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Posted By Topic: 2.5mm paralleled

Nov 17 2019 06:41

House was built in 2007. Was 2.5mm paralleled in the switchboard to feed rcd's and other mcbs from the load side of the main switch allowed in the rules back then? I saw a total of 6 cables coming out of the load side of main switch feeding three groups of sub circuits.

Nov 17 2019 20:55

paralleled or Looped ?
6 wires out of the main switch would sound about right.

Nov 17 2019 21:30

2007 wasnt 'Back Then' !!

The first edition of AS/NZS 3000 was in 2000, and apart from some amendments and tweaks, is still basically the same rules we have now.

1976 was 'Back Then' !

Nov 17 2019 22:13

There was also a NZS3000:1997 but that doesn't help to answer the question.

I am going to go with the fact that the wiring to the line side of the protective device is not part of the "circuit" and therefore 3.4.3 doesn't apply to that wiring as it only applies to "circuits".

According to the definitions, a final subcircuit and a submain etc are a "circuit" and they originate at the load side of the protective devices.

The definition of "circuit" indicates that the circuit includes the live conductors (so both active and neutral) as well as protective conductors (if any) - so doesn't necessarily seem to include the line side of protective devices - but obviously includes FSCs and submains.

Despite whatever the interpretation of the rules might be, the 2.5mm2 conductors are rated at 30+ amps each, so the combined rating is 60+ amps and more than enough for the job.

Nov 18 2019 07:26

I think I agree with Doug in thinking that clause doesn't apply to switchboard wiring

Although I could be way off but I though AS/NZS 3000 only applied to installation wiring

If you look at note 3 under 3.8.1 it says

"3 Switchboard wiring is not regarded as installation wiring but the
AS/NZS 3439 series restricts the green/yellow combination to the
identification of earthing conductors."

To me this note seems like a general statement and not just directly related to this clause

So I figured it would be AS/NZS 3439 that applied to this wiring and not AS/NZS 3000 but I have no idea what AS/NZS says about paralleling cables

Nov 18 2019 07:48

3439 has been updated as AS/NZS61439.x series

There's nothing in those standards about parallel conductors on the line side, or any restriction on conductor size to be paralleled.

Nov 18 2019 09:00

AS 3439 series and AS/NZS 61439 series do not apply to switchboards constructed from a plain switchboard panel and component parts MCBs fuses below 100 amps as was used in domestic installations in cira 2007 or thereabouts.

Some of the switchboard internal wiring methods used at that time relied solely on the maximum current in any conductor that is part of the switchboard internal wiring.

This wiring is, in general, is in free air so will have elevated current ratings and is also very short lenghts (300 mm max) so that voltdrop will be very small.

A ring-main type wiring 2.5 mm2 of all protective devices was often used to supply a maximum of 63 A HRC fuse used by the line company at the point of attachment to their network or a 63 A main fuse (or 63 A MCB) on the switchboard. In some line company areas, but not all, you had both fuses provided.

Nov 18 2019 11:34

The note about switchboard wiring not being regarded as installation wiring is located in clause 3.8, in order to emphasise that colour ID for installation wiring does not apply to swbd wiring.aspects; it isn't installation wiring.
However in terms of wiring a switchboard safely the same rules for CCC etc that apply to installation wiring remain valid - it's not as if the physical properties of the conductor change just because you're using it in a switchboard.

Table 4 of "3008.1.2" gives the in-air CCC for 2.5 as 34 A.
2.5 isn't allowed for conductors in parallel in installation wiring [3.4.3]; but since this isn't installation wiring the "min 4 mm2" restriction does not apply. So as conductors in parallel, the combined CCC will be 68 A - and therefore the 63 A supply fuse will provide overload protection. So 2 X 2.5 parallel as feeder to an RCD is adequately protected against overload. A ring circuit isn't exactly the same as conductors in parallel, but close enough.

The other relevant question is whether it's capable of carrying the max demand of the circuits it supplies; and for most cases of a single dwelling, 3 such groups will be fine.