NEC 110.14 (C) Conductors with temperature ratings higher than specified for specified for termination shall be permitted to be used for ampacity....

[danmark2002]

NEC 110.14 (C) Conductors with temperature ratings higher than specified for termination shall be permitted to be used for ampacity adjustment correction, or both.

Let say we have a conduit with 4 current carrying conductors. The correction factor is 80%. ( Table 310.15 (c) (1)
The current is 70 amps.
According to the code we shall use the 60 degrees but allowed to use a HIGHER rated wire ( THHN) to make the adjustment.

How would you determent what size THHN wire to use then ?

 

[AC\DC]

the Terminals be it on the breaker or equipment are the determining factor. they are usually 60/75 Degrees. You would size the wire off that factor.
You do correction factor from the coulomb your wire would land in either 60-75-90

 

[Carultch]

The rule to use 60C column is more academic, than practical. In practice, most terminals are rated for 75C, so a lot more often than not, you get to use the 75C rating for your termination ampacity. You still have the burden of proof to confirm this, but it is very likely to find 75C or 60/75C ratings. One place the 60C termination rule does apply in practice, is for Romex (i.e. NM Cable) that has its own 60C termination rule.

The derate factors for bundling and ambient temperature do not apply to the terminations. You use the ampacity of the terminations directly from the NEC ampacity table. So if using 90C wire on 75C terminals, you get to take credit for the 90C rating as the starting point for the derate calculations. This is the primary reason why there is value in 90C rated wire, despite 75C terminals being the industry norm.

 

[Fred B]

The rule to use 60C column is more academic, than practical. In practice, most terminals are rated for 75C, so a lot more often than not, you get to use the 75C rating for your termination ampacity. You still have the burden of proof to confirm this, but it is very likely to find 75C or 60/75C ratings. One place the 60C termination rule does apply in practice, is for Romex (i.e. NM Cable) that has its own 60C termination rule.

This is why many times you might end up, having to use a larger NM than you might use if you ran say THHN.

The derate factors for bundling and ambient temperature do not apply to the terminations. You use the ampacity of the terminations directly from the NEC ampacity table. So if using 90C wire on 75C terminals, you get to take credit for the 90C rating as the starting point for the derate calculations. This is the primary reason why there is value in 90C rated wire, despite 75C terminals being the industry norm.

Correct, also the conductor after derating, could be forced to a larger size if using the 75C wire to derate than if you use a 90C wiring. This is how sometimes you could legitimately use a 12 awg wire on a 30A breaker.

 

[augie47]

Keep in mind, the 1st thing you need to address is that the conductor must carry the load so, assuming it's a non-continuous load, you need a conductor with an adjusted rating of 70 amps. 310.15(B)(3) notes 4 conductors in a conduit calls for a derating to 80%. so your non-adjusted value would be 88 amps. In a 60° conductor that would be #2, 75° a #3, and 90°, a #4.
If the terminations are actually rated 60°, then the conductors 60° rating must be taken into account and not exceeded.
In this case, the #4 THHN, with an adjusted ampacity of 76 amps (95 x .8) is sufficient for the 70 amp load and has a 60° rating of 70 amps so it does not exceed the 60° terminal rating,

 

[david luchini]

This is how sometimes you could legitimately use a 12 awg wire on a 30A breaker.

I'm not seeing how ampacity adjustment would allow a #12 awg wire on a 30A breaker.

 

[jaggedben]

NEC 110.14 (C) Conductors with temperature ratings higher than specified for termination shall be permitted to be used for ampacity adjustment correction, or both.

Let say we have a conduit with 4 current carrying conductors. The correction factor is 80%. ( Table 310.15 (c) (1)
The current is 70 amps.
According to the code we shall use the 60 degrees but allowed to use a HIGHER rated wire ( THHN) to make the adjustment.

How would you determine what size THHN wire to use then ?

Simply...for copper conductors...
-If the terminals are rated 60C then the table requires min 4awg cu.
-If the terminals are rated 75C then the table still requires #4. Probably your terminals are rated 75 for this application but in this case it makes no difference, so no need to double check the equipment.
-THHN is 75C wire. If that's really what you're using then for the number of current carrying conductors derating take the 75C rating and multiply by the 80% factor. For #4 that's 85×.8=68 which is not quite high enough for your current, so you need to use #2. However mostly likely you will actually buy wire that is 90C rated (THHN-2/THWN-2), in which case for #4 it's 95×.8=76, #4 is still fine.

Whichever applicable requirement requires the largest size wire, that's the one you have to go with. For example if your current were 80A, then for 75C terminals you'd still be fine with #4, but with the 80% factor on using 90C wire you wouldn't be, so you'd have to use #2.

Hope that helps.

 

[infinity]

THHN is 75C wire.

THHN is actually a 90° C conductor.