310.15(a)2 exception

[tboyer]

The wording is fuzzy to me in 310.15(a)2's exception. The debate in class, taught by a state inspector, was whether the 10% is 10% of the circuit length, or 10% of the portion of the circuit which is rated at the higher ampacity. Common wisdom around here declares that it is 10% of the total circuit length, but Mike Holt's video and the way I read it seem to indicate the latter.

Example of a 100' run, a section of which runs along a rooftop. I do not have to adjust the conductor's ampacity if it runs through this hotter section no longer than (a) 10', or (b) 9'?

I've been searching the web and the forums for a while but have not found this question addressed. Apologies if I missed it. Thank you.

 

[augie47]

IMO, on a 100 ft run, the distance would be 10ft. 10% of the circuit length.
On a 150 ft run the distance would be 10 ft., the lesser number.

Welcome to the Forum !

 

[infinity]

IMO, on a 100 ft run, the distance would be 10ft. 10% of the circuit length.
On a 150 ft run the distance would be 10 ft., the lesser number.

Welcome to the Forum !

I agree with Augie, it's 10% of the entire circuit length or 10' which ever is less.

 

[tboyer]

Thank you both for the replies. The problem I have with the "10% of the entire run" answer is that the wording of the article doesn't seem to point to that. It seems to point to "10% of the portion of the run which would have a higher ampacity". I'll quote it below, and use bold text to make it easy to see how this interp could be arrived at.

This is from 310.15(a)2 exception.

Exception:? Where two different ampacities apply to adjacent portions of a circuit the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less.

The "3.0 m (10 ft)" is one distance. The "10 percent of the circuit length figured at the higher ampacity" is the second distance. Whichever of these two is less, this is the distance where "the higher ampacity shall be permitted to be used". So it's either ten feet, or ten percent of this value: the-circuit-length-figured-at-the-higher-ampacity.

You have a hundred foot run. Ten feet of it is across a roof. Ninety feet of it is figured at a higher ampacity, due to temps, than the ten foot roof section. Because ten percent of that ninety foot higher-ampacity section is nine feet, the whole run must be adjusted down to the ampacity of the roof section, because it is ten feet, which is more than ten percent.

I don't mean to go on. Just trying to show how it could be read. I really appreciate your time.

 

[charlie b]

Just trying to show how it could be read.

I agree with the way you are reading it, tboyer. And to complete your example, let us consider a 150 foot run, of which 10 feet is across the roof. That means 140 feet of the run has a higher ampacity. Ten percent of that length is 14 feet. We have to use 10, as it is lower than 14. So we can use the higher ampacity for the entire 140 feet that is not on the roof, plus we can use the higher ampacity for the first 10 feet that is on the roof, which is to say the entire 10 feet. As a result, we can use the higher ampacity for the circuit in its entirety.

 

[eprice]

Ok, let me throw in a related question then. The code section says "the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to...". What if there are two points of transition? Suppose we have a run that consists of 30' at a lower ampacity, then 90' and a higher ampacity, then 20' again at a lower ampacity. Does that mean we can use the higher ampacity for 9' past each transition? I think that is what the code language says.

 

[GoldDigger]

Ok, let me throw in a related question then. The code section says "the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to...". What if there are two points of transition? Suppose we have a run that consists of 30' at a lower ampacity, then 90' and a higher ampacity, then 20' again at a lower ampacity. Does that mean we can use the higher ampacity for 9' past each transition? I think that is what the code language says.

Looks that way to me.
And if the lower ampacity section is in between two with high ampacity, you would add the two high ampacity segments together and then apply the 10% factor.

 

[Smart $]

Looks that way to me.
And if the lower ampacity section is in between two with high ampacity, you would add the two high ampacity segments together and then apply the 10% factor.

I don't agree with that, at least not how I interpret the way you wrote it...

The lesser of 10' or 10% limitation applies to each transition, using the length of the higher ampacity section to determine the distance excepted.

For example, if you had 50' of highest ampacity, 10' of lowest in the middle, then another 50' at the highest ampacity on the other end. You could except up to 5' of each highest ampacity section towards the middle, a total of 10', and thus render the entire circuit length at the highest ampacity.

 

[Smart $]

...

For example, if you had 50' of highest ampacity, 10' of lowest in the middle, then another 50' at the highest ampacity on the other end. You could except up to 5' of each highest ampacity section towards the middle, a total of 10', and thus render the entire circuit length at the highest ampacity.

However, if the last 50' section had an ampacity in between the highest and lowest, the entire circuit would be limited to this in-between ampacity value.

 

[GoldDigger]

I don't agree with that, at least not how I interpret the way you wrote it...

The lesser of 10' or 10% limitation applies to each transition, using the length of the higher ampacity section to determine the distance excepted.

For example, if you had 50' of highest ampacity, 10' of lowest in the middle, then another 50' at the highest ampacity on the other end. You could except up to 5' of each highest ampacity section towards the middle, a total of 10', and thus render the entire circuit length at the highest ampacity.

An interesting point. Depends on whether the "circuit length figured at the higher ampacity" applies only to that portion of the circuit which is a continuous segment adjacent to the transition or really means the entire length of the circuit at that ampacity. And if three different ampacities end up being involved, then what happens.
Your idea of confining the calculation to the transition clears that up. But it leads to an even more, IMHO, interesting scenario:

100' at higher ampacity, low ampacity segment, 100' at higher ampacity.
Now at each transition you can extend the high ampacity segment conductor size 10' beyond the the transition, resulting in a 20' length with the lower rated ampacity. How do you feel about that?
The 10' maximum would still be applicable in towards the center from both transitions, just as the 10% would.

And to use your most recent scenario, if the first section (the high ampacity) were 100' long, then you could extend that ampacity 10 feet from the first transition but you could not apply that ampacity at all from the second transition back. It boggles the mind. The results depend on which transition you calculate first, since if you do the left one first the ampacity of the middle segment on the left of the second transition is now effectively the high value rather than the low value, and you can extend that 5' to the right of that transition, into the intermediate ampacity section. :slaphead:

There are things man was not meant to know....

 

[Smart $]

.... But it leads to an even more, IMHO, interesting scenario:

100' at higher ampacity, low ampacity segment, 100' at higher ampacity.
Now at each transition you can extend the high ampacity segment conductor size 10' beyond the the transition, resulting in a 20' length with the lower rated ampacity. How do you feel about that?
The 10' maximum would still be applicable in towards the center from both transitions, just as the 10% would.

I agree, a total of 20' in the middle would be excepted to the higher ampacity.

And to use your most recent scenario, if the first section (the high ampacity) were 100' long, then you could extend that ampacity 10 feet from the first transition but you could not apply that ampacity at all from the second transition back. It boggles the mind. The results depend on which transition you calculate first, since if you do the left one first the ampacity of the middle segment on the left of the second transition is now effectively the high value rather than the low value, and you can extend that 5' to the right of that transition, into the intermediate ampacity section. :slaphead:

It doesn't matter which transition you determine first. And there is nothing which says the excepted ampacities cannot overlap. For any two consecutive transitions, you will either have an overlap of excepted ampacities, an abutting of excepted ampacities, or a portion of the middle section which is not excepted. At most, you can eliminate only the lowest ampacity of the three sections from the overall circuit ampacity rating.

And no, you cannot cascade or pyramid the excepted ampacity value to the next-most-adjacent section...

 

[Smart $]

...

It doesn't matter which transition you determine first. And there is nothing which says the excepted ampacities cannot overlap. For any two consecutive transitions, [the middle section] will either have an overlap of excepted ampacities, an abutting of excepted ampacities, or a portion [] which is not excepted. [At most, you can eliminate only the lowest ampacity of the three sections from the overall circuit ampacity rating.] scratch this sentence... it is possible to eliminate the two ampacities to each side of the transitions if the highest ampacity is in the middle.

And no, you cannot cascade or pyramid the excepted ampacity value to the next-most-adjacent section...

EDITS as noted... :ashamed1: