Help with 310.10(H) Circuit conductor quantities per conduits

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andykee

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Scenario - Two 600mcm AL per phase, two 4" PVC conduits available.

Paralleled conductors of A and B phase in one 4" conduit and C phase in the another. Apart from necessary derating for number of current carrying conductors and running 2 separate EGC's. Are there other issues with the difference in conductor quantities per PVC conduit?
 
Put A,B and C in one conduit and the same in the other. IOW one parallel set in one conduit and the other set in conduit #2
 
If you want to do iso-phase with PVC conduit you need to comply with the 300.3(B)(1)Exception.

300.3(B)(1)Exception: Conductors installed in nonmetallic raceways run under‐
ground shall be permitted to be arranged as isolated phase, neutral,
and grounded conductor installations. The raceways shall be installed
in close proximity, and the isolated phase, neutral, and grounded
conductors shall comply with the provisions of 300.20(B).
Click to expand...
 
andykee said:
Are there other issues with the difference in conductor quantities per PVC conduit?
Click to expand...


With the exception of isolated phase installations, which have their own aforementioned requirements, you need a balanced arrangement of all circuit conductors inside each conduit. This way the magnetic field from the conductors adds up to zero (because the current adds up to zero, when accounting for direction), and doesn't induce heat in a steel conduit or anything in the surroundings, from cycling a magnetic field.

For conductors in parallel, you need to maintain the symmetry of each parallel set, so that no intentional differences in construction method would cause current to not divide equally. Adding up the ampacity of conductors in parallel assumes that current divides uniformly among the sets. If one path carries current better than another, it means disproportionately more current will follow that path, which could overload that conductor. So you need to terminate all conductors in the same manner, and have an equal number of sets in each conduit, if multiple conduits are used. It is OK to distribute 6 sets among 3 conduits, with 2 sets in each, but you will find it common to have a dedicated conduit for each set. It would not be OK to distribute 5 sets among 3 conduits, because one conduit would only have one set, while the others would have 2 sets.
 
While most code "experts" don't agree, the language in 300.3(B)(3) permits the installation described in the first post.
 
Don, I won't ask for the title of "expert," but you can add me to the “don’t agree” group.

300.3(B) starts out by requiring all conductors in the same conduit, unless addressed in (1) through (4) below. Paragraph (3) talks about conductors in separate conduits. But it does not say you can put some conductors in one conduit and other conductors in a separate conduit. Rather, what it tells me is that we can put A, B, C, and N in four separate conduits, if we do the installation as described.
 
I think it is arguable that if and only if you can put a single circuit in the two conduit configuration, then you can do the same thing with parallel pairs of conductors.
The two conductors of each pair will still be in identical configurations.

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GoldDigger said:
I think it is arguable that if and only if you can put a single circuit in the two conduit configuration, then you can do the same thing with parallel pairs of conductors.
The two conductors of each pair will still be in identical configurations.
Click to expand...
Could also be cheaper and easier to pull.
 
andykee said:
Scenario - Two 600mcm AL per phase, two 4" PVC conduits available.

Paralleled conductors of A and B phase in one 4" conduit and C phase in the another. Apart from necessary derating for number of current carrying conductors and running 2 separate EGC's. Are there other issues with the difference in conductor quantities per PVC conduit?
Click to expand...

Two separate EGC's (i.e. one in the first conduit with the A an B phases and the other in the second conduit with the C phase) would be parallel conductors, and therefore I believe should be subject to the same restrictions as other parallel conductors. Also, there would be some circulating current induced in the loop formed by two parallel connected EGCs because each has a different amount of magnetic coupling to the three phases.
If parallel conductors are used for the EGC then I think they should be in the same conduit unless they can be symmetrically distributed together with all of the other conductors across all conduits (e.g., A, B, C, and EGC in each conduit with multiple conduits).
 
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From the physics of the situation it is preferable to run the installation as Dennis describes in post #3, running a complete balanced 3 phase set in each conduit. This will minimize 'loop area' and circuit inductance. Additionally you will have 3 phase conductors plus EGC in each conduit, rather than 4 in one and 2 in the other, which will probably make the pull easier. On top of this the conduit with 4 phase conductors will be subject to derating whereas if you do the 'normal' install you don't have to derate.

However from the same physics perspective I believe that the install as described by the OP is as acceptable as any 'isolated phase' installation. The code seems to imply the binary of all the phases together or all completely isolated, so the installation might not meet the letter of the code as written, however it introduces no additional physics problems not already addressed for 'isolated phase' installations.

-Jon
 
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