3 phase service to MRI clinic

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I have a question concerning a 120/208 3 phase 4 wire service. This question has come about because of the very expensive and time prohibitive nature of running a new service.
The existing service is 208 3 wire 400amp with three 500kcmil that the plans call for 120/208 4 wire. There are two parallel 3" EMT's running a few inches apart over 150' through the bowels of a hospital which have 3 500kcmil in each. One conduit is a spare with cables installed.
We realize we cannot use the spare conduit with the three 500's ( spares) for a neutral due to transformer action and heating of the pipe.
However if we used one conduit for phases A & B and used the included third phase as a neutral then use the second conduit as C phase and used a second conductor in that conduit as a neutral which would be connected at both ends ( paralleled) will it cancel out the objectionable fields in both conduits having a neutral in each pipe? :?
 
You still have the inductive heating issue as you do not have all of the conductors of the circuit in the same raceway.

You could install one of these at the load end to get the required neutral.
 
I have a question concerning a 120/208 3 phase 4 wire service. This question has come about because of the very expensive and time prohibitive nature of running a new service.
The existing service is 208 3 wire 400amp with three 500kcmil that the plans call for 120/208 4 wire. There are two parallel 3" EMT's running a few inches apart over 150' through the bowels of a hospital which have 3 500kcmil in each. One conduit is a spare with cables installed.
We realize we cannot use the spare conduit with the three 500's ( spares) for a neutral due to transformer action and heating of the pipe.
However if we used one conduit for phases A & B and used the included third phase as a neutral then use the second conduit as C phase and used a second conductor in that conduit as a neutral which would be connected at both ends ( paralleled) will it cancel out the objectionable fields in both conduits having a neutral in each pipe? :?
If your loads are all single phase and are all wired in wye (line to neutral), and there is no connection at the load end between the neutral wire used for loads on A and B and the neutral wire used for loads on C, then yes, you could get zero net current in each of the two conduits.
It might even be NEC compliant if you consider it to be one partial MWBC and one single circuit. You would not then be violating any requirement to keep all phases and the neutral(s) in the same raceway.
But you could not safely terminate both sub-circuits in the same panel for distribution, since it would be too confusing to try to keep track of the two separate insulated neutral bars.
And any three phase wye load could not be properly connected, since the three center ends of the loads would almost certainly be tied together internally inside the load equipment. Even if they are not, the existence of a three phase load would definitely invoke the requirement for all of the circuit conductors to be in the same raceway.
 
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