Wire size for supply to small consumer unit in shed

[JGralton]

Hi,

I want to get a small consumer unit fitted in a shed I will add to my garden in the next few months. I am about to lay laminate flooring my my house and therefore want to run the wires under the floor ready for the connection to the shed.

To save money on the armoured cable I want to run twin core and earth to a RDC inside the house nearer the shed then the main consumer unit and then run the armoured cable from there to the new consumer unit.

I only require 1 or 2 double sockets in the shed, an interior & exterior light hence I will only need a small unit in the shed.

My question is not so much about the set up (although comments on a better way to do this are very welcome) as I will most likely get a sparky to do the connecting up etc, instead I want to know what size TC&E wire I should run under the floor from the main consumer unit to the position for the rdc and then what type/size?? armoured cable I should buy to run from the RDC to the shed.

All comments much appreciated.

Cheers,

James

[David Hansen]

On Fri, 27 Jul 2007 07:06:37 -0700 someone who may be [e-mail] wrote this:-

What would the length of both sorts of cable be?

[JGralton]

The TW&E would be about 12-15m and the Armoured cable would be about10m.

Cheers,

James

[Dave Plowman (News)]

One or two double sockets *could* mean a load of 40 amps or so. You need to know how much current you are likely to take. You might, for example, want to run a 3kW heater and an arc welder together while the pottery oven is heating up. Or only a desk light and soldering iron.

[John Rumm]

This is a subject that crops up from time to time and is a bit deeper than many anticipate since there are a number of aspects of the design that need thinking about.

Have a read of:

proposed setup sounds like one of the simpler arrangements, but think through your requirements carefully before deciding on a solution.

[JGralton]

Hi,

I will only use the sockets for a small water feature pump and lawn mower and the lights will be a simple 100w inside and about 150w outside.

Cheers,

James

[john]

[John Rumm]

With respect, this does seem to be glossing over a number of potentially important points.

The OP s requirements sound like they are unlikely to exceed 3kW, so even if we allow 4kW for expansion, a 20A supply would be more than adequate. From a voltage drop point of view 2.5mm^2 cable would just about be in spec at 25m for that load.

You also need to decide where you will site the RCD. In this circumstance at the head end of the supply would probably be acceptable. Depending on the earthing system in user you may or may not have a choice.

[JGralton]

Thanks for you comments,

I was going to site the RCD where I change from TC&E to the Armoured Cable is this not suitable. Do I need to run a separate earth cable will the one in the TC&E nor suffice?

Cheers,

James

[John Rumm]

You can have the RCD here if you want so long as you have a TN-S or TN-C-S earthing system in use at the head end. With a TT system at the head you would need some form of RCD protection at the head end to protect the T&E from the consumer unit to the point of changeover to SWA.

With RCD protection on the SWA there is no real worry about the size of the earth. This only becomes an issue when you have a long run that is not protected by a RCD at the head end, and you need to make sure that disconnection times can be met in the event of a fault to earth on the SWA. The RCD makes this issue go away.