2.5mm swa current rating

[Andrew Mawson]

According to the TLC web site 2 core 2.5mm SWA is rated at 36 amps, and 3 core 2.5mm is rated at 31 amps. If I am using 3 core with the third core as a cpc (connected to the wire armour ) can I reasonably use a 32 amp breaker to feed it? The cable is buried.

(It is 30 meters long, fed off a 30mA rcd board, and feeds four 13a sockets in the control cabinet for my front gate, with very minimal load (a wireless hub, three cat5 cameras and the gate itself which is only 3 amps peak. However it would be very convenient to use another socket on this circuit for the occasional power tool, concrete mixer, 9" angle grinder etc to save running extension leads, and I know my 9" grinder will trip a 20 amp breaker on start up)

AWEM

[Tim Watts]

On Tuesday 14 May 2013 17:37 Andrew Mawson wrote in uk.d-i-y:

I think for 30m you are going to get done by voltage drop requirements - I suspect 6mm2 will be nearer the mark.

Also, be careful as XLPE cable is often quoted as "X-amps" on the assumption of running at 90C. Most accessories it terminates in will only be rated for

70C - unless you are using metal boxes with appropriately rated connectors.

I assume you will have RCD protection at the supply end?

Would you like me to look up the numbers properly (or someone else might have them in their head)?

Cheers

Tim

[Tim Watts]

On Tuesday 14 May 2013 18:09 Tim Watts wrote in uk.d-i-y:

Oh crap.

There are so many "30s" above I got blinded to the bit that said "20A breaker".

You're probably OK with that then...

And yes, the CPC doesn't count - treat as 2 core.

[Stewith]

According to the book, 90degree C SWA cable is rated at 36A when in free air or on perforated cable tray, clipped direct it is 33A. Buried is not listed in the table but the 70 degree SWA is rated at 1A higher buried than it is clipped direct, so it seems logical that your cable should also be OK at 1A higher (34A).

[Fredxx]

I think doubly blinded!! Andrew mentioned using a 32A breaker because his 9" grinder would trip a 20A breaker!

[John Rumm]

He did say 32A MCB - I think the 20A bit was just an aside that the AG trips one.

Indeed. And this is also one of those circumstances where one could argue that the overload protection is enforced by the nature of the load (i.e. they are not general purpose sockets, but designed to feed very specific bits of kit), so all the head end MCB needs do is provide adequate fault protection.

[Tim Lamb]

In message , Andrew Mawson writes

Must be a Makita! Mine does that on a 20 amp ELCB. I rather wondered if the build up of dust on the windings was causing 30mA of leakage?

Can you not use a 25 amp breaker or go to *C* rating?

BTW the trencher has reached my budget plus transport and is still a long way off the last 10 seconds:-) Must be lots of people planning GSHPs!

[Tim Watts]

On Tuesday 14 May 2013 19:15 Fredxx wrote in uk.d-i-y:

Gawd...

In which case, 6mm2 probably - maybe 4mm2 at a stretch - will have to check the numbers properly.

No way will 2.5mm2 be adequate.

[Andrew Mawson]

Well at the time of writing there are 94 people watching it and it's north of £1300. The number of people who have rung me up asking me to stop the auction must now exceed ten !

AWEM

[Andrew Mawson]

Well rest assured that it's not getting 6mm - it's getting 2.5mm as it's in the ground and has been for a year! Now I'm about to commission the gate it's the fusing I was asking about NOT the cable. Standing load of the electronic bits is less than an amp. Normal 'peak' load is something like 3 amps which happens when the gate opens - maybe 6 to 10 times a day for 25 seconds, then once every blue moon I >MAY< plug an angle grinder in which >MAY

[ARW]

Never let logic get in the way.

[ARW]

But it is not 3 phase is it? (rhetorical)

So Tim was right that you can ignore the 3rd core and treat it as 2 core.

Your CCC is 33A if the cable is buried.

If it was me I would try using a C type 20A MCB and see how things go.