Correct me if I'm wrong in my thinking, please. I have a customer that currently has 3 seperate runs of 4/0 URD triplex. Each run is in its own Sch 40 PVC pipe. They travel 175 feet underground from the main service to a control building. One of the triplex runs are feeding a 200 amp(120/240)single phase panel. The other two are paralleled into a 400 amp disconnect feeding a phase converter.
My dilemma is, I am removing phase converter and installing true 3 phase service (120/240). Can this be done(rightfully) with the 3 runs. My thinking is to use all 3 runs to feed 400 amp switch. Can I parallel these 3 runs in the following manner?
Run #1: A,B,N Run #2: B,C,N Run #3: C,A,N
I would then run a new service to the single phase panel. Or should I just bite the bullet and start over with quadraplex?
Presuming that the POCO can give you three phase, I'd say that you would have to replace the wire. Each of the 3 neutrals that in parallel will carry 1/3 third of the unbalanced portion of the total. This could introduce counter electromotive force CEMF or impedece in the triplex.
A good way to look at it in an extreme condition example. 100 amps on any one phase would put 50 amps on it's respective phase in only two of the triplexes and 33 amps on each of the three triplex neutrals. Another way to look at it is that 100 amps between two phase will have one of the triplex balanced while the other two triplexes will be severely unbalanced.
Each of the unbalance triplexes will generate a megnetic field around them and impede current flow. Worse case, something in the triplexes can burn up. The only way your proposed plan would work if there was an even load on all three phases all the time. That is why it is very important to have all the conductors of a circuit in the same raceway.
Have you done a neutral load calc? I'm just wondering if those triplex URD with reduced neutrals might possibly be a problem on a 3-phase system with multiple single-phase loads. You said 120/240 3-phase, but is that correct?