AC wire size

[mbrooke]

Couldn't we calculate this with Nehr-McGrath?

 

[paulengr]

Sorry, I'm still inclined to disagree. The full definition of ampacity is "The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."

I don't find a definition of "condition of use" anywhere. If you can demonstrate that "conditions of use" means only "ambient temperature and number of current carrying conductors in a cable/conduit" then I will agree with you. Otherwise, I find it more rational to say that "conditions of use" includes temperature limitations of the conduit carrying the conductors. As well as possibly other limitations.

Which leads me to another ambiguity in the use of the word "ampacity": when the allowable current on a conductor is controlled by the temperature rating of one of its connections, terminologically, does that reduce the conductor's ampacity? Or is the ampacity still the same, and the termination limitation is an additional limitation? The distinction might matter for applying something like 240.4, "shall be protected
against overcurrent in accordance with their ampacities."

The wording of 240.4(D) "small conductors" is an example where the OCPD size is limited, but the language involved doesn't change the conductor ampacity. But 110.14(C) says "The temperature rating associated with the ampacity of a conductor shall be selected and coordinated . . ." Which suggests to me that the ampacity itself is changed by termination limitation.

Cheers, Wayne

Conditions are defined in UL or CSA or NEMA, such as standard conditions. It’s not spelled out but condition of use means how you are using it. For instance ampacity (Neher-McGrath) tables include an a maximum temperature. There are derating tables for use outside of those temperature conditions. You can’t really read NEC in isolation without considering the underlying conditions of Listing. There are a lot of gotchas hidden in UL standards.

 

[Alwayslearningelec]

I believe that you're correct.

Even if the LFNC is less than 3' or less?

 

[PGreg]

I have an air conditioner, the name plate says max breaker size of 25A. I can get by with #12 CU since its a motor, is that right?

240.4D ocpd for wire size. In 310.15b16 the ** next to #14,#12&#10 point you to 240.4. These 3 wire sizes do not apply in motor wire sizing.

 

[Dennis Alwon]

240.4D ocpd for wire size. In 310.15b16 the ** next to #14,#12&#10 point you to 240.4. These 3 wire sizes do not apply in motor wire sizing.

welcome to the forum and yes but for clarity I thing your last question should state these 3 asterisks do not apply to motor wire sizing.

 

[PGreg]

welcome to the forum and yes but for clarity I thing your last question should state these 3 asterisks do not apply to motor wire

Not meaning that they do not apply, meaning that 240.4 states OCPD for 14,12&10, you cannot protect larger than 15a for #14, 20a for #12 and 30a for #10 copper respectfully. Thus with the AC in question, #10 copper should be ran and protected at 25a.

 

[infinity]

Not meaning that they do not apply, meaning that 240.4 states OCPD for 14,12&10, you cannot protect larger than 15a for #14, 20a for #12 and 30a for #10 copper respectfully. Thus with the AC in question, #10 copper should be ran and protected at 25a.

The 15, 20, and 30 amp limits of 240.4 do not apply to the AC unit conductors.

 

[Dennis Alwon]

Not meaning that they do not apply, meaning that 240.4 states OCPD for 14,12&10, you cannot protect larger than 15a for #14, 20a for #12 and 30a for #10 copper respectfully. Thus with the AC in question, #10 copper should be ran and protected at 25a.

No that is not the case #12 can be run for the a/c as long as the wire is rated 75 C or 90C. Look at 240.4(G) which is referenced in 240.4(D)

 

[don_resqcapt19]

Not meaning that they do not apply, meaning that 240.4 states OCPD for 14,12&10, you cannot protect larger than 15a for #14, 20a for #12 and 30a for #10 copper respectfully. Thus with the AC in question, #10 copper should be ran and protected at 25a.

(D) Small Conductors.
Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed that required by (D)(1) through (D)(7) after any correction factors for ambient temperature and number of conductors have been applied.

Conductors that supply AC equipment are covered by 240.4(G), which sends you to Article 440.

 

[infinity]

No that is not the case #12 can be run for the a/c as long as the wire is rated 75 C or 90C. Look at 240.4(G) which is referenced in 240.4(D)

That's correct, the one caveat is using NM cable which limits the ampacity to 60° C. Run #12 MC and you can use a 25 amp OCPD.