Potential Feeder Imbalance

I'm having trouble seeing what the actual technical downside to the install in the OP would be. We can take as our baseline "worst acceptable imbalance" the isophase installation allowed under 300.3(B)(1) Exception. So if we had (3) 3" conduits in a row, we could install 3xA in the first, 3xB in the second, and 3xC in the third.

Now delete the third conduit and move the C conductors into the A and B conduits. This is going to decrease the mean distance between conductors of different phases, and is therefore an arrangement that seems to me closer to having all the conductors in a single conduit. Doesn't that mean that the imbalance in impedances will be less than in the isophase baseline case?

If someone with more physics that me can point me to the relevant equations for capacitance and inductance per unit length for parallel conductors at a distance d from each other, I'm happy to do the math comparing the various spatial arrangements.

Cheers, Wayne
 
wwhitney said:
I'm having trouble seeing what the actual technical downside to the install in the OP would be. We can take as our baseline "worst acceptable imbalance" the isophase installation allowed under 300.3(B)(1) Exception. So if we had (3) 3" conduits in a row, we could install 3xA in the first, 3xB in the second, and 3xC in the third.

Now delete the third conduit and move the C conductors into the A and B conduits. This is going to decrease the mean distance between conductors of different phases, and is therefore an arrangement that seems to me closer to having all the conductors in a single conduit. Doesn't that mean that the imbalance in impedances will be less than in the isophase baseline case?

If someone with more physics that me can point me to the relevant equations for capacitance and inductance per unit length for parallel conductors at a distance d from each other, I'm happy to do the math comparing the various spatial arrangements.

Cheers, Wayne
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With isophase you have equal impedance on individual conductors of each parallel set.

With OP's described setup you have two conductors of one phase in one conduit and one conductor of same phase in the other conduit. The impedance difference will cause current to divide unevenly over those three conductors.

OP at very least needs to find a way to get every conductor of a phase in the same raceway. This could still lead to overall phase imbalance but won't result in much for unbalance on the individual conductors of any phase set. This may result in two phases in one raceway and one phase and grounded conductor in the other raceway. Maybe not ideal arrangement, NEC may or may not apply, probably better situation than what he currently has though.
 
rezonatefreq907 said:
Unusual existing install have not encountered before. 2ea 3in PVC conduits buried under ground about 75ft. Each conduit has 5ea 3/0 & 1ea 2/0 xhhw alum. Intent was to have 3 parallel runs of 3/0 w/ 2/0 ground in copper not alum. 3 phase 480vac. No neutral needed. No ferrous metal around conductors. Stubbed up through floor into equipment. Switchgear one end. Adjustable inductive load bank (reactor) to mitigate negative kVARs. Yes, I know lots of mistakes or short cuts appear to have been made or taken by contractor who installed but this is the cards that were dealt. Of course lack of time, money with very remote logistics get sprinkled on. 2 parallel runs is easy. ABC in each conduit. Leave 2 ea 3/0 unconnected or grounded to prevent coupled voltage. How to get the third run if needed?

Yes I could remove, repull and/or replace. Not acceptable unless we cannot make existing work. Discussed with engineer and we can likely make just two runs work by limiting the adjustable reactor. I want the third run if it possible. Heating should not be an issue since no ferrous metal near conduits. Will there be a voltage imbalance caused by the following propose conductor termination layout?

3 runs in 2 conduits: Conduit 1: 2 x Phase A, 2 x Phase B, 1 x Phase C, 1 x Equipment Ground. Conduit 2: 1 x Phase A, 1 x Phase B, 2 x Phase C, 1 x Equipment Ground.
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Update: Needed com out to inductor. Ended up only actually having 2 conduits available back to switchgear. Pulled all conductors out. Use one conduit for control and com the other for the feeders. Reused existing conductors and ended up with 3 parallel runs in one conduit. Used smaller conductors for ground. No neutral needed. Inductor corrected the negative 250kVARs and allowed the genset to go online.
 
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