1/C with 3/C in parallel?

D

Dale001289

Senior Member
Location
Georgia
winnie said:
The code doesn't say 'same materials'. It also doesn't say 'same capability'. It says 'same characteristics'.

The conventional interpretation of 'same characteristics' is same material, same insulation, same conductor arrangement, same type of conduit, etc.

This is based on having current divide evenly between the paralleled conductors. Small differences in impedance between the two conductors will result in uneven current sharing.

If you picked two random conductors, both of 150A ampacity (say 1/0 Cu with a 75C rating of 150A in a steel conduit and a 1/0 Cu with a 75C rating as part of an Aluminum MC cable, and put them in parallel, then they would have _different_ characteristics and you would expect uneven current sharing.

-Jon
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Slightly irregular impedance that “might” result in a couple amps differential isn’t enough to worry about. I’ve never seen parallel conductors carry the exact same current or voltage.


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I

Isaiah

Senior Member
Location
Baton Rouge
Occupation
Electrical Inspector
winnie said:
The code doesn't say 'same materials'. It also doesn't say 'same capability'. It says 'same characteristics'.

The conventional interpretation of 'same characteristics' is same material, same insulation, same conductor arrangement, same type of conduit, etc.

This is based on having current divide evenly between the paralleled conductors. Small differences in impedance between the two conductors will result in uneven current sharing.

If you picked two random conductors, both of 150A ampacity (say 1/0 Cu with a 75C rating of 150A in a steel conduit and a 1/0 Cu with a 75C rating as part of an Aluminum MC cable, and put them in parallel, then they would have _different_ characteristics and you would expect uneven current sharing.

-Jon
Click to expand...

If you triplex the single conductors in triangular shape within the conduit it should offset the small differences in impedance


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W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Dale001289 said:
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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Can you be a bit more specific, maybe a pointer to the spec sheet or catalog page?

A search for "okonite 2kv cable" brings me to https://www.okonite.com/product-catalog/section-4 which covers "Multi-Conductor Low Voltage Cables to 2kV". But none of the product there are described as both "okoseal" and "2kV". Indeed only the control cables there are rated 2kV.

As for interpreting (2020) 310.10(G) on parallel conductors and how it applies to the OP where one conduit has 3 individual conductors and the other identical conduit has a 3 conductor cable, it seems to me the possible sticking points are:

310.10(G)(2)(4), which says that the conductors should have the same insulation type. The question is whether insulation type means just the material, or also the overall thickness. As the 3 conductor cable will have an overall jacket, and then arguably the conductors do not have the same insulation thickness in both conduits. Also a potential issue if the jacket material does not match the underlying individual conductor insulation.

310.10(G)(3), which says in part that "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." For this, do we compare the raceway containing the single conductors with the cable sheath containing the equal number of conductors in the other raceway? If so, does the raceway have the same electrical characteristics as the cable sheath? Perhaps if the raceway is PVC, not sure.

Otherwise I see no issue in terms of what is written in 310.10(G).

Cheers, Wayne
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Dale001289 said:
Or, maybe because it meets the intent of the code and is completely safe?
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Oh, I get it that that is your opinion, but if you install it and come up against an inspector who does not agree, you may have a problem.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Dale001289 said:
Or, maybe because it meets the intent of the code and is completely safe?


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I think most/all of us think it will work fine and is safe. I think what most of us are debating is whether it is code compliant. I'll be one of the first people to say something not code compliant does not mean it's unsafe. But If you're getting this inspected I would say there is a significant chance the inspector will reject it.
 
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