Studies engineer (PE)

[jimmy Antoun]

NEC 2008 states in 392.8 D paralell conductors has to be grouped as A, B, C and N or G as group1 same as group 2 and so on but not as A,A, .., B,B, .., C,C,.. , N/G,N/g, .. . If the contractor for physical limitations cannot grouped like that can derating be used Fore.g. 8 sets of 500KCM are laid 8 phase A , 8 phase B, 8 phase C ,8 N /G side by side and not as group ( which is supposedly a violation but he has no other alternative) does he has to undo everything or leave as is and derate the circuit ( due to the unbalanced reactances and the heat which will be generated). this is especially true for 392.11 B
Any body comments is greatly appreciated

 

[rbalex]

The grouping required is not a derating issue but a magnetic induction issue.

 

[Julius Right]

There are standards to follow in order to calculate the unbalance between parallel
single-core cables-IEC 60287-1-3 for instance. The unbalance could be up to 50%
that means one cable current could be 150 A and another 50 A for a total 800A [100A average per cable]
Since the calculation of cable ampacity in a cable tray is based on supposition all the
cables are equally loaded a derating for 150% load will be very conservative.
See J.Stolpe-Ampacities for Cables in randomly filled trays/1971-and"
ICEA PUB. P-54-440-Ampacities of Cables in Open-top Trays/1980"
If one could calculate the actual heating of the cable unequally loaded then it is possible to derate correctly the ampacity .

 

[GoldDigger]

If one could calculate the actual heating of the cable unequally loaded then it is possible to derate correctly the ampacity .

One of the possible side effects of the magnetic induction issues (as rbalex said, the real reason for the grouping requirement) is that it can increase the current imbalance among the wires of a single phase. It can also potentially cause inductive heating in the cable tray or other wire way components. And it will cause a greater stray magnetic field near the cables which could affect other sensitive equipment.

BTW, a variation from 150 amps to 50 amps is far more than a 50% imbalance.

 

[Smart $]

...

BTW, a variation from 150 amps to 50 amps is far more than a 50% imbalance.

Not familiar with the standard he is referring to, but I would think 50% unbalance would be a measure of the actual unbalance between conductor currents rather than a comparison of balanced to unbalanced currents. For the example presented that would be a 133.3A to 66.7A imbalance on conductors otherwise balanced at 100A ea.

 

[Smart $]

... If the contractor for physical limitations cannot grouped like that ... (... but he has no other alternative) ...

I'm curious what physical limitations make the required grouping "impossible"?

 

[GoldDigger]

Not familiar with the standard he is referring to, but I would think 50% unbalance would be a measure of the actual unbalance between conductor currents rather than a comparison of balanced to unbalanced currents. For the example presented that would be a 133.3A to 66.7A imbalance on conductors otherwise balanced at 100A ea.

One way of measuring the imbalance would refer both to the average, so 125 would be 100 + 25% and 75 would be 100 -25%, for a total imbalance of either 25% or 50% depending on your definition.

The perhaps more commonly accepted measure of imbalance is the difference divided by the lower of the two currents. In that case 125 divided by 75 would be 1.66 for a 66% imbalance.
By that same definition, a 50% imbalance would be 80 and 120.

And this whole discussion is still irrelevant to the original problem, since heating is not the primary issue.