Running in a spur to garden office

25 Sep 2007
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United Kingdom
I am in the process of landscaping the garden, with an intent at completion of having a small log cabin.

I'd like this to have electric and data connection to the house.

Cable run will be approximately 17 metres and I am assuming S.W.A. buried would be the simplest method with a reaosnable level of protection.
Intent would be if this could be spurred from the existing ground floor ring of the house, which is RCD protected at main CU, would 2.5MM cable be sufficinet? My logic being that any attempt to laod with greater than 16A would trip the breaker for that circuit in the main CU before any cable current capacity over the suggeted run would be a concern.

Not looking to connect anything up, I just want to verify & purchase the correct size of cable so I can get it laid in before any more of the garden work proceeds. I'll have someone in to complete work at each end once things are further along.

Happy to buy 4mm cable as the difference is only a few quid - though also don't want to over engineer if there is truly no point.
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27 Jan 2008
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Llanfair Caereinion, Nr Welshpool
United Kingdom
Although I can see the point in burying cable there is a problem with requirements of Part P. The person signing the installation certificate needs to view the cable before it is back filled and if there is to be LABC involvement this has to be started before any work is done not after.

As to cable I would assume the ring main has a 32A supply reference method D would therefore need 4mm SWA. Using a fused connection unit could reduce cable size. But forgetting about current carrying capacity you also need to consider volt drop so you need to know the volt drop already at the point you intend to connect to first.

Using an earth loop impedance meter you need the measure the prospective fault current at the point where you intend to take supply from. Line to neutral measurement then convert to ohms and take it away from ohms permitted for the protective device you have selected then work out the ohms of the cable selected to see if it is heavy enough.

Supply the figures and may be we can help with calculations but without figures it's like asking length of piece of string.

Calculating cable length and volt drop.
We are permitted 5% with power and 3% with lighting on top of the DNO’s permitted volt drop so if we find the PFC at DNO’s head we can take that away from the PFC at the end of final circuit.
The easy way is to work out the figures with a scenario then replace the figures with real ones.

So lets consider a ring main with a radial coming off it to a shed down the garden. At this point not considering the sockets limit of a spur.
At the DNO we measure 0.35 ohms or 657 amp. (Standard limit on TN-C-S supply) at the point where we intend to spur off we measure 0.575 ohms or 400 amp this means the cable already has a resistance of 0.23 ohms for a 3% volt drop (We will assume lights in the shed). So we are allowed a 6.9 volt drop which means for a 32 amp supply 6.9/32 = 0.21 ohms clearly we can’t use the full 32A. So working on 13A (largest fuse one can fit in a fused connection unit (FCU) then we get 0.53 ohms so have 0.3 ohms to spare. The resistance of 2.5mm cable at rated temperature is 0.018 ohms per meter but since we will not be drawing full current there is a correction factor however since there will be wiring inside the shed we will ignore the correction factor which will give us a margin for slight error. So maximum length will be around the 16 meters.

If we went to 4mm then we would be allowed 27 meters.

Since the sockets are permitted a 5% volt drop we could find that we have a minus figure as permitted length assuming lights in the shed. It may be one just can’t come from ring main.
One must also consider what the extra load will do to rest of ring main. Normally we consider 2000W as largest size of fixed appliance to be powered from a ring main so really the fuse size in the FCU should be 8.7A well can’t get a 8.7A so 7A would be maximum and even 7A fuses are hard to get so more likely 5A.

With these considerations most electricians would not power the garden shed from the ring main. They would come all the way back to the consumer unit. What they have to remember any of their work which needs Part P registration may be tested and they can’t afford it to fail or they could lose their Part P status. Years ago we would look at a job and say he only wants a bulb not worth the hassle going back to CU we will come of ring. But now we have the Part P police and common sense has gone we have to follow regulations by the letter. The LABC does not have the same problem no one checks their work and the building inspector can use some common sense but often it is not the building inspector who inspects and tests but a third party and he has to report all defects. The same when a periodic inspection report is done with change of occupant or every ten years.

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