De-rating when there is very little current

scrypps

Member
Situation: Lighting Control. A single 20a circuit feeds 8 lighting loads in a room. The contractor ran 8 hots and 8 neutrals for a total of 16 current carrying conductors in a 20' run of 1-1/2" emt from the control module to a j-box in the ceiling where lighting loads are. According to 310.15(b)(3), these should be de-rated by 50%.

My issue is that per 310.15, I can install 4 fully loaded 20a circuits and have 80 amps of power going through a conduit and the code is fine with it. In this case, we have 16 conductors carrying a maximum of 7 amps, and the code is not ok with that if the conductors are 12awg. Furthermore, if this wasn't lighting control, there would just be a hot and a neutral running to a box with 8 switches in it.

I'm inclined to think of it as not a big deal, I just can't imagine a heat damage situation occurring.

As a though experiment. What if I had one 20a circuit serving a permanently connected load of 7amps and just for the heck of it, I decided to parallel my #12 conductors in a long raceway from box to box, so the run had 16 pretty conductors spliced together from end to end. Would this be a violation of 310.15(B)(3) and would the intent of that section be met if this were considered a violation?

infinity

Moderator
Staff member
Might not ever be a problem but it's still a violation. Even mulitple switch legs on the same circuit require derating. It is what it is, a basic one size fits all rule that doesn't always make sense.

jap

Senior Member
Situation: Lighting Control. A single 20a circuit feeds 8 lighting loads in a room. The contractor ran 8 hots and 8 neutrals for a total of 16 current carrying conductors in a 20' run of 1-1/2" emt from the control module to a j-box in the ceiling where lighting loads are. According to 310.15(b)(3), these should be de-rated by 50%.

My issue is that per 310.15, I can install 4 fully loaded 20a circuits and have 80 amps of power going through a conduit and the code is fine with it. In this case, we have 16 conductors carrying a maximum of 7 amps, and the code is not ok with that if the conductors are 12awg. Furthermore, if this wasn't lighting control, there would just be a hot and a neutral running to a box with 8 switches in it.

I'm inclined to think of it as not a big deal, I just can't imagine a heat damage situation occurring.

As a though experiment. What if I had one 20a circuit serving a permanently connected load of 7amps and just for the heck of it, I decided to parallel my #12 conductors in a long raceway from box to box, so the run had 16 pretty conductors spliced together from end to end. Would this be a violation of 310.15(B)(3) and would the intent of that section be met if this were considered a violation?
You'd be in violation of 310.4 first.

JAP>

Barbqranch

Senior Member
Doesn't the fact that they are all one one single 20 amp breaker negate the need to derate?

LarryFine

Master Electrician Electric Contractor Richmond VA
Installing a 15a breaker would make this compliant.

winnie

Senior Member
I thought there was an exception for non-coincident loads, and this is sort of that situation.

Because these are all fed from a single circuit, any current on one 'leg' means that much less current available for the other 'legs'. The worst case heating scenario is when one 'leg' carrys all the current, in which case you would only have 2 cccs.

Jon

kwired

Electron manager
Take a look at 240.4(B)(1).

OP's arrangement (before deciding to make some parallel conductors) is NEC compliant.

90C 12 AWG with 50% required ampacity adjustment can carry 15 amps, OP says they are only carrying 7 amps,

12 AWG is sufficient for this and 20 amp OCPD is still acceptable here as well.

14 AWG on 15 amp OCPD also would still be compliant as it would have a derated ampacity of 12.5 amps.

If he had multiple outlet receptacle outlet circuits instead of fixed loads he would have to assume that each circuit could draw up to what the 20 amp breaker would allow and then would need to increase conductor size because of the adjustment.

winnie

Senior Member
But he doesn't have multiple circuits. He has 1 circuit split into 8 parallel paths all in one conduit.

Jon

infinity

Moderator
Staff member
But he doesn't have multiple circuits. He has 1 circuit split into 8 parallel paths all in one conduit.

Jon
As dumb as it sounds that would still be at least 9 CCC's (depending on if the neutral were run with the ungrounded conductor) so derating is required

LarryFine

Master Electrician Electric Contractor Richmond VA
90C 12 AWG with 50% required ampacity adjustment can carry 15 amps, OP says they are only carrying 7 amps,

12 AWG is sufficient for this and 20 amp OCPD is still acceptable here as well.
Why would a 20a breaker be compliant with 15a breakers available? See 240.4(B)(2).

infinity

Moderator
Staff member
Why would a 20a breaker be compliant with 15a breakers available? See 240.4(B)(2).
It wouldn't be compliant with 50% derating of #12 conductors. 30 amps*50%=15 amps which is a standard size.

texie

Senior Member
If you have 9 switch legs fed from the same breaker I don't believe this requires derating. If anything the circuit would run cooler as the load would be spread over all conductors. I recognize that the language of the code may not say it this way but it just seems to violate the common sense rule to ignore the physics. I would never flag somebody for this.

infinity

Moderator
Staff member
If you have 9 switch legs fed from the same breaker I don't believe this requires derating. If anything the circuit would run cooler as the load would be spread over all conductors. I recognize that the language of the code may not say it this way but it just seems to violate the common sense rule to ignore the physics. I would never flag somebody for this.
It doesn't matter that they're on the same circuit, you count the number of CCC's and derate based on that. I agree, split a 9 amp load over 9 conductors and it will generate almost no heat.

kwired

Electron manager
Why would a 20a breaker be compliant with 15a breakers available? See 240.4(B)(2).
Correct on that, would need to have 15 amp breaker.

I have run into this sort of situation more frequently with multiple motor circuits in one raceway, but with those 240.3 sends you to art 430 where overcurrent protection rules are different from the general rules, reality is motor conductors are protected to same level as the motor overload protection even though the short circuit/ground fault protection is usually higher than the general rules

kwired

Electron manager
As dumb as it sounds that would still be at least 9 CCC's (depending on if the neutral were run with the ungrounded conductor) so derating is required
and it still ends up with each conductor carrying less than the derated ampacity. Overcurrent protection required can be impacted though as was mentioned in my other reply.

don_resqcapt19

Moderator
Staff member
It doesn't matter that they're on the same circuit, you count the number of CCC's and derate based on that. I agree, split a 9 amp load over 9 conductors and it will generate almost no heat.
This would be a good place for a 2023 PI, but not sure what the language would look like to make it understandable.

16 conductors each carrying 5 amps, would produce the same total heat as 2 conductors of the same size each carrying 20 amps.

tortuga

Senior Member
Situation: Lighting Control. A single 20a circuit feeds 8 lighting loads in a room. The contractor ran 8 hots and 8 neutrals for a total of 16 current carrying conductors in a 20' run of 1-1/2" emt from the control module to a j-box in the ceiling where lighting loads are. According to 310.15(b)(3), these should be de-rated by 50%.
They do make smaller than 15 amp breakers that are sometimes used in lighting control panels.
These fall under supplementary over-current protection.
This would be a good place for a 2023 PI, but not sure what the language would look like to make it understandable.

16 conductors each carrying 5 amps, would produce the same total heat as 2 conductors of the same size each carrying 20 amps.
Yeah some kind of wording recognizing supplemental protection like smaller breakers for lighting.

wwhitney

Senior Member
This would be a good place for a 2023 PI, but not sure what the language would look like to make it understandable.
How about something like "where two or more current carrying conductors are protected by a single overcurrent device, and the currents in them are additive at the overcurrent device, they may be counted as a single current carrying conductor."

Cheers, Wayne