DIY Doctor

Garden Electrics - cable size & small consumer units

Postby spruce » Mon Apr 25, 2011 10:23 pm


I'm currently sorting my garden and want to provide electrics outside, a couple of garden lights and maybe a socket and light in my shed. My shed is approx 20 metres from the house and socket will be used for lawnmower/pressure washer. What size cable and how many cores does it need as have seen many variations on ebay! Also regarding connecting it in the house - I ideally don't want to run the cable all the way to the junction box as that is another 30 metres from the back of the house as its a long thin terrace with no easy route. I have a heavy duty cooker cable (10mm) already running from the main junction box(MCB-1B50) to the rear of the house (25m) for when we put an electric oven in. Can this cable be used to to put a small consumer unit with 2 breakers in the kitchen - one for cooker and other for garden cable. What size MCB's would the small unit in the kitchen need? Any help would be greatly appreciated - I am costing this up at the moment and would do all running of cables and get a qualified electrician in to do the connecting up.
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Simply Build It

Postby ericmark » Tue Apr 26, 2011 10:02 am

Although the work is simple the calculations are complex. In the main an electrician will look at a job and compare it with other similar work. He will consider how the last time he was a bit near the limit or how he was well within the limit and will take a chance and not calculate but will measure once complete. 99 times out of 100 it will pass so no problem.

However for a one off you would need to work it out. Important data is the supply to cooker point size and the loop impedance where it terminates. This will tell us how much we have to play with. Then you need to work out load in shed and distance. Working out the volt drop for available cables sizes can then be done.

I have a very simple system. A 13A 10ma @ 40ms RCD socket in garage feeding the 1.5mm SWA to shed using a FCU to move from Flex to SWA. In the shed it feeds double socket and a switched FCU with 3A fuse which works the lights.

This is ample and there is no need for consumer units a simple Fused Connection Unit (FCU) does any reduction in available current and works as light switch.

However the supply was tested with a loop impedance meter and with a low ohm meter (200ma) and insulation tester (500v) plus RCD tester. And the figures were checked with Excel before work started.

The problem with DIY today is although we can all buy the gear to do the job the meters required cost £75 to even hire. Add to that the £100+ LABC charge and DIY is very restricted.

My shed was planned before 2004 when Part P came in and I own a full test set. Even so with my daughters house which she did not own in 2004 all work outside has to be registered so it's cheaper to use an electrician who is a member of a scheme.
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