Look at this picture.

[a7]

Electrical equipment.....

Electrical equipment.....

....such as circuit breakers, panelboards, receptacles, switches, terminal blocks, wire nuts, lugs etc. have "temperature ratings" that must be observed.
The termination provisions are based on the use of 60C ampacities for wire sizes #14-#1 , and 75C ampacities for wire sizes #1/0 and larger. If using wire sizes #14-#1 in equipment marked "75C only" or "60/75C" , it is intended that 75C wire may be used at its full ampacity.
If the equipment is not marked, it is 60 deg. C rated for 100 amps and less. Over 100 amps is 75C rated. If the equipment is rated higher, it would have to be marked to indicate the higher temperature.

 

[a7]

Nec

Nec

110-14 (C).
I hope you guys didn't cause too many fires.

 

[augie47]

I fail so see the problem. The 60? ampacity of a #10 is 30 amps.
The load is less than 30 amps. The ambient temperature does not change the load and only effects the wiring insulation.

 

[a7]

i think that....

i think that....

I fail so see the problem. The 60? ampacity of a #10 is 30 amps.
The load is less than 30 amps. The ambient temperature does not change the load and only effects the wiring insulation.

If the ambient temperature is higher than 30C, it won't simply let the heat from the wire dissipate and the wire will exceed its 60 deg limits.

 

[RICK NAPIER]

I think augie47 is right in his computations and #10 would suffice. I disagree that the ambient temperature would be 122*. The only code reference in this section that describes ambient temperature are the notes to the new table 310.15(B)(2)(c) where both notes describe ambient temperature as an averaging of temperatures as this is the only description of ambient temperature we would need to use this as a guide. Since 122* is the maximum the ambient temperature would be less.

 

[a7]

temp

temp

I think augie47 is right in his computations and #10 would suffice. I disagree that the ambient temperature would be 122*. The only code reference in this section that describes ambient temperature are the notes to the new table 310.15(B)(2)(c) where both notes describe ambient temperature as an averaging of temperatures as this is the only description of ambient temperature we would need to use this as a guide. Since 122* is the maximum the ambient temperature would be less.

You disagree that the ambient temperature would be 122*, that's OK. It was on the picture I posted, that's all. I will not argue if it will ever reach 122* at the attic.

 

[roger]

The only code reference in this section that describes ambient temperature are the notes to the new table 310.15(B)(2)(c)

I don't think those notes could be found anywhere in the NEC prior to the 02.


Roger

 

[Dennis Alwon]

Okay you all have me confused but my feeling is if we use the problem given as NM cable then #10 won't work. If we use piping with 90C conductors than a #10 will work. The problem does state NM so a #8 would be needed.

Do we agree on this?

 

[RICK NAPIER]

No the notes don't appear prior to 2008 but they are the only guidance I see and they are an ASHRAE standard.

I think #10 romex works. As per augie47's calculations.

 

[Dennis Alwon]

No the notes don't appear prior to 2008 but they are the only guidance I see and they are an ASHRAE standard.

Yes, my mistake. I was thinking the load was at 32 amps. duh. #10 NM is good. I agree. Wow brain dead today-- I had it right then went zany with it.

 

[roger]

No the notes don't appear prior to 2008 but they are the only guidance I see and they are an ASHRAE standard.

Well, since the study material is pre 2002 it would be nice to see the authors new material to se if it takes the notes into consideration.

I still say the way the author shows it is correct.

BTW, after some looking I think the book is Tom Henry's

Roger

 

[480sparky]

............BTW, after some looking I think the book is Tom Henry's........

I'd say so too. I image looks like those in his books, plus the TH next to the page number.

 

[mivey]

Forensics at its best.

 

[dana1028]

just some opinions

just some opinions

I'm not sure if the OP is trying to answer a 'code quiz' question or perform an actual A/C installation....2 very different critters.

Real life: The 25A figure for a 3 ton AC is for the OCPD, not the FLA [which is probably in the 16A range...most of the 3T units I'm seeing installed have a 20A max HACR breaker].

The real attic temperature on an 88 degree day is over 140 degrees - I have tested many attics with a Fluke digital thermometer. So you would be required to derated for actual environment temperature...that would really blow your NM away.

As an inspector, if I ever tried to make a contractor derate his NM in a residential setting for true environment temps I'd be thrown out the door [by both the contractor and the Building Official].

That said, I agree with Augie's approach; NM is good for 90, derate at .82 [per OPs info] and you get 32A. 60-degree column is good for 30A, well above the supposed 25A load.

 

[don_resqcapt19]

We don't adjust the load. The ambient temperature does not change the load, it changes the ampacity of the conductor. We need a conductor that is suitable for a load of 25 amps at an ambient temperature of 122?F. NM has 90?C conductors and a #10 NM has an adjusted ampacity of 32.8 amps. This is greater than the load current and is a suitable conductor for this application.

 

[Dnkldorf]

Isn't 60C=140F?

If the attic is 122F, and the conductors are rated for 90C, and the insulation is rated for 60C, why is there a problem?

 

[dana1028]

Isn't 60C=140F?

If the attic is 122F, and the conductors are rated for 90C, and the insulation is rated for 60C, why is there a problem?

Table 310.16 requires you to derate the ampacity value of your conductors using a factor of .82 for a temp of 122F. Not that big a deal if you are using #12 NM for a general lighting load, but if you were using the NM for a mechanical/motor load requiring a rating higher than 24.6A then it would be an issue.

 

[Dnkldorf]

Table 310.16 requires you to derate the ampacity value of your conductors using a factor of .82 for a temp of 122F. Not that big a deal if you are using #12 NM for a general lighting load, but if you were using the NM for a mechanical/motor load requiring a rating higher than 24.6A then it would be an issue.

Thanks. Got it now.

 

[housefull]

I accept with information: I totally overlook that NM has 90? conductors as far as derating in concerned..