Derating wire in conduit for branch circuits

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
I made a PI for 2023 that would add the following sentence to the end of the note you quoted: "Where two or more conductors are protected by the same overcurrent device, and their currents are additive at the overcurrent device, they may be counted as a single conductor." Which seem logical to me, 20A spread over 5 conductors generates less heat than 20A on 1 conductor.

The panel response was "Loading and overcurrent protection could be changed in the future based on installed conductor sizes." To which my PC basically says "sure it could, but the possibility of future changes applies to any rule in the NEC, so give me a substantive technical reason to reject this particular proposal." I'm not holding my breath.

Cheers, Wayne
That is a good proposal and electrically it makes sense. But you said logical which unfortunately seems to escape much of the PI process. :rolleyes:

Responses like that are one reason I stopped wasting my time with PI's.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
That is a good proposal and electrically it makes sense.
Actually, I see now that the proposed language is a bit unclear on the case of parallel conductors. So it should be "Where two or more conductors are individually protected by the same overcurrent device. . ."

Cheers, Wayne
 

texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
Ah, his is an issue has has come up in my work. I'm in the camp that says you can use as many conductors as will fit as long as they are on the same OCPD. After all, the physics of this are kind of obvious. As an inspector I've have no quarrels approving this. I do agree that the code language should be modified and have often thought of submitting a PI. I think wwhitney is on the right track.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Ah, his is an issue has has come up in my work. I'm in the camp that says you can use as many conductors as will fit as long as they are on the same OCPD. After all, the physics of this are kind of obvious. As an inspector I've have no quarrels approving this. I do agree that the code language should be modified and have often thought of submitting a PI. I think wwhitney is on the right track.

you mean as many as the code allows for fill without derating, I assume. You still have to comply with conduit fill
 

kwired

Electron manager
Location
NE Nebraska
All in one raceway it counts as 8 CCC's. The fact that they're on one circuit is irrelevant. Something I've been complaining about for decades.
Yes it counts as 8. But the ungrounded supply and the neutral are the only two that possibly might need change in size after the adjustments.

For example if feeding six 2 amp loads, the switched legs only draw 2 amps each but the common supply and common return both have 12 amps on them. The switched lines aren't even close to needing a change in conductor size compared to the two commons.
 

kwired

Electron manager
Location
NE Nebraska
I made a PI for 2023 that would add the following sentence to the end of the note you quoted: "Where two or more conductors are protected by the same overcurrent device, and their currents are additive at the overcurrent device, they may be counted as a single conductor." Which seem logical to me, 20A spread over 5 conductors generates less heat than 20A on 1 conductor.

The panel response was "Loading and overcurrent protection could be changed in the future based on installed conductor sizes." To which my PC basically says "sure it could, but the possibility of future changes applies to any rule in the NEC, so give me a substantive technical reason to reject this particular proposal." I'm not holding my breath.

Cheers, Wayne
AFAIK the process as is isn't exactly precise to begin with when it comes to figuring actual potential for damage to conductors due to heating within a raceway/cable the results likely conservative enough you probably can get away with conductors that are improperly adjusted per the current rules and never really have any problems in most instances, especially if there is some load diversity that the code doesn't really address much at all. If everything is continuously loaded and especially at/near the base conductor ampacities, it maybe is good practice to assure you did follow what code does say for adjustments though.
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
Yes it counts as 8. But the ungrounded supply and the neutral are the only two that possibly might need change in size after the adjustments.

For example if feeding six 2 amp loads, the switched legs only draw 2 amps each but the common supply and common return both have 12 amps on them. The switched lines aren't even close to needing a change in conductor size compared to the two commons.
If there all on one circuit the adjusted ampacity for each conductor ( disregarding next size up rules) still needs to be greater than or equal to the OCPD size.
 

kwired

Electron manager
Location
NE Nebraska
If there all on one circuit the adjusted ampacity for each conductor ( disregarding next size up rules) still needs to be greater than or equal to the OCPD size.
Yes, this example somewhat a poor one because if either a 15 or 20 amp circuit, only 8 conductors vs say 20 conductors, plus along with the small conductor OCPD rules you likely don't need to increase conductor size anyway.

But say you in a situation where the adjustment factor is 50%. Those common conductors carrying 12 amps may need to be increased in size where the ones only carrying 2 amps don't even though they are on the same ocpd.
 
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