Swipe the diagram to see the switch loop
- Start derating at the 90 degree C ampacity only if the conductor is genuinely 90 degree C rated in the location it is installed in.
- Whatever the derated figure comes out at, it may not exceed the ampacity from the termination-rated column.
What 110.14(C) actually says
Conductor ampacity has to be determined using the temperature rating of the lowest-rated component in the circuit: the conductor, the termination, or the device.
A 90 degree C conductor landed on a 75 degree C lug is a 75 degree C connection. The insulation is not the weak point. The lug is, and heat at the lug is what starts fires in panels.
Circuits 100 A or less, 14 through 1 AWG60 degrees C column 110.14(C)(1)(a). Unless the equipment is listed and identified for a higher rating, which much modern gear is. |
Circuits over 100 A, or larger than 1 AWG75 degrees C column 110.14(C)(1)(b), again unless the equipment is listed for higher. |
Equipment marked 75 degrees C75 degrees C column Check the marking on the breaker or the equipment label. Most current gear is 75 degree C rated. |
This is a heat problem, not a paperwork problem
Sizing off the 90 degree C column when the terminations are 75 degree C does not overload the wire. It overloads the connection, which is where the heat builds and where the failure happens.
So what is the 90 degree column for
It is the legitimate starting point when you are correcting for ambient temperature or adjusting for the number of current-carrying conductors in a raceway.
You may begin from the 90 degree C ampacity, apply the factors, and use the result. What you may not do is finish above the ampacity the termination allows. Both tests have to pass, and you take whichever is lower.
Why this is worth having straight
Used correctly the 90 degree C column often saves you a conductor size on a long derated run. That is real money on a job. It is worth knowing exactly where the allowance stops rather than avoiding it or overreaching with it.
Ampacity values live in NEC Table 310.16, which is copyrighted and not reproduced here. This page explains which column applies and why. Work from the table your AHJ enforces.
Common questions
Can I use the 90 degree C column to size my wire?
Not for the final ampacity. 110.14(C) ties you to the lowest-rated component in the connection, which is normally the 60 or 75 degree C termination. The 90 degree C column is for starting a derating calculation.
Why does my breaker have a temperature rating?
Because the lug is where heat concentrates. The rating tells you what the connection can tolerate, and the code sizes the conductor to protect the connection, not just the insulation.
Does this mean 90 degree C wire is pointless?
No. It is what lets you begin a derating calculation at a higher ampacity, which on a long run with several current-carrying conductors can save a conductor size. The allowance is real, it just has a ceiling.
How do I know what my terminations are rated?
The marking on the equipment. Breakers and panelboards carry a temperature rating on the label or in the listing. If there is no marking, 110.14(C)(1) gives you the default by circuit size.