3 Phase Wiring

[DASTX]

I was recently told of 480V 3-Phase wiring mistake made at a plant. The run was 3-350MCM per phase through 3-conduits. The mistake was that all of one phase was run in each conduit, 3-A Phase in one, 3-B Phase in one and 3-C Phase in another. The 3-conduits terminated into an electrical enclosure.
One problem was heat created at the enclosure where the 3-conduits entered the enclosure. The solution was to break the connection at the enclosure, in other words a rectangular area was cut out around the conduits where they entered the enclosure, thus eliminating the metal to metal connections of the conduit and enclosure.
The question is what was causing the heat build up at the conduit/enclosure connection?
And what consequences if any exist in the conduits that still have all of 1-phase run in each?

 

[George Stolz]

It is not necessarily a mistake - see 300.3(B)(1), exception.

What was the wiring method employed?

 

[tlaidman]

When 1 phase is run through each knockout the magnetic field around the conductor causes inductive heating between the knockouts this is eliminated by putting the three phases in the same conduit threby canceling the field.

 

[etszap]

300.3(B)(1), exception doesn't apply to metallic raceways.
"cut out around the conduits where they entered the enclosure, thus eliminating the metal to metal connections of the conduit and enclosure."

 

[Psychojohn]

The metal conduit will act as the core of an inductor which will throw off the PF, increase the inductive reactance of the conductors, therfore reducing the current to the motor.
I also believe that the conduit will still get hot.

 

[nakulak]

you didn't mention neutrals - is this a delta ? sounds like an engineer should check it out, but it might be in the realm of possibility to simply rewire so that you have the 3 phases in each conduit ? (if enough slack).