I need to install a 20A supply plug outdoors for an air conditioning unit by means of a rotary isolator switch, like

http://www.screwfix.com/app/sfd/cat/...15054&ts=04598

it will be wired directly to the consumer unit.

For that (given the length and the amperage, and to be conservative) I 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 the steel armour as the earth cable?

2. I assume that I can terminate the SWA cable directly into the consumer unit? Or do I need one of those special amoured cable junction boxes like http://www.screwfix.com/app/sfd/cat/...16877&ts=04235

3. To terminate the SWA cable directly into the outdoors isolator switch, do I need one gland like http://www.screwfix.com/app/sfd/cat/...16877&ts=04235

Thanks,

Antonio

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

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

1. Do I need three core cable?

No.

Or can I get two core cable and use the
steel armour as the earth cable?

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.

2. Can I terminate the SWA cable directly into the consumer unit?

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.

3. To terminate the SWA cable directly into the outdoors isolator
switch, do I need the glands like
http://www.screwfix.com/app/sfd/cat/...18738%26p%3Dx4

Gives a 404 error.


--
David Hansen, Edinburgh
I will *always* explain revoked encryption keys, unless RIP prevents me
http://www.opsi.gov.uk/acts/acts2000/00023--e.htm#54

David Hansen wrote:

3. To terminate the SWA cable directly into the outdoors isolator
switch, do I need the glands like
http://www.screwfix.com/app/sfd/cat/...18738%26p%3Dx4

Gives a 404 error.

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.

--
Andy

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.

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.

http://www.tlc-direct.co.uk/Products/TLB20S.html

See "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.

"Christian McArdle" wrote in
message ...
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.

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.

http://www.tlc-direct.co.uk/Products/TLB20S.html

See "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.



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

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.

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.

"Christian McArdle" wrote in
message ...
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.

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.



....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

Andrew Mawson wrote:

[...] 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.

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

--
Andy

Christian McArdle wrote:
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.

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.

http://www.tlc-direct.co.uk/Products/TLB20S.html

See "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.

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