Electrical question: cable size and earthing for outbuilding

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I have a 2-wire overhead sub-main to a large shed 40-45m away. The house earthing is TN-C-S (with an earth rod, ref other append) and the shed earth comes from a local earth rod; there's an RCD in the shed CU. I plan to replace the overhead feed with 3 core SWA in the ground (0.5m deep, with warning tape above it at 0.3m deep), terminating the SWA internally in IP65 adaptable boxes at each end and running the final couple of metres in T&E.

The feed will come from a non-RCD protected MCB in the house CU. From the TLC cable size calculator it seems that buried 2.5 SWA is OK up to about 3.6kW for non-lighting circuits (about 2.2kW for lighting circuits). I plan to either feed the SWA from a 16A MCB in the house and to use a 10A MCB in the shed or or to use a 16A type C MCB in the house and a 16A type B MCB at the shed. Any comments?

I believe the correct earthing in this situation is to export the house earth down the SWA (hence 3-core, the armour will only be earthed at the house end) and to remove the shed earth rod. Once again, have I missed anything?

[John Rumm]

ok. Although in reality with the current configuration, two core SWA would be adequate in the circumstance.

Your limitation here is down to voltage drop. 2.5mm^2 SWA gives around

18mV/m/A of voltage drop. So 45m of 2.5mm^2 is going to give 10.53V on a 13A load. i.e. Just out of spec for a supply that will run lights... (how much you care again will depend on the use case for the building)

At that distance I would run a larger CSA SWA - the difference in cost of cable is small compared to the effort in laying it in. So allow for some future proofing and expansion unless you are sure that the current demands are unlikely to ever change.

As to MCB selection, that rather depends on what the power requirements in the shed are going to be?

There is no "correct" answer as such, which option you choose will depend a bit on circumstance. With a PME head end, if you elect to export the earth, then you also need to export the main equipotential zone. For that to make sense, the outbuilding should not have access to any other sources of earth potential that can't readily be bonded into the zone. (so for example, it would not be possible to extend the equipotential zone into a greenhouse with a bare soil floor).

Also if extending the equipotential zone, the earth connection of your submain will also need to function as a main bonding conductor and hence meet the (copper equivalent) CSA requirements for that (typically 10mm^2 with PME - although that assumes size of the neutral of the supply is of

35mm^2 CSA or less, and where the supplier has not specified another size be used).

If you are using 2.5mm^2 SWA, then its armour will typically be around

17mm^2 CSA. Divide that by 2.255 to get a copper equivalent of 7.55mm^2. So if you add a 2.5mm^2 core to that, you just squeak in to meet the minimum CSA for a PME main equipotential bond.

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The building is mostly a store with a powered hacksaw, log splitter and small welder. Electrical loads are modest and not going to exceed a couple of kW.

I've already got enough 2.5 SWA to do it.

I chose 10A at the shed to ensure the installation stayed compliant and

16A at the head to protect the cable and avoid nuisance trips.

The point about the floor is new to me, and very relevant to another task on the tuit list. I have a greenhouse with an overhead T&E feed (without local earch rod) that was signed-off by someone before I bought the house. Is there really a difference between a soil floor and a concrete floor that doesn't have a DPM? Where do I find the details?

Yes, and I would also need to add supplemental cable between the main earth and the SWA termination.

It's starting to look like I'm better to keep the shed earth rod and not to export the house earth.

Thanks for your comments.

[John Rumm]

Personally I would still design for at least 13A capacity at the socket, then you know you can plug it most loads without any difficulty.

(things like a 110V 3KV site transformers may still cause problems though - they will often trip 20A MCBs with inrush)

Well your call then ;-)

The head end only really needs fault protection in this case - so the protective device could be sized well above those in the shed. Even 32A would probably still protect it...

Yup, that's a harder one with a PME supply. You can't realistically make the greenhouse part of the equipotential zone, so in the event of a loss of combined earth / neutral on the supply side, you would be at risk in the greenhouse with your independent earth connections at very different potentials.

You would probably have to find a way of measuring the earth impedance of the concrete to be sure. You could argue that if its 23K Ohms or higher, then its not an extraneous conductive part that would otherwise need to be included in the main bonding.

A variation on the test procedure here might work:

(personally though, if there was any doubt, I would make the outbuilding TT and be done with it)

Yup, this is true if you are not running the SWA right to the CU.

Its what I normally do in most cases unless its a simple run close to the main building.

[Brian Gaff]

Makes my brain hurt, For many years we got used to the lights dimming as a fan heater cut in at our shed end. No people or animals came to any harm in over 20 years. grin. Brian