral and so can give you a different fault current to a TT system in the same installation. PME can also flow to earth and isn't consistent enough to use in hazardous installations.

steveb's method puts the armoured wire sheath on the PME inside the shed, but isolates it from the TT inside a plastic box.

mark's method puts the lot on the PME, without TT

Mixing the two in the wrong way exposes people and equipment to problems

"extraneous conductive parts" adds another fault current route, e.g. a metal water pipe buried underground. You can't rely on that as an "earth" and so it's bonded to the local earth system with a separate cable, eliminating another difference

To a detached garage for me rather than wooden shed... but sounds like we wanting to do the same things ish. 32A breaker in the house  + armoured cable is what I have. Then a small satellite consumer unit there for the sockets (16a breaker) and lights (6a breaker) 

As an aside, definitely use SWA cable but run it at the surface if you can, preferably cleated along a fence etc. While it has a high degree of mechanical protection, it wouldn't be impervious to, say, a sharp prong of a garden fork on a direct hit. Unless you want to bury it down at 450mm, and use cable tape/bricks etc, then just install it where you can see it, to avoid future issues. Alternatively bury it somewhere where there's no chance of it being disturbed, under the patio springs to mind (although if you have a 'Louise' in your life even that's not guaranteed).

SWA’s do have limited mechanical protection but the main function of the steel wire is to provide an earth to create a fault and blow the protection asap! Make sure swa’s are correctly terminated!

Run is likely clipped to the side of the house then under patio flags into the shed. 

You're describing what is possibly a TT (Terra Terra) system in the literal earth. Fault currents flow to earth, but this depends on the earth and can vary with ground conditions, water content, etc.

Yes. That's why there are two earth stakes. There's an earth resistance you have to get below to comply (100 ohms), it was too high (measured by an electrcian: below 100 but still too high in his opinion) in a drought so I added a second stake. So long as the earth resistance is low enough it's a perfectly safe system. You can do a simple check youself with a multimeter: measure the voltage between earth and neutral and it should be close to zero, even a couple of volts tells you the earth is poor. (all this said from France where DIYing electrics on your side of the meter is perfectly legal and acceptable).

I've found the leccy car charging block is a quick test of any unknown system, if the earth resistance is too high it shows a fault and the car won't charge. I plugged it in in one place and it wouldn't charge - the socket had no earth cable at all!