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What Size Armoured Cable for Shed Electrics?

Postby Jak68 » Mon Mar 25, 2019 8:46 am

Hi, I am planning to have a shed in the garden for putting in a tumble drier, a chest freezer a couple of sockets and some form of lighting, what size armoured cable would you recommend me to use for this? Thanks
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Postby ericmark » Mon Mar 25, 2019 12:07 pm

Unless using an inverter freezer the volt drop is rather important, so if coming from the consumer unit about 42 meters is the limit with 2.5 mm SWA cable with a 16A supply.

With 1.5 mm SWA same current then drops to around 24 meters.

Where is starts to get hard is when not coming from the consumer unit but from a 13A FCU off an existing circuit, with the ring final a load in the centre is not a problem as each leg will share the load, but as we get closer to either end then the load is no longer shared equal, so rule of thumb is 2 kW only for fixed appliance and your shed is in essence a fixed appliance. But from centre there would not be an overload problem as the MCB/RBCO/Fuse would open stopping an over load.

However from the centre you will have more volt drop, we have a meter to measure loop impedance and prospective fault current, they are two different units but tell us the same thing, line to earth we normally use the loop impedance, and line to neutral we use PFC but ohms law ohms = volts/current or current = volts/ohms so 0.35 ohms = 660 amps so with a TN-C-S supply we will often see ELI = 0.35 and PFC = 660 on the test sheets.

So at your sockets in the shed we are looking for 1.06 ohms or 216 amps PFC to show the volts will not dip over the 5% allowed for power, it actually says 3% for lights, but unless using old fluorescent fittings it is unlikely to be a problem.

The odd bit is under building regulations your permitted in England to extend a circuit to garden shed which as you see needs so measuring with expensive meters, but from the consumer unit as a new circuit you either have to pay the LABC or use a scheme member, in Wales the garden is a special location so LABC is required for both.

I would not mind paying if the LABC did something for their money, however the one job I did they did not inspect after complete, and when I asked for copies of the certificate when selling the house they could not produce it for 3 months and would cost £70 an hour for them to hunt for it. So in real terms it is simply a tax. I am talking about Part P notified work.

However it would be wrong not to tell you about it.

I have found 6 mm SWA is often cheaper than smaller sizes, down to amount made, it is the thickest SWA which will fit a 20 mm gland and where I worked we kept a role of 6 mm SWA in stock, any other size we had to order in special. So I would say no bigger than 6 mm, personally if price was right likely use 4 mm so I can add latter if coming from consumer unit. However even 2.5 mm will take over 20 amp it depends if thermal plastic or thermal setting exactly what it will take, and 1.5 mm will clearly take the current you could get from a FCU with 13A fuse.

So 1.5 mm will likely take the current, the question is volt drop.

Non inverter freezers will take well over the rated current on starting often around 10 amp and if the volt drop is too great, or it has only just switched off, the pressure from the compressor can be too great for it to start, so it is fitted with a over load device which will trip and wait for around 5 minutes before auto resetting and trying to start again.

This overload device is not intended to operate many times during the life of the freezer, and if it does will quickly wear out, the worry in your case is if the tumble drier is running when the freezer tries to start will the volt drop be too big? If so the over load device may wear out prematurely. And the tumble drier uses power for a long time.

With out upright freezer it has a three phase motor driven by an inverter to run the compressor so the start current is around 0.3 amp not 10 amp and the inverter will correct any volt drop, so it would run without a problem, except since frost free it has a minimum room temperature so a shed would be too cold.

Unfortunately freezers don't say what volt drop they will stand, they do often say don't use an extension lead which is because of the volt drop, even the fuse in a 13A plug causes a volt drop.

Commercial I would take a chance, fit 1.5 mm SWA and test and cross my fingers 99 times out of 100 it's OK, in fact only once have I needed to replace a cable due to volt drop being too high, one gets a feeling for what will and will not work, so you use 2.5 mm you see it was close to edge, so next time you use 4 mm I have rarely actually worked it out first. But this is experience which you don't have. And also test meters you don't have to verify I got it right.
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Postby collectors » Wed Mar 27, 2019 4:21 pm

If your coming from the fuse board there is a handy calculator here.
But for the sake of the price of the cable, go the next size up. IE: 2.5mm £1.26 a met or 4mm at £1.76 a meter all + vat. It will give you a little more flexability.
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