conductor sizing

[Smart $]

Didn't say it did. I said terminal temperature rating has a bearing on the selection of ampacity, per 110.14(C).

A conductor that has an ampacity of 65 cannot be protected at 75A, as 70A is the next standard size up.

But the conductor's [actual] ampacity is 75A.

 

[david luchini]

But the conductor's [actual] ampacity is 75A.

Yes, it is, but that does not change the requirements of 110.14(C).

If the lowest temperature rating of a termination, conductor or device is 75C, then the temperature rating associated with the ampacity of the #6 THWN-2 will also be 75C. The ampacity of THWN-2 at 75C is 65, not 75. And 240.4 doesn't let you protect a conductor with an ampacity of 65 at 75Amps.

 

[augie47]

I hesitate to step into any of Smart$ "philosophical posts" and I may bow out quickly but will add my thoughts that 110.14 limits the termination ampacity while allowing conductors with a higher temperature ratings to be used so long as they terminate at the 110.14 prescribed ampacity.
So, to me at least 110.14 determines the amapcity even if the conductor has a higher temperature rating and 210.19 states the conductor must have an ampacity equal to or greater than the load and 240.4 states the conductor must be protected at it's ampacity.

So if 110.14 allows a 75? termination, the #6 amapcity is 65 amps at 75? regardless of it's temperature rating. So we have a maximum load of 65 amps per 210.10 and a maximum OCP of 70 per 240.4

 

[Smart $]

Yes, it is, but that does not change the requirements of 110.14(C).

If the lowest temperature rating of a termination, conductor or device is 75C, then the temperature rating associated with the ampacity of the #6 THWN-2 will also be 75C. The ampacity of THWN-2 at 75C is 65, not 75. And 240.4 doesn't let you protect a conductor with an ampacity of 65 at 75Amps.

Okay... I've been having a hopefully momentary lapse of reasoning today, and needed the reassurance. Thank you for your assistance in that regard. :happyyes:

When I answered...

No to one of 'em. :angel:

...I was referring to one of the two realistic scenarios presented, that being the one we are discussing.

Anyway, the actual section which confirms your assertion is the latter part of 310.15(B) initial statement...

The temperature correction and adjustment factors shall
be permitted to be applied to the ampacity for the temperature
rating of the conductor, if the corrected and adjusted
ampacity does not exceed the ampacity for the temperature
rating of the termination in accordance with the provisions
of 110.14(C).

The maximum OCPD rating is 70A per 240.4(B).

 

[Smart $]

I hesitate to step into any of Smart$ "philosophical posts" and I may bow out quickly but will add my thoughts that 110.14 limits the termination ampacity while allowing conductors with a higher temperature ratings to be used so long as they terminate at the 110.14 prescribed ampacity.
So, to me at least 110.14 determines the amapcity even if the conductor has a higher temperature rating and 210.19 states the conductor must have an ampacity equal to or greater than the load and 240.4 states the conductor must be protected at it's ampacity.

So if 110.14 allows a 75? termination, the #6 amapcity is 65 amps at 75? regardless of it's temperature rating. So we have a maximum load of 65 amps per 210.10 and a maximum OCP of 70 per 240.4

No need to bow out.

Your assessment is correct (see my reply to David).

An indirectly associated matter which I question is circuit rating being based on the OCPD rating (210.3), when the true limitation is the lesser of the terminal temperature limitation or the OCPD rating. Going by the OCPD rating may give others at a later date a false impression.

 

[david luchini]

Anyway, the actual section which confirms your assertion is the latter part of 310.15(B) initial statement...

The actual section that confirms my assertion is 110.14(C) which is referenced by 310.15(B), and which I mentioned in posts 38, 40 and 42.

The temperature correction and adjustment factors shall
be permitted to be applied to the ampacity for the temperature
rating of the conductor, if the corrected and adjusted
ampacity does not exceed the ampacity for the temperature
rating of the termination in accordance with the provisions
of 110.14(C).

 

[Smart $]

The actual section that confirms my assertion is 110.14(C) which is referenced by 310.15(B), and which I mentioned in posts 38, 40 and 42.

Hmmm...

po ta to
po tah to



110.14(C) doesn't say the circuit ampacity is limited to the terminal temperature limitation. Only 310.15(B) says that.

 

[david luchini]

po ta to

Boil 'em, mash 'em, stick 'em in a stew.

...in accordance with the provisions of 110.14(C)

 

[electrofelon]

So we are in agreement that breaker setting cannot exceed its terminal rating amps, even if load does not, except for next size up cases. ?

 

[Smart $]

So we are in agreement that breaker setting cannot exceed its terminal rating amps, even if load does not, except for next size up cases. ?

Correct.

And load cannot either.

 

[Carultch]

So we are in agreement that breaker setting cannot exceed its terminal rating amps, even if load does not, except for next size up cases. ?

In cases where the next size up rule, 240.4(B), doesn't apply, the terminal rating of the conductors cannot be less than the trip rating of the breaker. Examples are tap conductors, transformer secondary conductors, and overcurrent devices exceeding 800A. For a 400A breaker on a transformer secondary, you need 400A of wire at 75C...not 380A of wire at 75C. From 310.15(B)(16), that would be 600 kcmil copper. Doesn't matter whether you have 380A of noncontinuous load, or 320A of continuous load, or 10 Amps of continuous load. When 240.4(B) doesn't apply, you need at least as much ampacity as you have overcurrent device. Both in terms of terminations and wire at conditions of use.

Once you are behind that transformer secondary breaker, then you can use the next size up rule. Feeders, as it is defined. Sections of conductors that are sufficiently protected at or near their ampacity, and against overload.

In cases where 240.4(B) does apply, very common in my scope of work, your conductor ampacity and terminal ampacity both need to not be less than the previous size OCPD below the one that you are actually using. The conductor terminations also have to have an ampacity for the load with the continuous load factor applied to the portion where applicable. In otherwords, both of these ampacities need to round up to the OCPD you are using, where required. And both need to be sufficient for the load.


If you are in doubt, and you'd rather spend the little extra money on material just to be safe than the time to research your uncertainty, you can simply make sure that all terminations and conductors at conditions of use, have an ampacity meeting or exceeding the OCPD. Pretending that 240.4(B) doesn't exist.