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Posted By Topic: 20 amp power circuit

Jul 15 2015 07:00

I have run a new 2.5mm power circuit to 3 new powerpoints and installed a 20 amp Rcbo the total cable run is 25 meters and run in conduit half the way clipped under the house the rest it is not run thru bats .
Could someone please tell me if this is ok or would I be safer to replace with a 16 amp Rcbo

Jul 15 2015 09:02

in residential you must assume bti, so must use partially-surrounded as best case.
2.5mm2 tps, in enclosure, partially, = 22 a

18.02 mv / a / m; with 10 a at 2nd last socket and another 10 a at last, you probably fail volt drop; and may fail efli if not on rcd.

Jul 15 2015 09:06

The problem with the 20A rating for that circuit is the voltage drop. You are at about 4% for that run. So unless the mains are very short, it\'s a no-go.

At 16A the VD is 2.9%. So probably ok with that.

But you need to check the VD of the mains to check.

Apr 14 2019 17:11

Sorry to dredge up an old topic. Since A2 in New Zealand domestic we've had to assume the presence of insulation when calculating current carrying capacity of cables. I was taught BTI and partially surrounded. But, 3.4.1 isn't specific about what type of insulation we must assume. Where is that written? Is it building code?

Apr 14 2019 20:10

There isn't any details. It's either partially or fully surrounded. The type of insulation doesn't make any difference.

Also, probably my earlier post about voltage drop is probably wrong... as you can work on 50% current rating for a distributed load.. 3.6.2 exceptions..


Apr 14 2019 20:44

Reason I ask is that I was taught to assume partially surrounded. Since the 2018 amendment Aussies are also subject to this assumption however chatter on FB suggests they’re assuming worst case and fully surrounded. I couldn’t find a reference for what I’d been taught and hence my question.

Apr 14 2019 21:55

Aussies have been known to be wrong on occasion... hope it's not the same FB pages I frequent ;)

Insulation only on one side is the key. So the cable should really be clipped to a stud - like that's ever going to happen..

3008 has the references for partially or fully surrounded.

Apr 15 2019 08:52

There are really only 3 ways you end up fully surrounded.
Either you've come along later and drilled through, in which case you know it's fully surrounded and rate accordingly.
Or you've run across existing BTI and someone then lays another layer.
Or the cables were there first, and someone uses that spray-in foam.

Installed in walls, I've never known or heard of an insulation installer bothering to cut the batt around the cables; they just push them away or tuck the batt behind the cables.
So usually safe to assume partially surrounded.

Point is you are not required to anticipate what sort of BTI may be used; and you're not required to assume worst case.
The rule is there to stop idiots rating for none when they know damned well BTI is going to be added before the wall is closed up.

Apr 15 2019 21:02

We're currently working on an old villa. New swbd with MCBs. But before we've finished electrical work for the renovations, an insulation installer haas filled all walls with blown-in fibreglass like cotton wool. The existing power circuits - probably dating from c 1980 - are only 1.5mm2.

Apr 16 2019 11:26

Haven't struck blown-in fibreglass before.
But it's the method of installation - sprayed in - rather than the substance sprayed, that leads to "completely surrounded".


Apr 16 2019 14:49

and another product i'm sure damages the electrical integrity of the house, many of the older power points and light switches i've taken out have loose fill trapped in their workings, surprised there's not a procedure required to prevent this, but given the lack of spine from our EWRB it figures that this has gone on like it is.