310.15C1

[Alwayslearningelec]

when using this table and you come up with % to reduce do you defer to the 90 degree column for ampacity?

 

[infinity]

Is Table 310.15(C)(1) correct?

 

[augie47]

As noted in the header, you use the column associated with the insulation rating. IF you are using a 90 deg temp conductor such as THHN-2 then you use that column. Similarly, if you are using a conductor with a 60 deg rating you would use that column.

 

[Alwayslearningelec]

got it but then you have to be concerned with terminal ratings. doesn’t that also drive what temp column you use? it seems guys i speak to , when deciding conductor ampacity, always use the 75deg column.

 

[augie47]

That's back to 110.14(C).. The adjusted conductor ampacity can not exceed the allowed ampacity based on equipment termination temperature.
If your equipment/device termination is 60° or 75° your allowable ampacity will be governed by that.
For instance, if you had a #8 THHN-2 with a 0.91 adjustment for fill the adjusted ampacity would be 50 amps (55 X .91) but the #8 at 60° is only rated 40 amps so it could not be terminated on a 50 amp, 60° device.

Here;'s a graphic Mike has on the subject:

 

[Carultch]

got it but then you have to be concerned with terminal ratings. doesn’t that also drive what temp column you use? it seems guys i speak to , when deciding conductor ampacity, always use the 75deg column.

The primary value in having 90C wire despite having 75C terminations being the most common, is that the 90C wire gives you headroom for your derate calculations for both bundling adjustments and temperature corrections. You are limited to the termination rating without derate factors considered (usually from the 75C column), but you get to use the 90C rating as your starting point for your ampacity derate calculations if the wire carries the 90C rating (which is most commonly the case for wire made today).

It is academic rather than practical to consider 60C being the default for 100A and less. E.g. a rule that matters when you are taking an exam, rather than a rule that governs a real life installation. You have a burden of proof to use the 75C termination ratings on equipment that is 100A and less, but it is the industry norm that most equipment is listed and labeled for 75C. One exception is when using NM cable (aka Romex), as there you are limited to 60C terminations even if the wire is 90C rated, and even if you have 75C terminations/equipment.

90C terminations are rare as part of manufactured equipment, even if the terminations are labeled AL9CU. It matters what the assembled product is rated for, rather than how the lug is stamped. They are common on separately-installed connectors, but need to be in a separate enclosure to take credit for the 90C rating of the connector.