SWA Cable

[asalcedo]

I need to install a 20A supply plug outdoors for an air conditionin

unit, like

will be wired directly to the consumer unit.

For that (given the length and the amperage, and to be conservative) need 4mm Steel Wire Armoured Cable (SWA Cable). I have a few questions

1. Do I need three core cable? Or can I get two core cable and use th steel armour as the earth cable?

1. Can I terminate the SWA cable directly into the consumer unit? Or d I need one of those special amoured cable junction boxes lik To terminate the SWA cable directly into the outdoors isolato switch, do I need the glands lik

Thanks,

Antoni

-- asalcedo

[David Hansen]

On Fri, 18 Aug 2006 13:28:28 +0100 someone who may be asalcedo wrote this:-

No.

Yes, provided that the steel armour provides a suitable disconnection time if there is a live to earth fault. This depends on the individual circumstances of the circuit.

It is fashionable to use one core of a three core SWA cable as an extra protective conductor, just in case. To my mind this just indicates a lack of confidence in one's workmanship.

Yes, provided that the consumer unit has a suitable place to terminate it. Depending on the type of consumer unit you will probably need an earth tag.

Gives a 404 error.

[Andy Wade]

Same here, but for exposed outdoor use you need a type CW gland. The more common BW type are for indoor or sheltered outdoor use only.

[Christian McArdle]

Not quite. It is just that the steel armour tends to rust, so it can go high impedence at the termination after several years, especially if any water gets in. Also, the default earth terminations that come with the glands are absolute crap. They are hard to get any clamping force on and don't allow actually have any obvious method of attaching the earth cable to. Aftermarket alternative earth glands are available and should be used.

"Piranha earth locking nut".

Despite all I said above, I would probably go 2 core for the air/con. I would be more concerned about a circuit supplying portable equipment sockets, where earth integrity might be more important, both now and in 20 or 30 years time when the steel has had ample time to oxidise.

Christian.

[Andrew Mawson]

air/con. I

On a related point 2 core SWA and 3 core SWA are rated differently for the same cross sectional copper area, so 10mm 2 core is rated at 85 amps whereas 3 core is rated at 73 amps - obviously due to the reduced heat radiating area per dissipating conductor. Now if 3 core is being used as 2 core plus earth, is it reasonable to uprate it (in this case) back to 85 amps as the earth will only carry current under fault conditions and then hopefully only momentarily.

AWEM

[Christian McArdle]

It is irrelevent, as it is extraordinarily rare to run SWA anywhere near its current rating. Normally, circuit voltage drop is the limiting factor. Typically, 10mm would be used for longer outbuilding runs, but at 32A, or possibly 40A. The highest rating in many ranges is 63A, so even then cable rating really doesn't matter.

Christian.

[Andrew Mawson]

factor.

...mmm ... you should see my 16KW kiln and my 15Kw three phase converter running at the bottom of the garden on 10mm, though not at the same time

AWEM

[Andy Wade]

Yes, entirely reasonable, it's the total I^2 * R that matters.

[ansalc]

Thank you Christian. That was a most informative response. I agree with you also on the advantages of the Piranha earth nut.

I have decided to go for a three core cable, it is just more straighforward and more reliable to earth and only a bit more expensive.

Antonio