Paralleling conductors in multiple conduits

[Jon456]

If parallel conductors are to be run in multiple conduits, is there a requirement to keep parallel legs together?

For example, for 3-phase (A + B + C, 4-conductors each) and neutral (N, 4-conductors) and ground (G, 4-conductors) in four conduits:

[AAAAG] [BBBBG] [CCCCG] [NNNNG]

or

[ABCNG] [ABCNG] [ABCNG] [ABCNG]

 

[augie47]

Take a look at 300.3(B). In all most all instances it's A-B-C-N-G in each

 

[infinity]

In 99.99% of installations you would need to use your second option.

 

[texie]

If parallel conductors are to be run in multiple conduits, is there a requirement to keep parallel legs together?

For example, for 3-phase (A + B + C, 4-conductors each) and neutral (N, 4-conductors) and ground (G, 4-conductors) in four conduits:

[AAAAG] [BBBBG] [CCCCG] [NNNNG]

or

[ABCNG] [ABCNG] [ABCNG] [ABCNG]

Except in rare cases that require other special rules, it should always be as you describe in your last line.

 

[Jon456]

Thanks for clarifying that for me. I have one more question regarding the EGC: If running two parallel sets of conductors through one conduit, are two EGCs required as well?

For example:

[ABCNG + ABCNG]

or

[ABCN + ABCN + G]

If the latter is acceptable, how does one size the EGC?

 

[infinity]

Thanks for clarifying that for me. I have one more question regarding the grounded conductor: If running two parallel sets of conductors through one conduit, are two grounds required as well?

For example:

[ABCNG + ABCNG]

or

[ABCN + ABCN + G]

If the latter is acceptable, how does one size the grounded conductor?

You've mentioned grounds and grounded conductor, are you referring to the EGC and neutral?

 

[Jon456]

I apologize for using the terminology incorrectly (and I do know the difference, so my only excuse is a tired mind and a rush to post).

I was referring to the EGC. In my diagram, G = EGC.

(I edited my question above to avoid any confusion.)

 

[don_resqcapt19]

The EGC must be full size in each raceway based on the rating of the upstream OCPD. 250.122(F).

 

[Jon456]

The EGC must be full size in each raceway based on the rating of the upstream OCPD. 250.122(F).

In my diagrams above, each set of square brackets represent a single conduit. So if I have two separate parallel conductors for each leg (A,B,C) and for neutral (N) inside a single conduit (for a total of 8 current-carrying conductors in the conduit), do I need two EGC conductors or will one EGC conductor suffice?

 

[masterinbama]

In my diagrams above, each set of square brackets represent a single conduit. So if I have two separate parallel conductors for each leg (A,B,C) and for neutral (N) inside a single conduit (for a total of 8 current-carrying conductors in the conduit), do I need two EGC conductors or will one EGC conductor suffice?

In the 8 conductors per conduit scenario only one EGC is required per conduit. But as Don said it must be sized to the breaker or fuses feeding the circuit.

 

[infinity]

In my diagrams above, each set of square brackets represent a single conduit. So if I have two separate parallel conductors for each leg (A,B,C) and for neutral (N) inside a single conduit (for a total of 8 current-carrying conductors in the conduit), do I need two EGC conductors or will one EGC conductor suffice?

Just one EGC per conduit is all that's required even if parallel sets of phase and neutral conductors occupy the same raceway.

 

[Jon456]

Ok, so that brings me to a (hopefully) final question on this topic. If there are multiple parallel conductors spread across multiple conduits, how does one size the EGC in each conduit? Must each EGC in each conduit be full-sized based on the rating of the OCPD? Or does each EGC in each conduit get sized to the capacity of the current carrying conductors in that conduit, provided that the sum of the capacity of all EGCs in all conduits matches the rating of the OCPD?

The reason I'm asking is this:

We have service entrance conductors from a 3-phase, 480V, 1,200 amp service panel to a distribution panel. The conductors run through four buried 4" PVC conduit. Each conduit contains four 500 mcm aluminum conductors (one for each of the three phases, plus one neutral) and one 1/0 aluminum EGC. So there are a total of sixteen current-carrying conductors and four EGCs.

We are trying to free up two conduits to run some additional conductors. By my calculations, running eight 500 mcm aluminum conductors plus two 1/0 aluminum EGCs in one 4" PVC conduit will put us at 40.32% fill. If we can consolidate the two EGCs into one, that may allow us to stay below 40%, but it will depend on the required size of the EGC.

(We are trying to avoid the expense of discarding our existing 500 mcm aluminum conductors and replacing with 350 mcm copper.)

 

[infinity]

Ok, so that brings me to a (hopefully) final question on this topic. If there are multiple parallel conductors spread across multiple conduits, how does one size the EGC in each conduit? Must each EGC in each conduit be full-sized based on the rating of the OCPD? Or does each EGC in each conduit get sized to the capacity of the current carrying conductors in that conduit, provided that the sum of the capacity of all EGCs in all conduits matches the rating of the OCPD?

The reason I'm asking is this:

We have service entrance conductors from a 3-phase, 480V, 1,200 amp service panel to a distribution panel. The conductors run through four buried 4" PVC conduit. Each conduit contains four 500 mcm aluminum conductors (one for each of the three phases, plus one neutral) and one 1/0 aluminum EGC. So there are a total of sixteen current-carrying conductors and four EGCs.

We are trying to free up two conduits to run some additional conductors. By my calculations, running eight 500 mcm aluminum conductors plus two 1/0 aluminum EGCs in one 4" PVC conduit will put us at 40.32% fill. If we can consolidate the two EGCs into one, that may allow us to stay below 40%, but it will depend on the required size of the EGC.

(We are trying to avoid the expense of discarding our existing 500 mcm aluminum conductors and replacing with 350 mcm copper.)

Parallel conductors would need a full size EGC in each raceway based on T250.122 and the OCPD ahead of the feeder.

Service entrance conductors should not have an EGC run with them. All grounding on the line side of the service disconnect is accomplished by a connection to the grounded conductor.

Also you didn't say what the load type is but it's likely that the neutrals are not considered CCC's.

 

[Jon456]

Shucks, I got my terminology screwed up again.

The utility company power comes from a ground-mounted transformer with underground service conductors to our service panel. Those would be the service entrance conductors, correct? For my question above, I was asking about the feeder conductors from that service panel to a remote distribution panel.

 

[Jon456]

Parallel conductors would need a full size EGC in each raceway based on T250.122 and the OCPD ahead of the feeder.

So it would appear that whomever did our original installation sized the EGC incorrectly. Each of the four conduits should contain a 250 mcm aluminum EGC (or a 3/0 copper EGC), for a total of four full-sized EGCs. Correct?

 

[infinity]

So it would appear that whomever did our original installation sized the EGC incorrectly. Each of the four conduits should contain a 250 mcm aluminum EGC (or a 3/0 copper EGC), for a total of four full-sized EGCs. Correct?

So the OCPD ahead of the feeder is 1200 amps? If so then yes.

 

[Jon456]

Yes, the OCPD ahead of the feeder is 1,200A.

Thanks again for all the assistance!

 

[david luchini]

We are trying to free up two conduits to run some additional conductors. By my calculations, running eight 500 mcm aluminum conductors plus two 1/0 aluminum EGCs in one 4" PVC conduit will put us at 40.32% fill. If we can consolidate the two EGCs into one, that may allow us to stay below 40%, but it will depend on the required size of the EGC.

Even if you could fit one properly sized EGC in each of your two conduits, you wouldn't be able to combine your four conduits into two, as you suggest.

Your 500 mcm aluminum has an ampacity of 310 Amps. You have 4 sets (4 * 310A = 1240A) for your 1200A feeder.

If you combined two sets in each conduit, the ampacity of the conductors would be 248Amps. Your four sets would then have a total ampacity of 992Amps, (or if the existing conductors are 90deg for we locations, the total ampacity would be 1120Amps.) You can't protect this feeder with a 1200A c/b.

(We are trying to avoid the expense of discarding our existing 500 mcm aluminum conductors and replacing with 350 mcm copper.)

350 mcm copper wouldn't work in your 2 conduits either. You would need at least 500mcm cu 75 deg (380 * 0.8 = 304A * 4/ph = 1216) or 400mcm cu 90 deg.

 

[Jon456]

Even if you could fit one properly sized EGC in each of your two conduits, you wouldn't be able to combine your four conduits into two, as you suggest.

If you combined two sets in each conduit, the ampacity of the conductors would be 248Amps. Your four sets would then have a total ampacity of 992Amps, (or if the existing conductors are 90deg for we locations, the total ampacity would be 1120Amps.) You can't protect this feeder with a 1200A c/b.

David, sorry for the late reply; I've been swamped. Previously in this thread, I had been worried about fill-ratios, and had not yet considered derating calculations due to the additional conductors in each conduit. In the end, the contractor did pull the eight 500mcm conductors plus one EGC into each of two conduits. As you correctly pointed out, this derated our capacity to 1,120A. So I removed the 1,200A rating plug from our disconnect and replaced it with a 1,000A rating plug. We aren't even using close to 1,000A, so we are only losing some excess capacity for future expansion.

There was a problem regarding the sizing of the EGC, but that will be the subject of another post.