Multicore cables in parallel - hysterisis (eddy currents)

Problems can occur with multicore cables, but only if there's an imbalance in the "going" and "return" currents in each cable. Normally such arrangements aren't permitted (521.8.1) and where cables are used in parallel things should be arranged so that there's a more-of-less equal division of currents - so again there should be a pretty good balance within each cable. I can see there may still be some small imbalance in practice - but as long as the imbalance is relatively small (<100A say) and we're talking thin steel sheet enclosures rather than 1/2" cast iron, the result is likely negligible (there is a suggestion that the whole 521.5.1 regulation can be safely ignored for DB where the imblance per hole is <100A (from memory).

   - Andy.

Thanks for the responses, the concerns are that inbalance could be greater than 100A as we have supplies feeding 800A rising bus bar systems in science labs to be used during a later fit out. the problem would be solved by installing a slot. And if 521.5 were to be ignored this would need to be recorded as a deviation from BS7671 sanctioned by the designer?

Let's make sure that we understand the situation. As I read it, you have two (or more) cables in parallel serving one circuit. Presumably, the cables run together and are the same length.

How does any imbalance occur?

ETA: I take it that each cable has 3 lines, possibly a neutral, and a CPC.

Let's make sure that we understand the situation. As I read it, you have two (or more) cables in parallel serving one circuit. Presumably, the cables run together and are the same length.

How does any imbalance occur?

ETA: I take it that each cable has 3 lines, possibly a neutral, and a CPC.

correct, cables up to 240mm 4c (L1, L2, L3, N) SWA's feeding panel boards or rising bus bar systems with circuit protection rated up to 800A. the three phases are not balanced, I can guarantee they won't be balanced apart from the parallel cables feeding chillers which are 3c (L1, L2, L3) 240mm SWA's. It is possible that the rising bus bars could feed single phase boards in the future, even if they were three phase boards they wouldn't necessarily be balanced.

The question really is a BS7671 compliance question.

Gareth Hayes said:

the three phases are not balanced

That does not matter because the imbalance will be going the other way in the neutral. So if you have two 240 mm² cables in parallel, half of the current in all four conductors will go in each cable (subject to minor differences in the impedance of the cores) and each cable will be balanced overall.

If the chillers are powered by 3-phase motors and there is nothing else fed by the circuit(s), they do not need a neutral, but otherwise the neutral is there to return the imbalance to the transformer.

Does this help?