Neutral in a separate conduit

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JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Is there a technical reason (i.e. not a Code reason) why you can't run the 3 phases in one conduit and the neutral in a separate conduit (assuming underground PVC conduit)? I understand why you wouldn't want to do it with steel conduit (induced currents, heating, etc.), but if the conduits are plastic, with no steel elbows or fittings, is there any reason why this can't/shouldn't be done?

So, here's the background, in case you're interested. I work for a power utility, so I'm not constrained by the NEC (contrary to popular opinion, we do have a lot of rules we have to follow; it's just that the NEC doesn't number among them). We have a large section of a circuit that we are going to move from overhead poles to underground conduits. This is currently a 3-phase, 4-wire circuit, and is what power utilities refer to as an "isolated neutral" system, meaning that the neutral conductor is only grounded at the source, instead of being grounded every x number of feet as is typical for a 4-wire utility installation. This allows us to protect the circuit from faults with what is essentially a large GFP. However, there are plans to change over to some new protection scheme a few years later, which will make the fourth wire unnecessary. The field crews are asking if they can install the phase conductors in one conduit and the neutral in another, so they can remove the neutral when it's no longer needed and have a spare conduit. So, that's the long version.
 

Smart $

Esteemed Member
Location
Ohio
I can't think of anything, assuming minimal separation. Even the NEC permits conductors of nonmetallic wiring methods in separate cables and conduits, and specifies no limitation on separation distance.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
I can't think of anything, assuming minimal separation. Even the NEC permits conductors of nonmetallic wiring methods in separate cables and conduits, and specifies no limitation on separation distance.
On the other hand, will the neutral fit in the first conduit? At some point the cost of pulling the wire to make the second conduit usable may be greater than the lost salvage value of the neutral cable if you just left it in place.
The number of CCCs in the first conduit would be the same either way and the pull might not be that much harder.
 

JRW 70

Senior Member
Location
Eastern Central Missouri
Occupation
Testing and Engineer
Protection ?

Protection ?

Do you need the neutral with the phase conductors
for your relaying for the C.B., etc. From an operational
perspective it shouldn't matter. However if there are
sensitive relays I wouldn't want to say it's ok.
How hard is this pull? Is the conduit big enough?
we have pulled MV cable with a Cat D8 using a strain
gauge to make sure we didn't get near the tensile strength.
I wouldn't go for this setup. You are the Engineer, I don't
know how it is in California any more (been gone a while)
BUT your decision should overrule the field crew. I make
sure my Engineering decisions are carried out at the plants
I'm responsible for. If the management will back you up,
stand your ground if you feel one conduit would be better.
It all comes down to line protection and what you're using.
A nearby conduit may be just far enough away to cause
problems or it may be totally fine. It just seems a little
risky to have your ground detection wire in a different
conduit, even if the plan is to go three wire in the future.
(I work in generation plants, so take my opinion from that
point of view.)

In any event, I hope your project goes well.

JR
 
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JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Thanks for the input, everyone. As of today we're changing areas, and another engineer is going to be taking over this project. I've passed along your suggestions to him.

Again, I really appreciate the information.
 

mwm1752

Senior Member
Location
Aspen, Colo
I can't think of anything, assuming minimal separation. Even the NEC permits conductors of nonmetallic wiring methods in separate cables and conduits, and specifies no limitation on separation distance.

Where in the NEC does it allow a non parraleled 3 phase circuit run in 2 condiuts?
 
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