NEC 110.14(C)

[Dale001289]

The MCB of this 480V SWGR has a temp rating of 65/75.
The ambient temperature at both ends of the cable is controlled at around 70Degrees F. Do I still need to derate the conductors based on ampacities of 310.16 75Degree C column?

 

[Carultch]

The MCB of this 480V SWGR has a temp rating of 65/75.
The ambient temperature at both ends of the cable is controlled at around 70Degrees F. Do I still need to derate the conductors based on ampacities of 310.16 75Degree C column?

The 60C part of this, means you can terminate 60C wire with 60C sizing.
The 75C part of this, means you can terminate 75C wire or higher with 75C sizing.

Had it been straight-rated for 75C, it would exclude the ability to connect 60C rated wire.

Derates apply to the wire rating and not to terminations, so ambient temperature and bundling are "out of the picture" of this issue. The most common advantage of 90C wire (despite 75C terminals being the most common) is that you can use the 90C column as the starting point for derate calculations, and terminate to 75C terminals.

 

[paullmullen]

Don't forget that you have ambient temp corrections 310.15(B)(2)(a) and potentially "more than three conductors" correction in 310.15(B)(3)(a) to attend to. At 75°F, and conductors in air, probably not much to worry about.

But @Carultch, I'm not sure I agree about the comment of 75°C terminals implying that 60°C wire could not be used. Is there a code reference from which I could learn something? My practice has been to look up wire ampacity in the table... If the terminals are rated for 60°C, then you have to use the 60°C column for ampacity regardless of how the wire is marked. If the terminals are labeled for 75° or 90°, then the ampacity is limited by the cable rating. But this doesn't preclude the use of wire that may only be labeled for 60°C connected to 75°C terminals... unless I'm about to learn something.

 

[Carultch]

Don't forget that you have ambient temp corrections 310.15(B)(2)(a) and potentially "more than three conductors" correction in 310.15(B)(3)(a) to attend to. At 75°F, and conductors in air, probably not much to worry about.

But @Carultch, I'm not sure I agree about the comment of 75°C terminals implying that 60°C wire could not be used. Is there a code reference from which I could learn something? My practice has been to look up wire ampacity in the table... If the terminals are rated for 60°C, then you have to use the 60°C column for ampacity regardless of how the wire is marked. If the terminals are labeled for 75° or 90°, then the ampacity is limited by the cable rating. But this doesn't preclude the use of wire that may only be labeled for 60°C connected to 75°C terminals... unless I'm about to learn something.

Schneider Electric has a white paper analyzing the code rules and UL standards together:

https://www.productinfo.schneider-electric.com/0110db9901wiretempratetermdb/0110db9901-wire-temp-ratings-and-terminations/0110DB9901%20Wire%20Temp%20Ratings%20and%20Terminations/English/0110DB9901%20(bookmap)_0000485497.xml/$/Introduction-D190ED6C

Under "The NEC Rules", and "Equipment Rated for 100 A or Less — NEC 110.14(C)(1)(a) items 1 through 4", it has a table that clarifies the various cases of 110.14 that you may encounter.

 

[paullmullen]

Yeah, that white paper consistent with what I think the code requires. I must have misread your intent in your previous post. Thanks!