Look at this picture.

[a7]

Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?
Thank you for your input.

 

[iwire]

Boy I never knew NM has an ampacity of 60 C.

What book is that?

BTW is there a question here?

 

[augie47]

I ask the same as Bob. What is that book ?
They totally overlook that NM has 90? conductors as far as derating in concerned..
You may use a #8, in fact, I see no problem with the #10.

 

[a7]

yes:

yes:

Boy I never knew NM has an ampacity of 60 C.

What book is that?

BTW is there a question here?

Yes, the question is on top: Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?

I don't know what book it is. It just ended up in my hands somehow.

 

[ty]

I ask the same as Bob. What is that book ?
They totally overlook that NM has 90? conductors as far as derating in concerned..
You may use a #8, in fact, I see no problem with the #10.

I agree.

And it says the ambient temp. Has Reached?
But what is the ambient temp?

 

[augie47]

Yes, the question is on top: Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?

I don't know what book it is. It just ended up in my hands somehow.

Based on their (incorrect) methodology, no.
Based on what I believe is NEC, yes. Iin fact, as stated, you could use a #10..

 

[Dennis Alwon]

I concur with #10 but I must state that I have never seen anyone use the ambient temp chart for resi work. 120 degree ambient is pretty hot. Sure most attics will get that hot but again what does ambient temp. mean?

 

[a7]

follow up....

follow up....

The only thing I see logical here is that they determine wire size based on 60C column according to terminals of the appliance - 60C (...mostly 60/75). I know I can size wires according to 75C column in table 310-16 if I size wires for motors because all motors has terminals rated 75C.
We must not forget that temperature rating of conductors is based on 30C ambient temperature!

 

[roger]

I say the book is correct.

What is happening is we must "adjust" the ampacity of the load to the heat in the attic (122 deg F) which becomes 43 amps, now 334.80 tells us that if we do use the 90 deg rating for "derating" purposes the conductor can not exceed 60 deg column but, we are not derating the cable (there are only three conductors in this cable) so we must use the 60 deg column straight up for theadjusted load

Meaning, a # 8 is only good for 40 amps so the # 6 is correct.


BTW, the book is pre 2002.

Roger

 

[ty]

I say the book is correct.

What is happening is we must "adjust" the ampacity of the load to the heat in the attic (122 deg F) which becomes 43 amps, now 334.80 tells us that if we do use the 90 deg rating for "derating" purposes the conductor can not exceed 60 deg column but, we are not derating the cable (there are only three conductors in this cable) so we must use the 60 deg column straight up for theadjusted load

Meaning, a # 8 is only good for 40 amps so the # 6 is correct.


BTW, the book is pre 2002.

Roger

Roger, I don't agree with the temp.

The highest it ever reached isn't the ambient, is it?

 

[a7]

Ambient temp.

Ambient temp.

I concur with #10 but I must state that I have never seen anyone use the ambient temp chart for resi work. 120 degree ambient is pretty hot. Sure most attics will get that hot but again what does ambient temp. mean?

The ambient temperature means that, if I use wire #8 THW and allow 50A current to flow through the wire, the insulation will (after some time) reach 75C if the ambient temperature is 30C like table 310-16 says.

 

[roger]

Roger, I don't agree with the temp.

The highest it ever reached isn't the ambient, is it?

Todd, I would say the highest temp would be the true ambient for the question as it is presented. We would have to answer the question with the information given and after the adjustment, we would be looking at 43 amps

Roger

 

[a7]

I say the book is correct.

What is happening is we must "adjust" the ampacity of the load to the heat in the attic (122 deg F) which becomes 43 amps, now 334.80 tells us that if we do use the 90 deg rating for "derating" purposes the conductor can not exceed 60 deg column but, we are not derating the cable (there are only three conductors in this cable) so we must use the 60 deg column straight up for theadjusted load

Meaning, a # 8 is only good for 40 amps so the # 6 is correct.


BTW, the book is pre 2002.

Roger

Yes, I think the book was before or around 2002. What do you think about my question?
"Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?"

 

[roger]

Yes, I think the book was before or around 2002. What do you think about my question?
"Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?"

Yes, you could use # 8 THHN but, that is not NM.

Roger

 

[ty]

Todd, I would say the highest temp would be the true ambient for the question as it is presented. We would have to answer the question with the information given and after the adjustment, we would be looking at 43 amps

Roger

I understand.

I take the book's example to use a blanket statement by using the highest temp that was ever reached, and not the actual ambient attic temp which could be alot lower.

 

[Dennis Alwon]

Yes, I think the book was before or around 2002. What do you think about my question?
"Would I be allowed to use #8 THHN copper wire instead of #6, if my appliance at the attic has wire terminals rated for 75C ?"

First off it is an a/c unit so it would be outside. Yes you could use #8 but I believe a #10 would also get it. Here is why.

the correction factor would be based on the actual rating of the wires in nm cable. That would be 90C. Thus you would use .82 not .58

 

[augie47]

Roger, I still agree with Dennis.
#10 NM @90? is 40 amps. In a 122? ambient the multiplier is 82 thus 32.8 amps.
The 60? rating is 30 so we can use whichever is less.
The load is 25 amps (or possibly 25 x 1.25).. less than the 30 amp ampacity of #10

 

[Dennis Alwon]

Roger, I still agree with Dennis.
#10 NM @90? is 40 amps. In a 122? ambient the multiplier is 82 thus 32.8 amps.
The 60? rating is 30 so we can use whichever is less.
The load is 25 amps (or possibly 25 x 1.25).. less than the 30 amp ampacity of #10

Gus I get 25 amps divided by .82 and I get 30.48 amps. I believe if it is less than .5 I can drop the fraction. Art. 220.5(B) #10 is good for 30 amps at 60C

 

[roger]

Roger, I still agree with Dennis.
#10 NM @90? is 40 amps. In a 122? ambient the multiplier is 82 thus 32.8 amps.
The 60? rating is 30 so we can use whichever is less.
The load is 25 amps (or possibly 25 x 1.25).. less than the 30 amp ampacity of #10

Gus, everybody is trying to derate the conductor, what we need to do is adjust the load ampacity and size the conductors to it, I still go with the book being correct.


Roger

 

[Dennis Alwon]

Gus, everybody is trying to derate the conductor, what we need to do is adjust the load ampacity and size the conductors to it, I still go with the book being correct.


Roger

That's a good point . #10 is rated 40 amps times .82 is 32.8. Since 10NM is only good for 60C- 30 amps then a #8 is required. I was following the books example which used the load.