electricalforum.nz

Hi,

I have a 15 amp spa and 20 amp sauna to go in at a house. Options I see are 6 mm out to sub board and then split from there, 15 and 20 rcbo, but then earth is only 2.5mm, just searching through rules but I thought PEC needed to be 6mm just cant find relevant rule. Distracted by hurricanes game, not sure if I can get that out there, may have to be an electrode. If so what do you do with earth in the 6mm feed. Would you still earth one end so cable had protection?

Thanks in advance.

It's fine to use the earth in the cable

The minimum 4mm size is for a main earth (between earth bar and electrode) the earth from a switchboard to another is not a main earth so table 5.1 may be used which says 2.5 is suitable for 6mm live conductors

As Jamie said, the 2.5mm² earth is fine for that submain.

But why not just run a 32A circuit and connect both?
Or just run two 2.5mm² circuits? Depending on the length to check the voltage drop.

Hey ive just been browsing the thread trying to answer a query and this is pretty on point. A work mate was adamant a submain earth was fine at 2.5mm. This is explained well enough here. But does a sub board (which is a switchboard and therefore has a neutral bar and a earth bar) really not need a 4mm minimum earth? The other query was bonding the metal sub board (wiring enclosure?) sureley needs a 4mm earth minimum? Any help would be great. Tried regs and getting lost.

Earthing conductors are sized according to the fault current they have to carry; for the short time they have to carry it.
That's broadly in proportion to the size of the actives. Clause 5.3.3.1 sets the requirements; and provides 2 methods of determining required size (from Table, or by calculation).,
Minimum size 2.5 mm2 applies only where the conductor is not incorporated into a multi-core cable; which is about avoiding physical damage.

Similar for conductors whose sole function is equipotential bonding, minimum size (generally 4 mm2) is about avoiding damage and not about ability to perform the function. the currents involved in equipotential bonding are low, so big conductors generally aren't needed. Hoiwever in some cases a higher minimum is set, eg 16 mm2 for hazardous areas; which is because bonding has to perform better there to ensure differences in potential between items of equipment are kept to an absolute minimum; to eliminate risk of sparks.

So yes, a 6 mm2 circuit is fine with a 2.5 mm2 PEC, regardless of whether it's a submain or a final subcircuit.
The earthing function is exactly the same; and being a submain doesn't alter the fault current level.

just to be clear, the wire connecting the swbd enclosure to the earthing system is not a 'binding" conductor; it's a protective earthing conductor.
The max current it can ever carry is the prospective fault current at end of submain - so why would it need to be any bigger than the submain PEC?