Ampacity vs. Allowable Ampacity

[Grouch1980]

Does the NEC define this difference? Is 'ampacity' after derating factors? 'Allowable ampacities' are noted on the tables in Article 310... so that's before derating factors. I came across a thread where this was mentioned. Does the term 'ampacity' take derating factors into account, if derating is applicable?

 

[Elect117]

The allowable ampacity in the table is before derating factors. The factors shall not reduce the allowable ampacity to less than the circuit's rating (branch circuit load, feeder load, etc.).

NEC 2020 - 310.15
"Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15(A) through (F) and 310.12. "


Ampacity is a vague term to describe any conductor's limits on electron energy (heat, electricity, etc.). There are various factors that can limit that value. I wouldn't get too hung up on the definition of ampacity because it requires context.

 

[wwhitney]

Does the NEC define this difference? Is 'ampacity' after derating factors?

Yes, as per the definition:

"Ampacity. The maximum current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."

The "under the conditions of use" part means that the word ampacity always refers to a value that has been adjusted for the number of conductors and corrected for temperature.

Unfortunately, several sections in the NEC misuse the bare term "ampacity" to refer to the table value without adjustment or correction. In particular, 110.14(C)(1), 210.19(A)(1)(a), 215.2(A)(1)(a), and 230.42(A)(1).

Cheers, Wayne

 

[ggunn]

Whenever I encounter the term "ampacity" elsewhere in the NEC I interpret it as being followed by "derated for conditions of use".

 

[Carultch]

Does the NEC define this difference? Is 'ampacity' after derating factors? 'Allowable ampacities' are noted on the tables in Article 310... so that's before derating factors. I came across a thread where this was mentioned. Does the term 'ampacity' take derating factors into account, if derating is applicable?

"Allowable" means after safety factors are applied. For instance, aluminum 6061-T6 has a yield strength of 35 ksi, and has an allowable strength of 21 ksi for the application I'm most familiar. Your design is limited to a working load of 21 ksi. Failure will happen at 35 ksi, and we limit working loads to 21 ksi to avoid this failure. Nothing necessarily will happen at 22 ksi, but design practices require us to avoid letting the working load get this high.

The NEC doesn't bother revealing these safety factors or giving you the ultimate ampacities at which your insulation would actually get to the temperature where the insulation would fail. It cuts straight to the point, and gives you the allowable ampacities, that keep the temperature within a safe margin of its limit. The Table 310.16 ampacities are based on laboratory conditions. Corrections and adjustments are for adapting a wire's rating under laboratory conditions to the conditions in the real world.