4.16kV feeder cable, each phase in separate conduit

[Designer69]

We have a situation where we are installing 3-1c 750MCM cables to a disc. sw. and the current design is to route each phase cable in a separate conduit.

An engineer raised a concern that this configuration would create eddy/circulating currents and it's better to use 1 conduit for all 3 phases or a tray.

I haven't heard this before so I was wondering what your thoughts are and if it's a valid concern.

Thank you

 

[david luchini]

We have a situation where we are installing 3-1c 750MCM cables to a disc. sw. and the current design is to route each phase cable in a separate conduit.

An engineer raised a concern that this configuration would create eddy/circulating currents and it's better to use 1 conduit for all 3 phases or a tray.

I haven't heard this before so I was wondering what your thoughts are and if it's a valid concern.

Thank you

NEC 300.35 addresses this type of installation.

 

[ron]

What kind of raceway and are they underground?

Checkout 300.20 & 300.35

 

[gar]

170831-1438 EDT

To minimize external magnetic fields you always want to run the supply and return path conductors as close as possible to one another so as to cancel their magnetic fields.

Changing magnetic fields about a conductor induce a voltage in the conductor.

If you have a single long straight wire with current flowing thru it and the return path for that current is a long way away, then the magnetic field around that wire will drop off as the inverse of the radial distance from the wire for radial distances small compared to the distance to the return current path.

Put a conductor, a hunk of metal, in that field and make the field oscillate (AC current), then that metal will heat from induced voltage and current.

.

 

[Julius Right]

At first, you may use one single-core cable per duct if the reinforcing bar[rebar] are only outside the group of ducts-in the envelope of the duct bank.
The ampacity will rise but the reactance between phases will rise also and so the voltage drop will rise. For short length it is o.k.