Neutral size for parallel underground service lateral

[Sometimeselectric]

We have six parallel runs of 750MCM aluminum USE conductors per phase, going between the utility transformer and an open bottom 2000A outdoor service disconnect.
My question is, how many neutrals (grounded conductors) do I need to run in that trench?
Using the 12 1/2% rule from table 250.102(c)(1) it would require roughly 600MCM.
250.24(C)(2) talks about parallel conductors in two or more raceways or cables, but no mention of direct bury installations.
I understand that if it was in conduit, each raceway would require 600MCM, but can't find anything that addresses this issue in a trench.
Thanks in advance!

 

[Sometimeselectric]

I spoke with our state inspector and they say if we have the parallel runs grouped together(A-B-C-N) and each group is separated in the trench, then we need a grounded conductor with each group.
If they are not separated, then we could get by with one grounded conductor, but would have to derate for 19 current carrying conductors as per 310.16

 

[ptonsparky]

Separate the bundles.
3 of A, 3 of B, 3 of C.
1 N or 3 smaller if preferred.

 

[electrofelon]

I spoke with our state inspector and they say if we have the parallel runs grouped together(A-B-C-N) and each group is separated in the trench, then we need a grounded conductor with each group.
If they are not separated, then we could get by with one grounded conductor, but would have to derate for 19 current carrying conductors as per 310.16

Unless there's a local or state amendment, the inspector is overstepping his authority. The NEC says nothing of the sort.

 

[don_resqcapt19]

Unless there's a local or state amendment, the inspector is overstepping his authority. The NEC says nothing of the sort.

If you separate the groups in the trench, I see the inspector as being 100% correct.

 

[electrofelon]

If you separate the groups in the trench, I see the inspector as being 100% correct.

Seems to me it says all conductors of the circuit must be in the same trench, nothing more, Charlie's rule. Where are you seeing it stated that they must be grouped within the trench?

 

[infinity]

I spoke with our state inspector and they say if we have the parallel runs grouped together(A-B-C-N) and each group is separated in the trench, then we need a grounded conductor with each group.

So if you do it this way (6 sets spaced in the trench) what size neutral conductor would you be ing using? IMO 6-#1/0 would be all that you would need.

 

[Sometimeselectric]

I understood that each set that is spaced in the trench would be treated like a conduit and would require the 600 MCM.

 

[electrofelon]

I understood that each set that is spaced in the trench would be treated like a conduit and would require the 600 MCM.

Got a code reference on that?

 

[infinity]

Although it mentions raceways since your grouping each set in the trench it seems like 240.24(C)(2) might apply when sizing the neutral.

 

[Sometimeselectric]

No code reference specifically for the sets spaced in a trench, but without finding anything addressing that specifically and trying to follow what the AHJ is suggesting, I would think 240.24(C)(2) would be the way to size the grounded conductor... I was originally reading that like each cable would need a grounded conductor sized off the total cross sectional area of all conductors making up the ungrounded conductor.
6 runs x 750MCM=4,500MCM x 12 1/2% = 562.5MCM.

Now that I read that more closely, it looks like you would size it based off the ungrounded conductor in each individual cable.
so right out of T250.102(C)(1) aluminum column 500 through 900 would require 3/0 aluminum in each cable. (using the term cable instead of "groups of USE conductors separated in the trench")

 

[ptonsparky]

Multiple groups of conductors does not equal cable

Although it mentions raceways since your grouping each set in the trench it seems like 240.24(C)(2) might apply when sizing the neutral.

Overcurrent Protection??

NM, I found it
250.24(C)(2)

 

[ptonsparky]

Are individual conductors laid in a trench considered cables?

 

[infinity]

Are individual conductors laid in a trench considered cables?

No, but the issue here is that the NEC doesn't directly address this either at least not in any language that I can find. The closest thing that I could find is the cable or raceway installations. Logically (if we even apply that word to the NEC) if the neutral brought to a service in a raceway can be sized by the conductors within the raceway then it would seem that you could apply that logic to groups of conductors laid in a trench.

 

[Engser18]

I spoke with our state inspector and they say if we have the parallel runs grouped together(A-B-C-N) and each group is separated in the trench, then we need a grounded conductor with each group.
If they are not separated, then we could get by with one grounded conductor, but would have to derate for 19 current carrying conductors as per 310.16

Just be careful, see annex B. if treated as single raceway then single N will be oki. If treated as multiple raceways then each raceway required N.
For single raceway, 50% derate factor (10-20)
For multiple raceways, seem like it is required separation 24" IAW Annex B to have full ampacity.

 

[ptonsparky]

No, but the issue here is that the NEC doesn't directly address this either at least not in any language that I can find. The closest thing that I could find is the cable or raceway installations. Logically (if we even apply that word to the NEC) if the neutral brought to a service in a raceway can be sized by the conductors within the raceway then it would seem that you could apply that logic to groups of conductors laid in a trench.

But...we are not required to put all phases and neutral in the same nonmetallic raceway. A one, B, etc. Why would we be required for individual conductors in a trench?

 

[infinity]

But...we are not required to put all phases and neutral in the same nonmetallic raceway. A one, B, etc. Why would we be required for individual conductors in a trench?

I touched on the reason in post #7. He said that the AHJ wants the ungrounded and neutral conductors run in sets in the trench.

I spoke with our state inspector and they say if we have the parallel runs grouped together(A-B-C-N) and each group is separated in the trench, then we need a grounded conductor with each group.
If they are not separated, then we could get by with one grounded conductor, but would have to derate for 19 current carrying conductors as per 310.16