1/C with 3/C in parallel?

D

Dale001289

Senior Member
Location
Georgia
Field wants to add a parallel feeder but can’t find single conductor for the 2nd feeder - they want to use 3/C instead. They’d both be routed in separate conduits.
If the 3/C size is the same 1/C I don’t see why this wouldn’t work in parallel. Comments?


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electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
Dale001289 said:
Field wants to add a parallel feeder but can’t find single conductor for the 2nd feeder - they want to use 3/C instead. They’d both be routed in separate conduits.
If the 3/C size is the same 1/C I don’t see why this wouldn’t work in parallel. Comments?


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Click to expand...
Review 310.10(H), particularly (2)(4). What exactly do you mean by 3/C? Is it a cable assembly or just plexed? IMO plexing means nothing in the NEC, although a real picky person could say it makes the length not the same.
 
D

Dale001289

Senior Member
Location
Georgia
electrofelon said:
Review 310.10(H), particularly (2)(4). What exactly do you mean by 3/C? Is it a cable assembly or just plexed? IMO plexing means nothing in the NEC, although a real picky person could say it makes the length not the same.
Click to expand...

It’s a factory assembled 3/C cable with an integral grounding conductor


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roger

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
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.
Click to expand...
 
roger

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
electrofelon said:
Can you expand on that roger, I'm not seeing where that is necessarily a violation. The way I look at it, if the inners of the cable assembly match all the conditions and the other run you can sort of ignore that it's a cable assembly.
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Well after rereading I see I was missing the fact that both conduits will in fact have the same number of conductors.
 
G

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
roger said:
Well after rereading I see I was missing the fact that both conduits will in fact have the same number of conductors.
Click to expand...
One thing that wasn't mentioned is the reason for the second parallel run. If they want to increase the OCPD on the feeder to take advantage of the increased ampacity, the ground wires will be too small. If it's just for voltage drop they need to be upsized as well.
 
electrofelon

electrofelon

Senior Member
Location
Cherry Valley NY, Seattle, WA
ggunn said:
One thing that wasn't mentioned is the reason for the second parallel run. If they want to increase the OCPD on the feeder to take advantage of the increased ampacity, the ground wires will be too small. If it's just for voltage drop they need to be upsized as well.
Click to expand...
Unless the original run didn't have a wire type EGC, then could one just not land the EGC in the cable assembly and call it a spare?

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

synchro

Senior Member
Location
Chicago, IL
Occupation
EE
When paralleled, the conductors of a three conductor cable will have slightly less resistance than a single conductor with the same total cross section due to the skin effect and proximity effect. Such reduction in resistance becomes more pronounced with increasing frequencies, which is why Litz wire composed of many individually insulated strands is sometimes used at frequencies up to a few MHz.
Whether these phenomena would make any significant difference in the OP's case would depend on the details.
 
D

Dale001289

Senior Member
Location
Georgia
synchro said:
When paralleled, the conductors of a three conductor cable will have slightly less resistance than a single conductor with the same total cross section due to the skin effect and proximity effect. Such reduction in resistance becomes more pronounced with increasing frequencies, which is why Litz wire composed of many individually insulated strands is sometimes used at frequencies up to a few MHz.
Whether these phenomena would make any significant difference in the OP's case would depend on the details.
Click to expand...

The runs are in separate rigid metal conduits and are only about 100 feet long. The system is a standard 60 hertz.


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T

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
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?
 
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