I have a service where I'm installing a 240V/200amp single phase distribution panel 400 feet away from the main service. My total demand load is 141 amps and is 95% 240 volt heat trace circuits. I sized the service for 160 amps with no more than 3% voltage drop on each circuit. When I sized this I came up with 2 parallel #4/0 conductors. Can I run this in one 3" conduit instead of using two parallel runs of conduits?
You can run multiple parallel sets in the same conduit, and obviously you'd also have to account for applicable ampacity adjustments. When you do this, you only need one EGC per conduit.
What you cannot do, is when you run more than 2 parallel sets in multiple conduits, you cannot run dissimilar numbers of sets of wiring in different conduits. For instance, if you had 3 sets in parallel, you couldn't run 2 sets in one conduit, and 1 set in the remaining conduit. All sets need to have the same conditions of use, which means you are required to uniformly divide the sets among the conduits.
Another thing to keep in mind when voltage drop is a governing factor, is that there is little advantage to running two parallel sets of the equivalent total KCMIL of a single parallel set. Resistance in general is inversely proportional to KCMIL. The slight performance advantage to paralleling smaller wires of the same total KCMIL as a larger one, is that the "AC Resistance" due to the additional inductive effects is slightly less in smaller wires. Paralleling is usually an advantage for ampacity reasons, because the surface area to volume ratio of smaller conductors is greater. But not nearly as much of an advantage for voltage drop curtailment.
