Parallel conductors of different sizes

[Steve-R]

Found an existing installation on a 240v delta high leg system. Two sets of paralleled conductors, some are 250MCM, some are 300MCM. I wasn't able to count quantities of each as it was hot.

So, what is the allowable ampacity of this feeder? It is currently fused at 600A (all three legs) and this is what I'd like to get from it, obviously that isn't possible. The primary problem I see is that current won't be evenly divided between the different size conductors. It is worth at least 255A (the ampacity of the 250MCM). Is there some legal way to calculate a higher allowable ampacity so it doesn't have to be removed?

Thanks!

 

[don_resqcapt19]

It is not uncommon to have three different size conductors on a high leg system. The high leg being the smallest, the grounded conductor larger and the other two phases larger yet, however that would require a smaller OCPD on the high leg as compared to the other two ungrounded conductors.
If you have two different size conductors on the same phase, it will have to be changed. There is no calculation that you can do with unequal size paralleled conductors. If the smaller conductors are only used for the high leg, you can install a smaller fuse for that leg. The high leg OCPD and conductors would be sized based on the three phase load and any line to line loads that are connected between the high leg and one of the other phases. The other two conductors would have the maximum unbalanced line to neutral load added to them and the OCPD and conductor sized to that total.