Underground cable for garden lights etc

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I want to put a ring of SWAC underground around my garden, with short spurs to aboveground sockets. The ring is going to be about 100 metres long, with maybe 4 - 6 short spurs, to which I will connect lights, maybe power for a fountain pump, etc. Total load about 5KW max if everything is connected at the same time.

Does anyone have any ideas about cable size?

If the circuit wasn't a ring, but just a spur from the consumer unit, I could make the total length smaller (perhaps 60 metres), but then I understand that the load would be greater because it wouldn't be a loop. Does anyone know how to work out the "tradeoff" between cable length and "loop versus spur"?

 

[mapj1]

5KW is about 20A, from a current handling consideration, a 4mm radial, with, or a 2.5mm ring, either fed with a 32A breaker are the two alternatives that will avoid nuiscence tripping at full load.
Given the mechanical handling problems of SWAC, you might prefer to go the radial route, particularly as the difference in price per metre is probably more than the difference length.
Voltage drop may, or may not, be an issue depending on how far from the feed point most of the load is, and certainly if all the load were all at the far end, then the 'next size up' -6mm would be in order to keep the drop below the 4% on load which is the greatest drop recommended for socket outlets, where the equipment to be supplied may be of unknown performance. There is a cable loss calculator on the TLC website, which will allow you to play off size against load.
Note also all outdoor installations should be protected by an RCD, and are now supposed to be building control notifiable under part P, if part of a private dwelling.
regards M.

 

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Thanks M:
I don't quite understand your 2nd sentence. Are you saying that it's easier mechanically to go for a loop with the smaller cable, even though that would be longer?

 

[mapj1]

No- the reverse actually. Unless you have had a lot of practice, you may find SWA difficult to gland in, and in any case the outer diameter doesn't change much until you get to the really thick sizes, nor does the price per metre. Given this 2.5mm is almost as unwieldy as 6mm, and not half the price as you might hope!
If you don't make a ring you have less cable to get dmaged or be buried, fewer terminations to worry about, and its more obvious if one point in the chain is flakey, which one is at fault.

Also I'd suggest that you use 3 core (gives you a separate earth conductor, in addition to the steel armour, which should also be earthed)
The tail will wag the dog though, I'd not recommend using plastic boxes, at least unless the cable run in is very straight and well clamped as the 'springyness' of the SWA can easily crack the casing, and I'd suggest getting a size with plenty of room inside to manipulate the ends.

Hope that made it clearer which way I was going...
regards
M.

 

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Many thanks for your advice M - will let you know how I get on!