So L1 & N through one hole and L2 & L3 through a different hole in a steel enclosure? to me that's a non-compliance with 521.5.1.

The heating effect with relatively small circuits and especially with thin-walled steel is relatively small however. I've seen all four supply conductors entering a fully loaded 100A TPN DB through separate holes and the heating effect has been unobservable (certainly less that the normal heating of the cables themselves). Several hundred amps through one of the old fashioned cast-iron switch-fuses might be quite a different situation though - but the regs don't distinguish, but apply the same principle regardless.

So for me, it's not compliant, but not a disaster. The old school solution was one big hole,with a sheet of paxolin or similar with a slightly smaller hole in it sandwiched in the gap between the two pieces of trunking (or trunking and DB enclosure) to keep the cables away from the sharp edges of the steel. I guess these days a length of flexible grommet strip might be used instead.

   - Andy.

Thanks for your reply! I’m just trying to distinguish where to draw the line on the non compliance.

These circuits then go on to exit and re enter for the connection of kWh meters all in a row but with 2 inch centres via pvc glands to maintain the ip rating of the trunking, then bush  individually into switch disconnecters for each circuit probably 6inches part, then exit again individually in swa cables all glanded in a row a banjo spacing apart.

To me everything apart from the initial pass though the bushes looks normal and compliant but in theory could have the same effect as when the circuits pass through the bushes? 

regs to one side, brass bushes are not affected by magnetic field, so the hole mayy be bigger than you think

(and in some quarters, when cutting the dog bone slits between holes in steel plates, the slot is then brazed shut for reasons of blast containment or ingress of contamination.)

Now  It is not too clear from your description, how the cables are paired - it would be OK if it was  L1A L1B N1     L2A L2B N2  L3A L3B N3 where N1 carries the return current from L1A and L1B etc.

But even if it is worst case, for thin sheet steel spaced off from the wire by several mm because of the brass bush, you need hundreds of amps of unbalanced current to give noticeable heating. If you had unbalanced cables in long runs of steel conduit it is a different story.

not compliant perhaps. but assuming not obvious heating, either a C3 or no action needed.

Mike.

Thanks for your reply! That’s exactly how they are paired. Both circuits on L1 all pass through one bush together, live and neutral. Then likewise for L2 and L3

I read it differently again with two possibilities: (1) L1, L1, 2 x N through bush 1; L2, L2, 2 x N through bush 2; L3, L3, 2 x N through bush 3. Or (2) something like L1, L2, 2 x N through bush 1; L2, L3, 2 x N through bush 2; L3, L1, 2 x N through bush 3. Either way, provided that the neutrals for each circuit go through the same bush as the line conductors for the same circuit, there will be no problem.

then its OK. Each bush carries currents that sum to zero

Mike

Thanks, appreciate your help.