neutral conductor on a separate conduit?

[mel127]

We are planning to install 2 additional neutral conductors on a 3 phase, 5 KV circuit (2 circuits for redundancy). The conductors are underground. The entire run will cover 11 manholes at about 3000 feet. We are looking at different options and the correct way to do it. First option is to pull out the feeders and run tne new neutral in the same conduit as the feeders. Second option is to run the neutral conductors on a spare conduit in the ductbank. If we are to consider option number two are we violating any codes, is there any other issues besides the code? Please advise.TIA.

 

[waross]

Running the neutrals in a different route will result in a disproportionately high impedance. If either the neutrals or the power conductors are in ferrous conduit the impedance may be so high as to render the new instalation pointless.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jghrist]

NEC 300.3(B) requires all conductors of the same circuit to be run in the same raceway. There is an exception, however:

Conductors installed in nonmetallic reaceways run underground shall be permitted to be arranged as isolated phase installations. The raceways shall be installed in close proximity, and the conductors shall comply with the provisions of 300.20(B).

As Bill noted, the impedance will be higher. You should check to see if the zero-sequence impedance will be too high. More ground fault current will flow in the cable shield if the neutrals are in separate conduits. Check that the cable shields will not be damaged by the higher current.

 

[potteryshard]

Can you use the existing neutral to pull in a new, over-sized neutral in with the existing phase conductors?

If pulling a neutral into a separate conduit, not only the conduit must be non-magnetic, but also the surrounding structures. Rebar in concrete duct structures and manholes has been known to cause losses and even heating and spalling of the concrete.

 

[mel127]

Thanks everyone for the prompt response.

@potteryshard, no we dont have any neutral at all.