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Posted By Topic: Query on submain earth size

Jan 24 2016 10:32

Situation is that an installation has a switchboard, 40A main switch, fed by 6mm. This board is being demoted to a subboard, fed from a new main board, breakered at 40A. The 6mm supply cable, now a submain, will be extended to the main board using 16mm. The question is what size should the earth conductor be.

Table 5.1 gives answers. If we assume the active conductor required is 16mm, then the earth is 6mm. If we assume the active conductor is 6mm, then only 2.5mm is required. It would be convenient to use 4mm. is quite clear, to use table 5.,1 one has to use the "largest active conductor supplying the portion of the electrical installation to be protected", so I need to use the entry for 16mm.

If this were a main rather than a submain, then would provide the necessary relief, I can legitimately use 4mm, or even 2.5mm. But there appears to be no such relief for submains.

Is there something I have missed?

I've done a quick calculation based on the measured PSSC of the sockets there today, and it comes out to 3.2mm, so if the PSCC doesn't go up dramatically as a result of the new supply, then that would be legal alternative. But I suspect the PSCC will irocket, as the sockets have 1.5mm final circuits, which I'm suspecting where most of the loop impedance will come from.

Jan 24 2016 15:12

"The 6mm supply cable, now a submain, will be extended to the main board using 16mm. The question is what size should the earth conductor be."

So you joining 6mm to 16mm?

Are you doing this for voltage drop?

Just wondering why you aren't running 6mm the whole distance?

Jan 24 2016 16:11

The 6mm run from the old board to the meter box is already there, it's being extended to where the new main board will be. 16mm ended up the lowest cost option. Oh yes, and to keep voltage drop within acceptable limits.


Jan 24 2016 17:02

So your joining in the meter box?


Jan 24 2016 17:11

Hell no! The old 6mm is being fished out of the meter box and crimped to the 16mm extension in the wall adjacent to the meter box. Or, at least, that's the plan.

Jan 25 2016 09:18

I would apply the exception (b) from, even though it's a submain and technically that Exception is only for mains.

the point is to have a PEC that is large enough to have an inpedance low enough to pass enough current to permit the overcurrent protective device to operate.
PEC sizing is usually done "simple" as per Table 5.1 in relation to size of active conductor; so you could comply by running large PEC for the new 16 mm2 part of the circuit, and a smaller one for the 6 mm2 section.

But Table 5.1 isn't the only permitted method, and if you apply calculation instead [] the answer will instead be matched to rating of protective device, which is what matters. Being a submain the required trip time for EFLI is 5 secs; and if you want to be absolutely thorough (or your a masochist), do the calc from for short circuit protection as well.

But rather than do the actual calculations (other than for the mental exercise), just apply Table 5.1 for the 6 mm2 section. The values on the table work for subcircuits requiring 0.4 sec trip time, so they are going to keep you on the right side unless the length of the 16mm2 section is huge.
For a confirmation; most cable specs will give you the resistance per km so you can easily find the resistance for the length of run, and you can compare that to Table 8.2 fr 40 A fuse (0.43 ohm) and know you are OK for EFLI.