Number of Conductors in a Conduit

TheElectrician

Senior Member
Hey Guys,

I am designing a solar system, and I wanted to get an opinion in the number of wires in an RMC. The design is as follows:

6 sets: [4 #400 MCM AL-XHHW2
1 #3/0 CU SSBJ
4" RMC]

According to NFPA 70 Annex C: I can put 9 #400 conductors in a 4" RMC

So is it recommended to use 3 RMC with 9 conductors in each along with the ground or use 1 conduit for each set?

Thanks
 

kwired

Electron manager
Hey Guys,

I am designing a solar system, and I wanted to get an opinion in the number of wires in an RMC. The design is as follows:

6 sets: [4 #400 MCM AL-XHHW2
1 #3/0 CU SSBJ
4" RMC]

According to NFPA 70 Annex C: I can put 9 #400 conductors in a 4" RMC

So is it recommended to use 3 RMC with 9 conductors in each along with the ground or use 1 conduit for each set?

Thanks
9 current carrying conductors in same raceway will require ampacity adjustments - may or may not require increase in conductor size. More than 9 conductors in one raceway is definitely going to require larger conductors as you jump to 50% adjustment factor or even less. You are looking at three times as many raceways for the other option, though they can be smaller.
 

Carultch

Senior Member
9 current carrying conductors in same raceway will require ampacity adjustments - may or may not require increase in conductor size. More than 9 conductors in one raceway is definitely going to require larger conductors as you jump to 50% adjustment factor or even less. You are looking at three times as many raceways for the other option, though they can be smaller.
The EGC is never counted as "current-carrying", and the neutral in general, does not count either. There are excpetions, but a solar inverter output circuit is usually not one of them.
 

TheElectrician

Senior Member
Welcome to The Forum.

What is your desired or required ampacity?
Thanks Fletcher, I am using 14 inverters with 72 A output on each, With 6 sets I am looking at 210 in each set, I have already upsized the wire. Also, is it a good practice to put two sets in a single conduit or one conduit for each set

Ps: 72 A*1.25 = 90 A
90 A*14 = 1260 A
1260/6 = 210 A

Thanks
 

TheElectrician

Senior Member
9 current carrying conductors in same raceway will require ampacity adjustments - may or may not require increase in conductor size. More than 9 conductors in one raceway is definitely going to require larger conductors as you jump to 50% adjustment factor or even less. You are looking at three times as many raceways for the other option, though they can be smaller.
Thanks for the input kwired, the wire has been derated to only 3 current carrying conductors, but my contractor insists on putting 9 conductors since its on Annex C. I just need an opinion on this to talk to him.

Thanks
 

kwired

Electron manager
The EGC is never counted as "current-carrying", and the neutral in general, does not count either. There are excpetions, but a solar inverter output circuit is usually not one of them.
The #3/0 he mentioned is the grounding conductor, then there was 9 other conductors. He didn't mention if it was single phase three wire or three phase three wire - that would make some difference on how many are current carrying conductors for adjustment purposes.
 

kwired

Electron manager
Thanks for the input kwired, the wire has been derated to only 3 current carrying conductors, but my contractor insists on putting 9 conductors since its on Annex C. I just need an opinion on this to talk to him.

Thanks
I'm not sure I understand what you are saying.

If you have three current carrying conductors there is no adjustment factor. If you have six (six hots, 1 neutral) current carrying conductors your adjustment factor would be 80%. If all nine are current carrying the adjustment factor is 70%.

If you are saying they were adjusted for three CCC's (100%- no adjustment) then you must adjust to 70% if you put all nine in one raceway but could have left them at 100% if they were in separate parallel sets in separate raceways. Now just so happens you normally don't need to increase size of 90C conductors when the terminal temp was 75C when applying 70% adjustment factor. But if ambient temp adjustment happens to be needed also - be careful, you may need larger conductor in that case.
 

electrofelon

Senior Member
Seem like you are good with two sets in each pipe. That would be 6 CCC. Actually still works even if you had 7-9 CCC. How many sets to put in each pipe is a preference, but I would say most of us would generally run one set in its own pipe. IMO it just gets to be a cluster firetruck with so many conductors going in a single pipe, plus you have the derating (although if you are already oversized for VD, the derating may not come into play). IF its not spelled out in design documents, I dont see how you can tell your contractor to do it a certain way.
 

kwired

Electron manager
Thanks Fletcher, I am using 14 inverters with 72 A output on each, With 6 sets I am looking at 210 in each set, I have already upsized the wire. Also, is it a good practice to put two sets in a single conduit or one conduit for each set

Ps: 72 A*1.25 = 90 A
90 A*14 = 1260 A
1260/6 = 210 A

Thanks
I am confused with the mentioning of six sets in OP but 9 conductors in the pipe?

I calculated that for 1260 amps you could use three sets of 400 aluminum for this if all conductors are in same raceway - that would be for where all nine conductors are current carrying conductors like for 3 phase 3 wire application. If it were single phase three wire like 120/240 the neutral isn't a current carrying conductor for ampacity adjustment purposes and you only have 6 CCC's and a 80% adjustment factor - I think you could use 350 aluminum if that is what you have, but either case is only thee sets of parallel conductors not six. Those the two situations I see where you would have 9 conductors not counting EGC/SSBJ.
 

TheElectrician

Senior Member
Thanks everyone for your information. It was really helpful. Since I am running 6 sets of A,B,C and N and using Steel Conduit, my Power Company requires the conductors to be phase isolated in the case of steel conduits(Phase A, B,C and neutral in separate conduits). So I am running 4 conduits.
 

Dennis Alwon

Moderator
Staff member
Thanks everyone for your information. It was really helpful. Since I am running 6 sets of A,B,C and N and using Steel Conduit, my Power Company requires the conductors to be phase isolated in the case of steel conduits(Phase A, B,C and neutral in separate conduits). So I am running 4 conduits.

You cannot do that with steel conduit. If you use PVC then it may be allowed to run all phase A conductors in one conduit. I believe it would have to be underground also.
 

TheElectrician

Senior Member
You cannot do that with steel conduit. If you use PVC then it may be allowed to run all phase A conductors in one conduit. I believe it would have to be underground also.

Yes this is underground, But the Power Company code book says the following:

"Three-phase secondary cables shall be phase grouped in the case ofconcrete ducts and HDPE conduits ( Phase A, B,C and neutral in thesame conduits) and shall be phase isolated in the case of steel conduits(Phase A, B,C and neutral in separate conduits)."

Let me know your thoughts
 

LarryFine

Master Electrician Electric Contractor Richmond VA
"Three-phase secondary cables shall be phase grouped in the case of concrete ducts and HDPE conduits ( Phase A, B,C and neutral in the same conduits) and shall be phase isolated in the case of steel conduits(Phase A, B,C and neutral in separate conduits)."
This sounds backwards to me, is against conventional electrical practice, and is NEC non-compliant.

Any suggestions on how I can proceed? Do I have to talk with them about this?
I would. Tell them "This sounds backwards to me."

NESC may not conform with NEC, but the laws of physics apply everywhere.
 

TheElectrician

Senior Member
This sounds backwards to me, is against conventional electrical practice, and is NEC non-compliant.


I would. Tell them "This sounds backwards to me."

NESC may not conform with NEC, but the laws of physics apply everywhere.

Thanks man, I will confirm with them again
 

tkb

Senior Member
Hey Guys,

I am designing a solar system, and I wanted to get an opinion in the number of wires in an RMC. The design is as follows:

6 sets: [4 #400 MCM AL-XHHW2
1 #3/0 CU SSBJ
4" RMC]

According to NFPA 70 Annex C: I can put 9 #400 conductors in a 4" RMC

So is it recommended to use 3 RMC with 9 conductors in each along with the ground or use 1 conduit for each set?

Thanks
Why is your neutral full size for solar?
 
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