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.

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?

Thanks for the responses, the concerns are that inbalance could be greater than 100A as we have supplies feeding 800A

I would hope that there was something very very wrong if two cables in parallel were supplying 800A and there was a 100A imbalance within each cable. Its not the balancing between phases that's the issue - on a 4-wire circuit any differences between L1/L2/L3 would be carried by N in the same cable - so still balanced overall as far as the cable is concerned.

The problem arises because the current might not divide equally between the two cables - and even then that's not a problem if the L and N currents divide in the same way in each cable. Using single phase number just for the sake of simplicity, if one cable carried 350A in both L & N and the other 450A in both L & N there still wouldn't be any imbalance overall in each cable - you'd need a situation where one cable was carrying 350A in L and 450A in N while the other cable carried 450A in L and 350A in N - which would need a very very peculiar set of circumstances given the normal rules for parallel cables (same c.s.a., length etc.) not to mention the physical natural tendency for conductor in parallel to self-balance (if one conductor handles more current than the other it runs warmer, increasing its resistance, so increasing the bias for more current to take the other conductor).

   - Andy.

It rather depends how those multiple cores are wired. If for example  there are loads wired between say the L1 of cable 1 and any other phase or N of say cable 3 then you do fall foul of the '7671 advice exactly as much as you would if they were singles doing the same thing. If however all the 3 phases and neutral of any given sub-main, are co-located in the same cable, they are already passing through the same hole, and the magnetic effects are cancelling. If they are not, and wiring cannot be re-arranged to achieve this, then the hole needs to be made common to both cables by breaking the magnetic loop that surrounds any un-cancelled current. To do that the fastest cheapest way is a slot, to make a dumbell / dog bone shape, and then backfill with braze metal and grind level, or epoxy (HY60 or at a push Araldite rapid ?) and flushtop,  or leave it open to acquire a mixture of fluff and iron filings, depending on the installation quality and location of the offending panel... can you tell I'm not a fan of open slots?  Outdoors it tends to be leaves and bits of bird nest.

If at any downstream point for example, the neutrals of two cables in different holes were exchanged or even combined, then there is scope for that out of balance trouble ..

Mike

mapj1 said:

It rather depends how those multiple cores are wired. If for example  there are loads wired between say the L1 of cable 1 and any other phase or N of say cable 3

That sounds rather like a borrowed neutral to me.

As I understand it, there are two (or more) identical cables in parallel, so each takes half the current of the combined conductor. Let's draw 100 A in L1, 200 A in L2, and 300 A in L3. Because of the imbalance, you get about -173 A in N. So now we have 50 A, 100 A, 150 A, and -86 A in the conductors of each cable. That still leaves each cable balanced, so the arrangement is compliant.

mapj1 said:

It rather depends how those multiple cores are wired. If for example  there are loads wired between say the L1 of cable 1 and any other phase or N of say cable 3

That sounds rather like a borrowed neutral to me.

As I understand it, there are two (or more) identical cables in parallel, so each takes half the current of the combined conductor. Let's draw 100 A in L1, 200 A in L2, and 300 A in L3. Because of the imbalance, you get about -173 A in N. So now we have 50 A, 100 A, 150 A, and -86 A in the conductors of each cable. That still leaves each cable balanced, so the arrangement is compliant.

Hi Chris,

You're understanding is correct. While the hysteresis would be negligible the wording of 521.5.1 suggests all circuit conductors are collectively surrounded by ferromagnetic material. so my concern is with compliance of the wording, it may well be that 521.5.1 is written in a way that causes confusion and ought to be re-worded with a caveat for multicore cables in parallel?

I see what you mean. That would be a very literal interpretation. I have no problem with that, but in practice, there will be no difficulty.

Being too literal could cause problems with any conductors in SWA in parallel. Would you have to combine their wire armour using some form of twin gland at the point of entry?

Even the armour itself would count as a ferrous enclosure - applying that interpretation of 521.5.1 would mean that using SWA for parallel applications at all would be out of the question.

  - Andy.

+ ½

I had thought about that and I think that the armour does satisfy the definition of "enclosure", etc. So, if you combine two or more enclosures, they become one enclosure. (We can ignore the gap which a brass gland provides.)

Not really - the wires in the SW armour are neither  magentically in good contact nor forming a closed magnetic path enclosing a net current.. Had you said welded conduit, I'd have no problem with the statement.

I'm still not seeing the problem, so long as the cores in each cable form a complete circuit, that is to say that all 4 current carrying cores that carry associated current, are together.

Actually, for sensiible thicknesses of steel box, and sensible hole sizes you need many tens of amps going out of one hole and coming back via another to raise things by a few degrees.

Far more common, with unbalanced currents and it does apply to SWA or AWA as well, is the transformer-like problem of armours earthed at both ends forming a one turn loop for voltage pick up. Dog bone slots don't help much there, as that really is an eddy current effect .

Its why single core SWA does not exist....

Strictly, the problem we are discussing is not eddy current, rather it is magnetic hysteresis, though i appreciate even some publications that really should know better get confuse the two.

Mike.

Hi,

I'm not disputing the physics here. The question is compliance with BS7671.

Gareth

The question is compliance with BS7671.

The issue then is the word "collectively" - does it refer to all conductors of the circuit or merely some suitable collection of them. Certainly ambiguous (online dictionaries suggest both "as a group; as a whole" as a definition - so could be either way). Given the physics, "a suitable group" might not be an unreasonable interpretation.

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

   - Andy.

AJJewsbury said:

Also consider how the reg could apply to other situations - say a steel adaptable box used as a JB for lighting - it would be quite normal to take just the L and SL (without N) through one hole for a switch drop, SL and N through another to the light - so neither contain "all" the conductors of the circuit - but do contain a suitable (balancing) group.

But 521.5.1 does not say that all of the conductors must enter (or leave) together. It simply says that the ones which do must go through the same hole.

Hi All,

Thanks all for your comments!

I have just spoken to the technical helpline at the NICEIC who have confirmed that all conductors of the circuit (even consisting of multicore cables) must enter the same aperture of a ferrous enclosure in order to comply with BS7671.

Cheers

Gareth

Again  this falls back to definitions, in this case 'circuit' which seems to mean something different in standards land, to the meaning for the broader world of electron herding. Current flowing out to a light switch and back does form a circuit in an electrical sense. - but in standards speak is only part of a circuit.