Workshop rewire

2 Sep 2008
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United Kingdom
hi all

I am looking to have my garage,workshop and shed rewired, I have had a look at what my houses previous owner had done and and the shock of my life when I looked at it basicaly a total bodge up I need some help with what needs to be done.
Here is the current setup my house has a new cu running from that to my single garage is a 5 core armored cable looks like 2.5mm, he has doubled up to pairs of 2.5mm ( I know i did say bodge ) in the single garage is a really old cu that that has a double socket running off it and a light tube know idea of any fuse values here, from the single garage runs again another 5 core cable to my workshop approx 25m away in here is another old cu 5 tuble lights and approx 10 double sockets in a ring the shed which is attached to the workshop currently has now power.

My Requirements are

Single garage 2m from house - light and double plug all it that will be used here is for power to my pond pump and for my mower when I cut the grass

Worshop 25m from house - this has a fridge ( For Beer ) but this workshop is used for working on cars so I regualy used a mig welder air compressor and various power tools although none of these are used at the same time I need about 10 sockets and 6 tuble lights .

Shed attached to workshop - just a light

what size SWA will be needed?

Do i need to have 2 seperate feeds from the house?

Thanks for your time
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Using conductors in parallel is unusual but perfectly okay (Regn 523.8 17th or 523.02 16th) Providing the cores are identified, colour sleeved ie two brown, two blue or marked L and N. It can be a cost effective way to run long SWA. The fifth core as an earth (cpc) in addition to the armour helps keep Zs down.

Your existing SWA if it is undamaged and passes testing should be adequate.

This job is covered by Part P and if you are having the work done the sparks should be able to do the calcs for you but here are a few thoughts:

You will need to provide RCD protection for the socket outlets and preferably treat the outbuildings as a separate TT installation with their own Earth Rod.

As you are using Welder and compressor both have high start currents and may trip B curve MCBs (check with Manufacturer) You may be able to fit C curve MCBs but you will need to check the loop impedance is low enough to operate them. A C20 MCB may be better than a B32 on your ring circuit.

Assuming the distances you gave are for the cable lengths:

Voltage drop is an issue, 27m of existing SWA from a 32A MCB at the house gives 32x27x0.018/2 = 7.8V okay for the sockets but too high for the lighting and thats before allowing for the final circuit wiring.

If the final circuits are 20A for the sockets and 6A for the lighting all is well
26x27x0.018/2 = 6.3V but remember to add the final circuit wiring.
Limits are 3% for lights and 5% for the sockets.

You can take the actual lighting load x1.8 rather than 6A if that helps.

Good luck.

Sailbad the sinner MIET
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