Potential Feeder Imbalance

R

rezonatefreq907

Member
Location
USA
Occupation
Utility
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.
 
If the install is subject to the NEC, you need to have the same number of conductors/phase in each conduit. You only have two conduits so your scheme will not work.

If this is a utility project you need to consult with your engineers, however I have never seen an approved real world installation like this.
 
Easy.

You set a 3 phase delta: 5 phase star transformer at the supply end, and a 5 phase mesh : 3 phase wye transformer at the other end, and each conduit gets A, B, C, D, E plus ground. (Tongue firmly in cheek...)

Seriously, you cannot get a balanced installation with all of the conductor as described. In fact, even with more conductors you couldn't get _3_ parallel runs. With 2 conduits you are restricted to an even number of parallel runs.

Jonathan
 
Since is in PVC there is no inductive effects from the raceway, but you likely would have better balancing had you ran 2 A phase and 2 B phase conductors in one raceway and 2 C phase conductors in the other raceway. May not have same balance per "phase" but would likely have better balance per parallel element as each element likely has same or very close to same impedance this way.
 
ptonsparky said:
310.10(G)(3)
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So I guess the question is whether this requirement has a technical basis when 300.3(B)(3) applies (nonferrous wiring methods, conductors of a circuit do not need to be in the same raceway/cable).

Seems to me if the conductors were direct burial rated single conductor cables, you could throw all 9 circuit conductors in a trench without worrying about grouping them into balanced sets. Using that as a baseline, what negative electrical effect would the unbalanced arrangement of conductors proposed in the OP have that is worse than the common trench case?

Cheers, Wayne
 
For a balanced arrangement, Annex C says 3" PVC can take up to 11 3/0 XHHW condcutors at 40% fill. So you could add one more 3/0 conductor to each conduit to get 4 parallel sets, 2 in each conduit.

Cheers, Wayne
 
kwired said:
Since is in PVC there is no inductive effects from the raceway, but you likely would have better balancing had you ran 2 A phase and 2 B phase conductors in one raceway and 2 C phase conductors in the other raceway. May not have same balance per "phase" but would likely have better balance per parallel element as each element likely has same or very close to same impedance this way.
Click to expand...
Thanks this is the type of discussion I was looking for. Will stick with only 2 runs and see but want options if possible.
 
jim dungar said:
If the install is subject to the NEC, you need to have the same number of conductors/phase in each conduit. You only have two conduits so your scheme will not work.

If this is a utility project you need to consult with your engineers, however I have never seen an approved real world installation like this.
Click to expand...
Violation of 310.10(G) is the answer.as you described.
 
wwhitney said:
So I guess the question is whether this requirement has a technical basis when 300.3(B)(3) applies (nonferrous wiring methods, conductors of a circuit do not need to be in the same raceway/cable).

Seems to me if the conductors were direct burial rated single conductor cables, you could throw all 9 circuit conductors in a trench without worrying about grouping them into balanced sets. Using that as a baseline, what negative electrical effect would the unbalanced arrangement of conductors proposed in the OP have that is worse than the common trench case?

Cheers, Wayne
Click to expand...
Regardless of what NEC might say, you still want as many of the same characteristics as possible from each individual conductor of a parallel set which will give them equal impedance and equal division of current. Air gaps and separation by the raceways in OP's situation will allow for more variances than if he had all the same conductors buried (in close proximity) in a trench.

Different raceways works and is done but normally is done with one raceway for every conductor of a phase in the raceway.

What OP has is asking for some impedance differences between the different conductors of each parallel set there.
 
kwired said:
Since is in PVC there is no inductive effects from the raceway, but you likely would have better balancing had you ran 2 A phase and 2 B phase conductors in one raceway and 2 C phase conductors in the other raceway. May not have same balance per "phase" but would likely have better balance per parallel element as each element likely has same or very close to same impedance this way.
Click to expand...
I consider your idea if backed into a corner to keep lights on. Will measure current on each conductor if we do.
 
rezonatefreq907 said:
I consider your idea if backed into a corner to keep lights on. Will measure current on each conductor if we do.
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I see I misunderstood what you have. You have three conductors per phase I for some reason when I posted before thought there was two.

I still think you would have better balance if you had 3A, 3B conductors in one conduit and 3 C and an EGC in the second conduit. You may not have overall balance of A, B and C but should have much better balance across the three conductors of each phase.
 
He has 5 conductors in each conduit.

Best balance will be to use 3 conductors in each conduit, 1 each ABC.

2A and 2B in one and 2C in the second would also be balanced, because all As see the same other conductors, etc. But this won't get you any additional ampacity.

What OP wants to do (which is an NEC violation) is some variation that gives 3 conductors per phase, eg AABBC in one conduit and ABCC in the other.

This is an NEC violation but might not be problematic if the current is well enough balanced. Since this is not an NEC installation there is some wiggle room, but I think we all agree that it doesn't meet NEC requirements.

Going down the road of permission to violate the NEC under supervision of the power company engineers, I'd look at AAABB in the first conduit and BBCCC in the second. This would cause imbalance in the B conductors but compensate with an additional B conductor.
 
winnie said:
He has 5 conductors in each conduit.

Best balance will be to use 3 conductors in each conduit, 1 each ABC.

2A and 2B in one and 2C in the second would also be balanced, because all As see the same other conductors, etc. But this won't get you any additional ampacity.

What OP wants to do (which is an NEC violation) is some variation that gives 3 conductors per phase, eg AABBC in one conduit and ABCC in the other.

This is an NEC violation but might not be problematic if the current is well enough balanced. Since this is not an NEC installation there is some wiggle room, but I think we all agree that it doesn't meet NEC requirements.

Going down the road of permission to violate the NEC under supervision of the power company engineers, I'd look at AAABB in the first conduit and BBCCC in the second. This would cause imbalance in the B conductors but compensate with an additional B conductor.
Click to expand...
5 conductors plus a ground conductor in each conduit.

In scenario I presented he could have ground conductor in the conduit with less conductors in it. Still a NEC violation as well though. Main problem it may introduce is increased impedance in certain ground fault situations should they occur.

I do like the scenario you presented in the last paragraph there as well.
 
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.
Click to expand...
Thanks for the thoughtful comments and specific code reference. My hope is we can use just two runs. If not the ideas on how to reterminate to add another are insightful. Another question on unused conductors in conduits. My intent is to connect them to equip ground on both ends rather than leave unterminated and have stray voltage on them.
 
rezonatefreq907 said:
My intent is to connect them to equip ground on both ends rather than leave unterminated and have stray voltage on them
Click to expand...
If connectedon both ends they would create parallel grounding conductors which would be a code violation.

Unterminated conductors are typically not a problem <1000V. Leave them alone or only ground one end.
 
You have parallel grounding conductors all the time when there is multiple raceways.

What you can't do is run multiple smaller grounding conductors with the intent of creating a single larger conductor.

"Conventional" parallel runs that we see the most often with one of each phase and neutral in each separate raceway must have full sized EGC from T250.122 in each raceway. You can't take the T250.122 conductor needed, and split it into equivalent smaller conductors like you do with the regular circuit conductors.

Not certain what OP has, he mentioned there is 3-3/0 in parallel and 2/0 grounding conductor. This sounds like the 2/0 grounding conductors possibly do need to be larger, but need more details to know for certain. Now if they are service conductors and are essentially a grounded neutral they are probably fine and possibly could even be smaller. Clear as mud now isn't it?
 
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