Thread: Derating wire

Looking for guidance on derating wire.

I know that if a wire is bundled it is subject to derating.

I know that if wire goes through a hot attic space it is subject to derating.

I believe that if a wire is bundled in an attic, both derating factors should apply.

What if a wire is bundled at some point, later unbundled, and then run through an attic? Seems to me it should be subject to whichever derate factor is greatest, but not subject to cumulative derating.

Is there specific guidance on this, code or otherwise?

Thanks

Originally Posted by Erik Pendleton

Looking for guidance on derating wire.

I know that if a wire is bundled it is subject to derating.

I know that if wire goes through a hot attic space it is subject to derating.

I believe that if a wire is bundled in an attic, both derating factors should apply.

What if a wire is bundled at some point, later unbundled, and then run through an attic? Seems to me it should be subject to whichever derate factor is greatest, but not subject to cumulative derating.

Is there specific guidance on this, code or otherwise?

Thanks

The conductors would be (re)-rated to the lowest ampacity..The temperature adjustment used would be the one that allowed the lowest ampacity although multiple places may need to be calculated. The largest (permitting the lowest ampacity) number of conductor bundled for 2 feet or more would be used. I must add that this is mostly never done in dwelling units/residences since it would result in ridiculously large conductors. And the diversity of use is not calculable. hence the load in amps is elusive.

I used (re)-rated on temperature because with low temperatures you would actually gain or increase the ampacity..

Originally Posted by Roland Miller

The conductors would be (re)-rated to the lowest ampacity..The temperature adjustment used would be the one that allowed the lowest ampacity although multiple places may need to be calculated. The largest (permitting the lowest ampacity) number of conductor bundled for 2 feet or more would be used. I must add that this is mostly never done in dwelling units/residences since it would result in ridiculously large conductors. And the diversity of use is not calculable. hence the load in amps is elusive.

I used (re)-rated on temperature because with low temperatures you would actually gain or increase the ampacity..

Ronald, I am not sure I understand your answer.

Here is how I think the factors should be applied:

Lets say the wire is 12 awg copper type THHN and is part of a 6 conductor bundle (3 x 12/2) which is bundled for 5 feet in the basement. The bundle would be subject to 0.8 derate from 30 to 24 amps.

Now say one of the circuits goes up to the attic with an ambient temperature of 130*F
max and runs through the attic for 20 feet. This wire would be subject to a derate of 0.76 for temperature, from 30 to 22.8 amps.

Since the lowest adjusted ampacity for this wire is 22.8 amps, 22.8 amps would be applied. Since this is above the normal 20 amp limit for 12 awg copper, the circuit would allow for max 20 amp breaker.

However, I have read some forum posts that seem to indicate that the wire should be derated for both bundling and temperature, even though the two derate issues are in separate areas. Doing this would result in the following 30 x 0.8 x 0.76 = 18.24 amp limit, resulting in max 15 amp breaker.

Which is the correct method?

The NEC requires all adjustment (derating) factors for each circuit be applied. So this is correct:

However, I have read some forum posts that seem to indicate that the wire should be derated for both bundling and temperature, even though the two derate issues are in separate areas. Doing this would result in the following 30 x 0.8 x 0.76 = 18.24 amp limit, resulting in max 15 amp breaker.

So if this were a required small appliance branch circuit or laundry circuit (or any other circuit required to be 20 amps), you would need to increase the conductor size until you have the required remaining ampacity. Remembering that you can only utilize the ampacity available in the 60 C column...

Originally Posted by Roland Miller

The NEC requires all adjustment (derating) factors for each circuit be applied. So this is correct:

However, I have read some forum posts that seem to indicate that the wire should be derated for both bundling and temperature, even though the two derate issues are in separate areas. Doing this would result in the following 30 x 0.8 x 0.76 = 18.24 amp limit, resulting in max 15 amp breaker.

So if this were a required small appliance branch circuit or laundry circuit (or any other circuit required to be 20 amps), you would need to increase the conductor size until you have the required remaining ampacity. Remembering that you can only utilize the ampacity available in the 60 C column...

But that doesn't make any sense. Do you have a quote from the code? Who could deny that bundling in the attic would not result in higher wire temps in the attic than bundling remotely from the attic? Yet you are saying to derate them the same?

If the conductors are both bundled and run through a high ambient temperature, the NEC requires both adjustments be made for those conductors affected. If part of the conductors only are bundled or are only ran through a higher temperature, only that single adjustment needs to be made. Both conditions add heat to the conductor as does the current flow. Since we are protecting the insulation (which is temperature rated and the weak link, usually) all adjustments need to be applied that the conductor is exposed to. There is not one specific statement that states this and that is one reason why the NEC is NOT a training manual..The other things to consider are the temperature limitations of the terminations, splices, raceway, etc..