Armoured core cable Size

[Scorpius]

In the US - the simplest, code approved method of providing power to an outbuilding is to install a single, underground, 120V - 20 amp branch circuit (2 conductors w/ground) or a multi-wire branch circuit (3 conductors w/ground) using non-metallic sheathed cable rated for direct burial - just be sure to observe the minimum burial depth which is 2 feet in most cases.

http://www.southwire.com/products/type-uf-b-direct-burial-water-well.htm

The circuit could then have multiple receptacles (wired in parallel) with the first one being a ground fault circuit interrupter (GFCI). This arrangement provides ground fault protection for all the receptacles but only requires purchase of one of the more expensive GFCI receptacles.

This will work fine if the load does not exceed 20 amps and you don’t need a separate circuit for lighting. If you do need more than one circuit, you can install a multiwire branch circuit (one for the receptacles & one for the lighting) fed from a double pole breaker in the house panel & meet code. This would still count as a single circuit eliminating the need for a separate subpanel & grounding system at the shed. However, multiwire branch circuits - which share a neutral between circuits - should be installed by someone who knows exactly what they’re doing or a dangerous condition could exist. Of course both these wiring methods assume the main panel box is properly grounded.

However, if you need more than two circuits in the shed, a secondary disconnecting means (subpanel) must be installed which has its own grounding system (ground rods). In this case, the neutrals (grounded conductors) need to be isolated from the equipment grounds (ground wires) by terminating them on separate bus bars in the subpanel. Isolation of neutrals from grounds typically involves removing (or not installing) the subpanel’s bonding strap & terminating all neutrals on one bar & all grounds on the other. Over here, hot is black, neutral is white & ground is bare copper but grounds may also be covered with green insulation or some other color having green stripes. Regarding wire size, #12 AWG should  be fine for the distance you stated however, you could always go with  #10 AWG on a 20 amp breaker of you’re concerned about voltage drop.

Here one ground fault circuit interrupter protects multiple duplex receptacles coming after it, known as multiple-location protection. Two-wire cable runs from the GFCI to all the following receptacles. The line terminals on the GFCI are connected to the circuit source and the load terminals are connected with a pigtail splice to each of the following receptacles to join them in parallel. This keeps each duplex connected directly to the GFCI.

An excerpt from the VA Residential Code which addresses separate grounding requirements for outbuildings. Note the exception to E3607.3 which permits a multiwire branch circuit (actually 2 circuits with a shared neutral) without requiring a subpanel or separate grounding system at the shed.

“E3607.3 Buildings or structures supplied by feeder(s) or branch circuit(s).

Buildings or structures supplied by feeder(s) or branch circuit(s) shall have a grounding electrode or grounding electrode system installed in accordance with Section E3608. The grounding electrode conductor(s) shall be connected in a manner specified in Section E3607.3.1 or, for existing premises wiring systems only, Section E3607.3.2. Where there is no existing grounding electrode, the grounding electrode(s) required in Section E3608 shall be installed.

Exception: A grounding electrode shall not be required where only one branch circuit, including a multiwire branch circuit, supplies the building or structure and the branch circuit includes an equipment grounding conductor for grounding the noncurrent-carrying parts of all equipment. For the purposes of this section, a multiwire branch circuit shall be considered as a single branch circuit.”

This is my understanding of US requirements however, I agree with others who recommended you consult a “spark-trician” familiar with UK codes, permits & inspection requirements...

[JamesF]

I agree that an RCD doesn't have anything directly to do with the earth.  However, my understanding is that if you have a short from live to earth (via a human body, perhaps) then if the path to earth has too high a resistance the RCD will not sense the difference between the live and neutral early enough (at a sufficiently low current) to break the circuit and protect the person across whom the short has occurred.

What I think you're supposed to do when you add a new earth is to choose an RCD rating of a suitable value such that it will trip fast enough based on the combined resistance (maybe impedance actually) of all the connected earth paths.  This may only apply to TT systems where there's a locally-provided earth.  I'm really not sure.

James

[Davey-T]

Have to say that when working on site on live wires (elf and safety, wha's that ) I have received a shock from RCD  protected circuits that have not tripped the RCD

Dave

[teifistar]

I agree that an RCD doesn't have anything directly to do with the earth.  However, my understanding is that if you have a short from live to earth (via a human body, perhaps) then if the path to earth has too high a resistance the RCD will not sense the difference between the live and neutral early enough (at a sufficiently low current) to break the circuit and protect the person across whom the short has occurred.

What I think you're supposed to do when you add a new earth is to choose an RCD rating of a suitable value such that it will trip fast enough based on the combined resistance (maybe impedance actually) of all the connected earth paths.  This may only apply to TT systems where there's a locally-provided earth.  I'm really not sure.

James

Hi James,

For your own safety I think you need to reassess your understanding of RCD's.

Please take this in the manner it is intended. Your post contains inaccuracies.

peter

[JamesF]

Perhaps you'd care to point them out then?

James

[JamesF]

As regarding the second part of my post that you say contains inaccuracies, BS 7671 Regulation 411.5.3 gives a formula for the current rating of an RCD as fifty divided by the resistance of the path to earth, the intention being that the RCD should trigger before any more than 50V appears across a short from live to earth.  If you export a house earth to an outbuilding and significantly increase the resistance of the path to earth but leave the cable protected by a standard 30mA RCD then that may become too high a figure in such circumstances more than 50V can appear across a short to earth before the RCD disconnects the circuit.  To come below the safe value you'd need to reduce the rating of the RCD so it will trigger earlier.

That seems to be consistent with what I've said, doesn't it?  If not, how not?

Of course the RCD doesn't need to measure the current on the actual earth.  If 30mA is going from live to earth then the neutral will be carrying 30mA less than the live, so the effect of measuring the difference between live and neutral is the same.

James