Originally Posted by jcccd
It's not so much of a conductor heating problem, but rather a conduit heating problem. A conductor carrying current will create around itself a magnetic field. If you have all phases in the same conduit (e.g., Phases A and B in a 120/240V single phase system, or Phases A, B, and C in a three phase system), then the magnetic fields created by each of the conductors tends to cancel out the fields created by the others. As seen by the conduit itself, then, the field is essentially zero.
But with only one phase in a given conduit, presuming the conduit is made of a magnetically permeable material, then there will be a significant field seen by the conduit. Therefore, current will flow in the conduit itself, and that is one bad thing. The other bad thing is that the conduit will get hot. That is why the rules limit the use of "iso-phase" installations to non-magnetic raceways.
Welcome to the forum.
Charles E. Beck, P.E., Seattle
Comments based on 2014 NEC unless otherwise noted.