310.10(H)(3)

[xformer]

Can someone give an example of installs where the cables don't match the physical requirements of another?

 

[roger]

You could use THWN copper for one phase, leg, or neutral and XHHW AL for another.

IOW's A phase conductors would have to have the same characteristics but B could be different.

Roger

 

[kwired]

The idea behind the requirement is to make sure each parallel set has same overall impedance in each sub unit. If not same the lower impedance unit will carry the most current, the goal is to keep them equal so they will carry equal current.

As mentioned it doesn't matter if phase A is copper, phase b is aluminum and phase C is copper but a different size, but all parallel conductors of A need to be same size, type, and length to ensure the current on A equally divides - same for B, C and the neutral.

Also you can not run a parallel set with ABCN in PVC and the second parallel component in steel conduit, they won't have the same characteristics and will have different impedance.

 

[GoldDigger]

Not that there is likely to be a reason to do so, but you can use a single conductor for one phase line and a parallel set for another phase line. But you cannot then run the parallel wires in separate raceways with the individual conductor in one of them.

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