Same Electrical Characteristics

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infinity

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Does the NEC define "same electrical characteristics"? Would parallel conductors runs in two different size raceways (3.5" & 4") be of the same characteristics if they're both EMT?


310.10(I)(3) 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.
 

GoldDigger

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Any objective measurement of wire impedance, etc., should be next best thing to identical if only the raceway size differed.
AHJ opinion, who knows?
 

gadfly56

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Putting on my Karnack the Magnificient turban and reading the minds of the code committee, I'm going to guess that "same electrical characteristics" means resistance and impedance, assuming negligible difference in conductor lengths. I would hazard a further guess that conductors in different sized raceways might have different impedance if the phases are separated since theoretically the distance from the conductor(s) to the wall would be different, causing different magnectic interactions. But that assumes the conductors are bundled in the dead center of the conduit. They're going to sit right on the bottom, and I would suppose (really going out on a limb now) that there would be scant difference between the two sets, depending on conduit fill.
 

GoldDigger

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One in EMT and one on PVC might be objectively "same electrical characteristics" but more likely to raise flags.
THXX in EMT and TC in open cable tray might really be stretching it. Thermal characteristics would be different.
 

don_resqcapt19

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Up to the 2008 code the wording required that the raceways for parallel installations have the same physical characteristics. In the 2008 code this was changed to electrical characteristics. The change resulted from proposal 6-7a that was made by Panel 6. The substantiation says:
Substantiation: The panel has reorganized this section in response to a number of public proposals indicating the need to reorganize the section to make it more usable without changing the intent of the section.
It appears that CMP 6 is saying that the word "physical" has the same meaning as the word "electrical" for the application of parallel raceways.

The 2014 Handbook commentary indicates that the parallel raceways must be of the same size.
 

gadfly56

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Up to the 2008 code the wording required that the raceways for parallel installations have the same physical characteristics. In the 2008 code this was changed to electrical characteristics. The change resulted from proposal 6-7a that was made by Panel 6. The substantiation says:

It appears that CMP 6 is saying that the word "physical" has the same meaning as the word "electrical" for the application of parallel raceways.

The 2014 Handbook commentary indicates that the parallel raceways must be of the same size.

Why didn't they just say so in the body of the code? This is rediculous; it's like they're doing their best to NOT make things clear. :rant:
 

infinity

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Up to the 2008 code the wording required that the raceways for parallel installations have the same physical characteristics. In the 2008 code this was changed to electrical characteristics. The change resulted from proposal 6-7a that was made by Panel 6. The substantiation says:

It appears that CMP 6 is saying that the word "physical" has the same meaning as the word "electrical" for the application of parallel raceways.

The 2014 Handbook commentary indicates that the parallel raceways must be of the same size.

If the two words are needed to explain what they really meant then maybe they should have used both the words physical and electrical in the requirement.

Does anyone have the 2014 NECH commentary that they can post?
 

don_resqcapt19

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Rob,
Here is the comment that follows 310.10(H)(3).
All parallel raceways or cables for a circuit are required to be of the same size, material, and length. ?Cables,? in this case, means wiring method?type cables such as Type MC. For example, the conductors in phases A and B may be copper, and those in phase C may be aluminum.
 

charlie b

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Does the NEC define "same electrical characteristics"? Would parallel conductors runs in two different size raceways (3.5" & 4") be of the same characteristics if they're both EMT?
I am going to take a stab into the darkness, and submit for your consideration that conduit does not have any "electrical characteristics" at all. It may have magnetic properties, and if so that could influence the outcome of a short circuit event. But it is the wires that have electrical properties, and the conduit that only has physical properties, magnetic permeability being one of them.

I will grant that the material of construction and the geometry of a conduit will have an effect on the circuit's overall inductive and capacitive reactance. But these are superficial effects on the currents that will flow in the circuit. Having two parallel A Phase conductors of different material, AWG size, or length can have a much more substantial effect. That is the concern over which this requirement is based.

 

jim dungar

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I will grant that the material of construction and the geometry of a conduit will have an effect on the circuit's overall inductive and capacitive reactance. But these are superficial effects on the currents that will flow in the circuit.

I do not think I have ever considered or 'entered' the size of conduits into any circuit analysis I have ever performed. All I have ever cared about is magnetic vs. non-magnetic. Also most circuit analysis programs do not care how many conduits are involved, 2 sets of paralleled conductors in (1) conduit are treated the same as if they were in (2) separate conduits.
 

don_resqcapt19

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...
I will grant that the material of construction and the geometry of a conduit will have an effect on the circuit's overall inductive and capacitive reactance. But these are superficial effects on the currents that will flow in the circuit. Having two parallel A Phase conductors of different material, AWG size, or length can have a much more substantial effect. That is the concern over which this requirement is based.
Those concerns are addressed in 310.10(H)(2). The raceway is specifically addressed in (H)(3).
 

infinity

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I am going to take a stab into the darkness, and submit for your consideration that conduit does not have any "electrical characteristics" at all. It may have magnetic properties, and if so that could influence the outcome of a short circuit event. But it is the wires that have electrical properties, and the conduit that only has physical properties, magnetic permeability being one of them.


A metallic conduit is conductive and a PVC conduit isn't, are those electrical characteristics?
 

iwire

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I think not. They are physical properties that have no bearing on the electrical circuit that they surround.

I will have to disagree.

The capacitance coupling of the circuit to the conduit is an electrical characteristic and will be different between PVC, steel or aluminum.
 

charlie b

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I will have to disagree.
I suppose there has to be a first time for everything. :lol:

The capacitance coupling of the circuit to the conduit is an electrical characteristic and will be different between PVC, steel or aluminum.
They will not have any measurable influence on the circuit. Therefore, I do not consider the conduit type to be relevant to the requirement that parallel runs have similar electrical characteristics.

 

iwire

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I suppose there has to be a first time for everything. :lol:


I will mark it on my calender as a special day. :D


They will not have any measurable influence on the circuit. Therefore, I do not consider the conduit type to be relevant to the requirement that parallel runs have similar electrical characteristics.

Perhaps I mean impedance? :? :(

But what I am getting at is that when I see voltage drop calculators I often see choices for PVC vs Steel. To me that indicates that the conduit does have a measurable impact on the circuit.

BTW are you just talking theory or code? Because code wise I think it is pretty clear I get a red tag if I run two parallel sets one in steel and one in PVC.
 

steve66

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Even going back to the 2008 handbook, the commentary says raceways must be of the same size, material, and length.
 

GoldDigger

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I will mark it on my calender as a special day. :D




Perhaps I mean impedance? :? :(

But what I am getting at is that when I see voltage drop calculators I often see choices for PVC vs Steel. To me that indicates that the conduit does have a measurable impact on the circuit.

BTW are you just talking theory or code? Because code wise I think it is pretty clear I get a red tag if I run two parallel sets one in steel and one in PVC.
Possibly the calculator only includes the effect of PVC versus steel if you select a one-way wire length, indicating unbalanced current. Such a configuration (uniphase pairing) would not be acceptable for paralleling anyway.
Have you tried changing the conduit type to see how the result changes?
 

charlie b

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I was talking theory, Bob. And your mentioning capacitive coupling and mentioning impedance essentially conveyed the same meaning. I don't know how any VD calculator would alter its results, if you selected a different material for the conduit. I just believe that unless you are dealing with many hundreds of feet, or perhaps even thousands of feet, the influence of the conduit material would be too small to measure. It is a different matter, when you push thousands of amps during a fault event. A magnetically permeable material, such as steel, will tend to act as a choke, reducing the amount of current, and thus delaying the tripping of the breaker.
 
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