Spec'd out; pipe on site, ready to dig and bury. Customer wants to increase amperage - future capacity. Loads are all motors - 2 soft start.

Spec'd at 1290A - 3 parallel 4" RMC runs, each containg A,B,C,N,G - 500 MCM Cu + 1 3/0 G.

He now wants 1600A. My solution: 4th pipe run - give me 1720A; he wants to add wire to the planned 3 runs.

Fill factor is ok to add 4th wire, but it would lead to A,A,B,C,N in pipe 1, A,B,B,C,N in pipe 2 and so on. I was always taught (in RMC)to keep A,B,C,N together in each pipe, or you end up with a "shorted turn" effect due to any imbalance, and heat up the pipe.

All I can find in a first look is Article 300.20 - "all phase conductors shall be grouped together".

You shouldnt have any induction problems as all the phases and neutral are present in each conduit. You should make sure the paralleled conductors are hte same length and size. Also, why not use the RMC as the ground?

You cannot run the conductors like that. It is a violation of 310.4 and will produce excessive heat. Also you don't have enough copper with 3 runs of 500kcmil for 1290 amps...that is only good for 1140 amps. You have to use the 75° column for this application unless all of the terminations are listed for 90°....very unlikely. Don

You could make it work for 1600 amps with 8 350 kcmil in each raceway, assuming that the grounded conductor is not required to be counted as a current carrying conductor. The 90° ampacity of 350 is 350 amps and we can use that for purpose of derating. The derated ampacity would be 280 amps and 6 conductors at 280 gives you a total ampacity of 1680 amps. Ten 350 kcmil THWN-2 will fit in the 4" rigid so you will have room for the EGC. Note that it will have to be a 4/0 for each raceway. Don

why would there be any heat from induction with all the phases present?

You have more than just all phases in each conduit...you have an extra conductor from a single phase with no conductors of the other phases to cancel the fields. This would be the same as putting A in one conduit and B in another conduit. Remember this is a ferrous metal conduit. If the conduit was non-ferrous, you would not have the heating problem, but there still would be a violation of 310.4.

yes but it is the A phase from the same system, which has the B,C,and N in the conduit. How would they not cancel out the magnetic effects in the condiut?

ASSume you have 100 Amps per phase CONDUCTOR. That will give 200 A phase A flux (2 conductors), 100 A phase B flux, 100 A phase C flux. The flux due to 100 Amps of A + B + C will cancel, but you still have an EXTRA 100 A of phase A flux. There is no more phase B or C flux to cancel it out.