1/C with 3/C in parallel?

[Dale001289]

While this proposed installation may meet the letter of the code, I don't think it meets the spirit of the code. Then there are the physics of this. I don't see how a triple factory twisted assembly of conductors would behave the same electrically as individual conductors in the other raceway. And are they of the same type of insulation? There is also the issue of the EGCs now being to small and/or different sizes for the new increased amperage (assuming wire type EGCs are used). And, as a practical matter, how do you add second raceway of the same approx. length when it was not planned on when the first raceway was installed?

There is no such thing as the spirit of the code. There is the letter of the code and there is the intent of the code. Also the code is not a design guide. The issue of EGC has already been mentioned several times in this thread.
Unless you’re able to post the physics you mentioned, this installation should be considered safe, adequately engineered and meets both the letter of the code and its intent.


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[winnie]

The requirement in code is that parallel conductors have the same electrical characteristics.

This means the same conductor material, of the same size, with the same insulation, in the same electrical environment, of the same length.

Electrical environment includes the surrounding conductors and type of conduit.

I would argue that it is impossible to meet the letter of the code, because there is no mention made of things such as measurement or conductor production tolerances. So in a real way, all parallel installations violate the letter of code yet most are safe and meet the spirit of the code.

If the '3C' being considered is constructed of the same conductive material, of the same nominal size, with the same insulation type, in the same type of conduit with the same number and phases of conductor, it would still be different from the existing conductor:

The different conductor arrangement will give different impedance, and the insulation would have aged differently.

In real world installation people usually don't control the conductor arrangement, they just pull conductors into the pipes. And while most parallel installations are done all at once, I doubt insulation aging is ever considered or particularly significant.

I think it is likely that the installation being considered would be safe and meet the spirit of the code (with the exception of the EGC size mentioned by others), but IMHO it doesn't meet the letter of the code.

Jon

 

[kwired]

How are you going to address this?

(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.

Same electrical characteristics comes into play for OP as well. The idea is you want equal overall impedance on each segment of a parallel set. If not equal more current will take the lesser impedance path. Too much difference may overload one path while the other path carries somewhat minimal current.

 

[Dale001289]

The requirement in code is that parallel conductors have the same electrical characteristics.

This means the same conductor material, of the same size, with the same insulation, in the same electrical environment, of the same length.

Electrical environment includes the surrounding conductors and type of conduit.

I would argue that it is impossible to meet the letter of the code, because there is no mention made of things such as measurement or conductor production tolerances. So in a real way, all parallel installations violate the letter of code yet most are safe and meet the spirit of the code.

If the '3C' being considered is constructed of the same conductive material, of the same nominal size, with the same insulation type, in the same type of conduit with the same number and phases of conductor, it would still be different from the existing conductor:

The different conductor arrangement will give different impedance, and the insulation would have aged differently.

In real world installation people usually don't control the conductor arrangement, they just pull conductors into the pipes. And while most parallel installations are done all at once, I doubt insulation aging is ever considered or particularly significant.

I think it is likely that the installation being considered would be safe and meet the spirit of the code (with the exception of the EGC size mentioned by others), but IMHO it doesn't meet the letter of the code.

Jon

I don’t think the intent is to cover every conceivable scenario. At some point, reason and sound engineering has to prevail. Purists will insist the letter of the code must always be met in every installation. I disagree


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[Dale001289]

(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.

Same electrical characteristics comes into play for OP as well. The idea is you want equal overall impedance on each segment of a parallel set. If not equal more current will take the lesser impedance path. Too much difference may overload one path while the other path carries somewhat minimal current.

I would argue the same characteristics have been met. The slight difference in impedance would not play a significant role in a 100’ feeder


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[winnie]

In post 21 you said the installation meets the letter of the code, and challenged people to post refuting physics.

Now you say that it meets the intent of the code even though purists would say it doesn't.

IMHO the burden of proof is on you to show that the impedance difference will be negligible.

Have you calculated the impedance of 3 conductors side by side in a conduit vs 3 conductors tightly bundled in a conduit? It is only 100 feet, so the absolute value will be low, but what matters is the percentage difference, and since the difference will be in magnetic inductance, the % difference may be swamped by the resistance of the conductors.

My hunch agrees with yours, that the difference will be negligible. But it is your installation and your problem if the hunch is wrong.

Jon

 

[electrofelon]

(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.

Same electrical characteristics comes into play for OP as well. The idea is you want equal overall impedance on each segment of a parallel set. If not equal more current will take the lesser impedance path. Too much difference may overload one path while the other path carries somewhat minimal current.

Is that the actual code wording? Which edition? I don't see the "shall have the same electrical characteristics" in the 2017

 

[Dale001289]

In post 21 you said the installation meets the letter of the code, and challenged people to post refuting physics.

Now you say that it meets the intent of the code even though purists would say it doesn't.

IMHO the burden of proof is on you to show that the impedance difference will be negligible.

Have you calculated the impedance of 3 conductors side by side in a conduit vs 3 conductors tightly bundled in a conduit? It is only 100 feet, so the absolute value will be low, but what matters is the percentage difference, and since the difference will be in magnetic inductance, the % difference may be swamped by the resistance of the conductors.

My hunch agrees with yours, that the difference will be negligible. But it is your installation and your problem if the hunch is wrong.

Jon

Rather than continue with conceptual and abstract arguments…
Let me look at some calculations…IF they favor my argument I will post them . Lol.
If not I will simply modify the installation and search for single conductors. As noted they’re currently out of stock so we will definitely take a hit on lead times which of course will affect schedule. Not good


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[wwhitney]

in the same electrical environment

That phrase does not appear in 310.10(G).

As to 310.10(G)(3), it's not clear to me how to apply the part about "the cables or raceways . . . shall have the same electrical characteristics." How do you compare the electrical characteristics of a raceway with the electrical characteristics of a cable? Or if you are relying on the cable being in a raceway as well, how do you get to ignore the electrical characteristics of the cable itself?

As to the basic issue of 3/C being available and 1/C not, if the insulation type is the same, why not just remove the jacket of the 3/C cable and run the individual conductors within the conduit?

Cheers, Wayne

 

[Dale001289]

That phrase does not appear in 310.10(G).

As to 310.10(G)(3), it's not clear to me how to apply the part about "the cables or raceways . . . shall have the same electrical characteristics." How do you compare the electrical characteristics of a raceway with the electrical characteristics of a cable? Or if you are relying on the cable being in a raceway as well, how do you get to ignore the electrical characteristics of the cable itself?

As to the basic issue of 3/C being available and 1/C not, if the insulation type is the same, why not just remove the jacket of the 3/C cable and run the individual conductors within the conduit?

Cheers, Wayne

What kind of labor-man hours would it take to strip say, 120’ of 3/C w/ground?
I’ve never seen this done before

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[roger]

What kind of labor-man hours would it take to strip say, 120’ of 3/C w/ground?


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So it's a cable assembly, why not tell everyone what it is?

 

[roger]

Is that the actual code wording? Which edition? I don't see the "shall have the same electrical characteristics" in the 2017

My quote was from the 11 because it's what I had convenient at the time.

 

[Dale001289]

So it's a cable assembly, why not tell everyone what it is?

Okonite, 1/0, 3/Cw/ground Okoseal ? Copper
90degree C unshielded 2kV rated
I don’t recall if it’s TC rated


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[electrofelon]

My quote was from the 11 because it's what I had convenient at the time.

I usually use the 2017 because it is in convenient PDF. I know I should start using something a bit newer.....

 

[wwhitney]

Is that the actual code wording? Which edition? I don't see the "shall have the same electrical characteristics" in the 2017

Yes, it's the 2020 NEC

I usually use the 2017 because it is in convenient PDF. I know I should start using something a bit newer.....

Or at least check the 2020 NEC when you have a discrepancy like this. You can read it as HTML here: https://up.codes/viewer/alabama/nfpa-70-2020

Cheers, Wayne

 

[electrofelon]

So it's a cable assembly, why not tell everyone what it is?

Okonite, 1/0, 3/Cw/ground Okoseal ? Copper
90degree C unshielded 2kV rated
I don’t recall if it’s TC rated


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Ok it must be a RHH/RHW-2 insulation if it is 2kv rated. Just a quick look at the okonite catalog , must be either their "okoclear" or "okolon" product.

Here is an interesting question: the "same insulation type" requirement, would one set being a 600v RHH/RHW-2 and the other being a 2kv RHH/RHW-2 be acceptable? What type and voltage rating of conductors are in the existing run?

 

[electrofelon]

Ok it must be a RHH/RHW-2 insulation if it is 2kv rated. Just a quick look at the okonite catalog , must be either their "okoclear" or "okolon" product.

Here is an interesting question: the "same insulation type" requirement, would one set being a 600v RHH/RHW-2 and the other being a 2kv RHH/RHW-2 be acceptable? What type and voltage rating of conductors are in the existing run?

Those cables I mentioned appear to only be single conductor. Only multi conductor I can find are XHHW and 1kv

 

[LarryFine]

The slight difference in impedance would not play a significant role in a 100’ feeder

That's the crux of the conversation, and it depends on how close to max paralleled ampacity the actual load is at any given moment. The greater the load, the more critical any impedance difference will be.

 

[ggunn]

Still struggling with why single conductors cannot be obtained, strange.

I am wondering the same. 1/0 wire is pretty common.

 

[electrofelon]

I am wondering the same. 1/0 wire is pretty common.

It may be a special type of cable, perhaps MV, but details are obviously very sparse.