A very long electrical supply to garden shed

[Cordless Crazy]

I am planning to run an electrical supply to my shed and wish to ge

some advice before calling in an electrician, so as to avoid hi baffling me with requirements and fleecing me of my hard earned cash!

The supply is to come from the CU in my garage, run surfaced fixe around the garage, through the wall, around the kitchen, through th wall and up the garden to the shed. The length of run through the hous will be 10m and then 45m up the garden (armoured cable I assume burie in a duct 150mm below ground).

It will be supplying 3 No. strip double fluorescent lights, 3 No internal double sockets, 1 No. external double socket and a water-but pump (only used occasionally though). The maximum amount of things will be using at the same time would be a couple of battery chargers, circ saw and the lights.

Questions:

1. Can he utilise a spare 32A MCB running through a 80mA RCD on my C in the garage?

1. If not, would is required?

2. What size of cable would be required on this 55m run with, what assume will be, a huge voltage drop?

1. Can he install a switch within the kitchen to turn off the suppl when I'm not in the shed? (This is to avoid the local thieving chav nicking my juice!)

2. Can he install T&E through the house and changing at a weatherproo junction box on the external kitchen wall, to save on the cost of usin Armoured all the way?

1. Will he insist on a separate CU in the garage running from th meter? (More money though to get electricity board involved)

2. Is it necessary to have a CU with separate breakers in the shed t control/isolate all the different components?

Any advice would be welcome to avoid being taken for a ride

-- Cordless Crazy

[Christian McArdle]

Yes.

Indeed. And you should really allow voltage drop between the garage and the main house CU as well (unless the garage CU is located at the service point). Also, there is voltage drop within the shed installation.

Assuming that there is no additional voltage drop to take into consideration and allowing 10m within the shed, 10mm SWA is indicated for the full 32A load.

If you design for a lower load, such as 20A, which will fulfill all your needs, you're on the margins of being able to use 6mm.

Yes. Use a 45A DP switch for this. Often sold as cooker or shower switches.

Yes.

No.

No, as it runs from an RCD circuit anyway, but you might as well. There's a Volex "garage" unit, which has an RCD and a couple of MCBs in a waterproof box. If you don't have one, the lighting circuit must come off an FCU, as

32A would be too much without further protection.

It may be inappropriate to export the house earth this distance. You may need a TT earthing system (earth rod) and RCD, with the house earth (i.e. SWA armour) isolated at the shed consumer unit.

Christian.

[Mr Fizzion]

Voltage drop is given by V=IR where I is the current in amps and R is the resistance of the wires in ohms. The resistance of the wires is found from the resistivity of copper - 1.68*10^-8 ohm metres: R = (length in metres * resitivity) / cross-sectional area of the wire in square metres.

Mr F.

[Ian_m]

You can use the voltage drop calculator here to find the size of wire you need.

Dad ran 6mm for 50m to his garden shed to power lights and power tools. Wire got there in a combination of tacked onto wall and burried underground. Came from a small CU (holding 63A RCD and 2 MCB's, on for garage) next to main CU (wired up using 10mm wires) so that you could isolate the supply easily and if there was a fault would not affect any of the house ring mains. Ran to another CU in shed with 63A switch and split into plugs and lighting via two MCB's. Not too sure what was done with earth but I think an earth rod was used.

[Will Dean]

You don't need to put armoured cable in a duct. It's designed for direct burial.

Will

[John Blessing]

Isn't that formula valid only for DC not AC current?

[jim_in_sussex]

SFAIUI (and that's only a little) Pirelli specify a maximum surrounding (ambient) temperature for their SWA (around 5deg C IIRC) & that it should be surrounded by something that will take the heat away. See

IIRC Pirelli specify depth min 450mm & surrounding the cable in soft sand (which should be moist, or at least not liable to dry out).

Modern SWA is not laid in ducts normally - presumably you'd have to derate it to take account of the reduced heat conduction. How much all that mattters in your case depends on whether and by how much the cable is overrated for your full load & what proportion of the time it is carrying full load amps (ie the duty cycle).

HTH

[John Rumm]

20A ought to be plenty

In fact 20A would do the whol building just fine then.

Yes, and sort of. Do you mean 80A RCD or 80mA Trip RCD? The former is a common total current rating for some RCDs, the latter would be an "odd" trip threshold (30mA or 100mA being far more common). For sockets that may reasonably be used for powering appliacnes outside, then you really want the sockets protected by a RCD with a 30mA trip current. (the total current carrying capacity is not relevant here).

6/10mm SWA for 20/32A circuit.

Yup

Yup

Noe, and whay would that involve the electricity board anyway?

It is adviseable - at least to separate the ligting circuit from the sockets. Also see below.

Since you want to use a circular saw in the shed you ought to pay close attention to what happens under fault conditions. When you accidentally snag your cable and end up with the saw kicking back at the same time as it trips the RCD, do you really want to be plunged into darkness?

The solution may be to run the cable from a non RCD protected supply [1], and then include a RCBO for the power circuit in the shed CU. That way a fault on the socket circuit will not kill the lights.

[1] You did not mention what type of earthing you currently have in the house, there are two particular implications that spring to mind: Firstly you may *need* an RCD at the head end for protection of the SWA and shed CU from phase to earth faults. In which case using a time delayed one would be stongly advised so that the downstream on can discriminate socket circuit faults and trip first before the upstream one looses your lights. At the distance you are talking about you will probably want to make the shed a TT install anyway, however if you have PME setup in the house, you may find it very difficult to meet the standards required in the shed to export the house earth anyway and be forced to go TT.

[Mr Fizzion]

If you are considering a section of cable then it's valid for AC too unless the cable is tightly coiled into an inductor.

You do however get the "skin effect" with AC where all the current is concentrated in the outside of the cable and none of it flows through the central core.

The skin depth in copper at 50Hz is around 10mm, which I believe in practice means that there isn't any point in having copper cable whose radius of cross section is larger than 10mm. (but many years since I studied that kind of thing!)

Mr F.