Air conditioner feed sizing

[Shawno]

Good evening all. I have a air conditioner question. I need to hook up a 480v unit with an mca of 33 and mop of 35. It currently has 10 thhn to the disco. 75^ degree column says I’m good to 35 amps, but I’m stuck on the 10 is only good for 30 amps mind set and should re-feed with #8. Any guidance would be appreciated.

 

[infinity]

240.4(G) allows you to disregard the 30 amp limit for #10 conductors.

Welcome to the Forum.

 

[Shawno]

Thanks for the response Rob, it’s a great forum with lots of great information. I’ve read through 440 III and VI again and still am not seeing where it allows for me to disregard the #10 rated for 30amps.

 

[electrofelon]

Thanks for the response Rob, it’s a great forum with lots of great information. I’ve read through 440 III and VI again and still am not seeing where it allows for me to disregard the #10 rated for 30amps.

It's not in 440, it's in 240.4 as Rob said. I prefer to think about it this way: the ampacity charts show 35 amps, so why would you be restricted to 30? That small conductor rule, #14=15 amps, #12=20amps, #10=30 amps, is a further derating for "general" uses.
It does not apply to 440.

 

[LarryFine]

I’ve read through 440 III and VI again and still am not seeing where it allows for me to disregard the #10 rated for 30amps.

Read 240.4, then 240.4(D), then 240.4(G) and its table, THEN go to 440 Parts III and IV, mostly IV.

The MCA defines the conductor sizing and MOC defines the OCPD selection; no calculating required.

 

[infinity]

Thanks for the response Rob, it’s a great forum with lots of great information. I’ve read through 440 III and VI again and still am not seeing where it allows for me to disregard the #10 rated for 30amps.

You find the 75° C ampacity in T310.15(B)(16) which is 35 amps for #10. If it's considered a "small conductor" you need to apply 240.4(D) which limits the ampacity to 30 amps but 240.4(G) tells you that for AC units you do not need to apply the small conductor limit of 30 amps so you can use the #10 at its 35 amp rating.

 

[augie47]

If I may muddy the water a bit. You do still need to keep 110.14 into account and make sure you do not exceed the termination temperature limits. To use your #10 at an ampacity greater than 30 amps requires termination ratings of 75°.
Likely the case with the over-current device but not necessarily with the utilization equipment.

 

[electrofelon]

If I may muddy the water a bit. You do still need to keep 110.14 into account and make sure you do not exceed the termination temperature limits. To use your #10 at an ampacity greater than 30 amps requires termination ratings of 75°.
Likely the case with the over-current device but not necessarily with the utilization equipment.

Right, and it only works with mc, se (maybe), and pipe and wire, not ramen which is restricted to 60 degrees.

 

[synchro]

Right, and it only works with mc, se (maybe), and pipe and wire, not ramen which is restricted to 60 degrees.

You can (and should) cook the ramen above 60 deg C, but wait till it cools down somewhat before consumption.
...sorry, I couldn't resist.

 

[electrofelon]

You can (and should) cook the ramen above 60 deg C, but wait till it cools down somewhat before consumption.
...sorry, I couldn't resist.

Well there is clearly a contradiction in the NEC. 310.15(B)(7) tells us NOT to bring ramen above 60 degrees, but the instructions on the package say to go above 60 (110.3(B)) :rant:

 

[LarryFine]

In this case, I believe the wet rating of the ramen comes into play.