Parallel Conductors and Conduit

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FewerSparks

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When running parallel conductors for a large service (say 1200 Amps using parallel 500 MCM conductors (90 deg C), three conductors per phase), if we run this in three seperate conduits, does the NEC or any other code require that we keep the phases together in each conduit or that each conduit has one of each phase?
 
Also, a set of three 500's will not suffice for a 1200 amp load. You can't take credit for the 90C ampacity. Reference 110.14(C).
 
I got one

I got one

I got a call the other day asking if one of the three can be run overhead instead of underground because the plumber had to sawcut, you know the rest of the story. The conduit and wire would be the same, even length. I couldn't find anything specific, but it wouldn't settle, I said not without an engineer signing for it for reasons of 'best practice' and safety when the next guy can't figure it out. He was trying to save the plumbers some extra coin. One could stretch the meaning to include being run in the same fashion. :confused: So, two questions I guess.

1. Agree or disagree with my direction?

2. Am I missing something in the NEC?
 
TCN said:
I got a call the other day asking if one of the three can be run overhead instead of underground because the plumber had to sawcut, you know the rest of the story. The conduit and wire would be the same, even length. I couldn't find anything specific, but it wouldn't settle, I said not without an engineer signing for it for reasons of 'best practice' and safety when the next guy can't figure it out. He was trying to save the plumbers some extra coin. One could stretch the meaning to include being run in the same fashion. :confused: So, two questions I guess.

1. Agree or disagree with my direction?

2. Am I missing something in the NEC?
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Agree with your decision. Although the NEC doesn't prohibit from doing it the way you are questioning. As long as they comply with 310.4 your ok. Have never seen an install like the one you mentioned.
 
chevyx92 said:
Agree with your decision. Although the NEC doesn't prohibit from doing it the way you are questioning. As long as they comply with 310.4 your ok. Have never seen an install like the one you mentioned.
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I see that as a violation of 310.4(C). By running two sets next to each other at the third set at a remote location they all do not have the same electrical characteristics.
 
don_resqcapt19 said:
I see that as a violation of 310.4(C). By running two sets next to each other at the third set at a remote location they all do not have the same electrical characteristics.
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That very paragraph states that if they run one phase per pipe, it can be done without matching physical characteristics.

310.4(C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
 
don_resqcapt19 said:
I see that as a violation of 310.4(C). By running two sets next to each other at the third set at a remote location they all do not have the same electrical characteristics.
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He said they would have the same electrical characteristics. The 3rd set in question if run remote from the other two doesn't mean they dont have the same electrical characteristics. Explain how they dont have the same "Electrical Characteristics"? Physical paths would be different but the electrical characteristics. IMO. Its not ideal by any means.
 
In general, each conduit in a parallel set must contail _all_ of the conductors associated with a given circuit. For a 3 phase wye feeder, this means all of A,B,C, and N.

The reason for this is to reduce the 'magnetic loop area' between circuit conductors. Such magnetic loop area can induce current flow in the conduit surrounding the conductors, and can cause 'EMF' issues. EMF issues are not restricted by the NEC, but code does have requirements in order to limit current being induced in the conduit.

If you ignore the requirements for dealing with induced current, and simply run each phase isolated in its own conduit, then enough current can be induced in the conduit system to cause real damage. There are stories on this board about exactly these sort of installations gon bad.

If you follow all of the rules about not using ferromagnetic raceway, providing suitable insulation between raceways, cutting slots in enclosure openings, etc, then you are permitted to run an 'isolated' phase installation, where each phase runs in a separate conduit. If you already have the wrong sort of conduit in place, then you can't try to salvage the situation by running an isolated phase installation.

Additionally, if you do have all of the correct conditions for an isolated phase installation, then IMHO it would still be a very bad design to run one phase leg in a physically separate location from the others. The space between the phases would be subject to significant 60Hz electromagentic fields. This might not be regulated by the NEC, but I would bet that the EMFs associated with a 1200A service with significant spacing between the phases would be large enough for someone to notice, and to require corrective action. You don't describe the surroundings, but I am imagining things such as large current flows being induced in nearby steel structures, problems with nearby communications circuits, etc.

-Jon
 
I dont see any nec violations however every electrician that see's it will probably laugh and wonder what kinda of poor planning on someones fault caused that:mad:confused:. Ive seen some strange things but never a 1/3 overhead, 2/3 underground installation.
 
TCN said:
That very paragraph states that if they run one phase per pipe, it can be done without matching physical characteristics.

310.4(C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
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The quoted section says it only applies to the conductors, not the raceways. In other words, the conductors of Phase A have to match the other conductors for Phase A, but do not have to match the conductors of B and C phases.
 
chevyx92 said:
He said they would have the same electrical characteristics. The 3rd set in question if run remote from the other two doesn't mean they dont have the same electrical characteristics. Explain how they dont have the same "Electrical Characteristics"? Physical paths would be different but the electrical characteristics. IMO. Its not ideal by any means.
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There will be changes in the impedance of the circuits when part of the circuit is remote from the other part. There are even differences where they are run next to each other, but these differences will be greater with the remote path. Will they be different enough to change the current division so that one set is overloaded? I have no idea, and is not really likely. That being said I still see it as a code violation.
 
don_resqcapt19 said:
There will be changes in the impedance of the circuits when part of the circuit is remote from the other part. There are even differences where they are run next to each other, but these differences will be greater with the remote path. Will they be different enough to change the current division so that one set is overloaded? I have no idea, and is not really likely. That being said I still see it as a code violation.
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I interpert it to mean that all conductors or one phase must be routed together in same conduit. No where does it state raceways have to be side by side.... Just my opinion.
 
jwjrw said:
I interpert it to mean that all conductors or one phase must be routed together in same conduit. No where does it state raceways have to be side by side.... Just my opinion.
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I agree with you for the particular section of code being discussed.

The quoted code section is specifically about making sure that all of the conductors of a given phase carry an equal portion of the load. As long as all of the phase A conductors are the same length, of the same characteristics, and in conduit of the same characteristics, then this portion of the code has been fulfilled.

If all of the phase A conductors are in one conduit, and all of the phase B conductors are in a different conduit following a different path, and all of the phase C conductors are in yet another conduit following still another path, then _this_ section of the code could still be met.

_However_ other code sections apply to parallel installations. In particular (and I don't have my code book with me and don't remember the number), all of the conductors in a circuit are required to be in the same raceway, with some exceptions. This requirement is not related to balancing current in the parallel phase conductors, but is instead related to magnetic induction of current in the raceway system. I mentioned this above in my previous post.

The net result of both these code sections is that you either have to run all of the phases in all of the conduits, in which case the conduits need to follow quite similar paths, _or_ you can isolate the phases each to their own conduits and then jump through the hoops to fit the exceptions to the general rule about having all conductors in the same conduit.

-Jon
 
Thank guys.

Thank guys.

winnie said:
...all of the conductors in a circuit are required to be in the same raceway, with some exceptions. This requirement is not related to balancing current in the parallel phase conductors, but is instead related to magnetic induction of current in the raceway system.
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Precisely why I required an engineer to sign off on design IF that ends up being the choice.:grin: I still wouldn't like it.
 
jwjrw said:
I interpert it to mean that all conductors or one phase must be routed together in same conduit. No where does it state raceways have to be side by side.... Just my opinion.
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And my opinion is that when you run two sets underground and one set overhead the electrical characteristics are not the same and the installation is a violation.
 
don_resqcapt19 said:
And my opinion is that when you run two sets underground and one set overhead the electrical characteristics are not the same and the installation is a violation.
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I believe that this is an apples and oranges situation.

I absolutely agree that a conductor run underground in a conduit will almost certainly have different electrical characteristics than one run overhead.

This means that if you are running parallel conductors, you could not put a phase A underground and parallel it with a phase A running overhead.

However the characteristics of phase A are not required to match phase B or C or N; only those conductors that are actually electrically in parallel are required to have the same characteristics. If you managed to arrange things so that you could legally and safely run all the A conductors in one conduit, all the B ones in another, and so on, then one conduit could be overhead and the others underground. They would have different characteristics, but this would be acceptable because none of the paralleled conductors would have different characteristics.

-Jon

P.S. I will repeat that I don't think that this would be a good installation, even if one could stretch things to make it legal.
 
Jon,
I don't think that we are talking about an isolated phase installation here. I am saying that even with each of the raceways containing all of the phases, that running one conduit of a set of parallel conduits remote from the other conduits that make-up the parallel circuit that the electrical characteristics do not remain the same across all of the sets of parallel conductors.
I see this as a clear violation of 310.4(C)
(C) Separate Cables or Raceways. Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics. Conductors of one phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall not be required to have the same physical characteristics as those of another phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor to achieve balance.
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don_resqcapt19 said:
Jon,
I don't think that we are talking about an isolated phase installation here. I am saying that even with each of the raceways containing all of the phases, that running one conduit of a set of parallel conduits remote from the other conduits that make-up the parallel circuit that the electrical characteristics do not remain the same across all of the sets of parallel conductors.
I see this as a clear violation of 310.4(C)
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I see your point now...... Why cant they make this a lil easier to understand. Like you must run all conduits within 1ft of each other blah blah blah?:D
 
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