110.14(C)
Temperature Limitations at Terminations
Even if you run 90-degree-rated THHN wire, the breaker it lands on may only be rated for 75 degrees, and the receptacle on the other end may only be rated for 60 degrees. Section 110.14(C) says you must pick the ampacity column in Table 310.16 that matches the lowest temperature rating of anything in the circuit path. For circuits 100 amps or less, most equipment defaults to a 60-degree-C rating unless it is specifically marked for 75 degrees. For circuits over 100 amps, 75 degrees is the normal default. The good news is there is an important exception: you ARE allowed to start your derating calculations at the 90-degree-C column and then derate down, as long as the final number does not exceed the ampacity you would get from the termination-temperature column. This is the entire practical reason electricians prefer THHN wire — it gives you a higher starting point for the math.
When You Need This
- Any time you are doing a full conductor sizing calculation that involves derating for temperature or conduit fill
- Selecting wire for a panel where the bus and breakers have different temperature ratings
- Justifying to an inspector why you are starting your derating math from the 90-degree-C column
- Upgrading older equipment that may only have 60-degree-C-rated terminations
- Answering exam questions that combine ampacity tables with terminal temperature limits
Key Points
Common Mistakes
Assuming all breakers are rated for 75 degrees C — many residential breakers and small devices are still 60-degree-C-rated
Using the 90-degree-C ampacity as the final answer without checking the termination temperature limitation
Not checking both ends of the circuit — the terminal rating at the source and at the load can differ
Forgetting that for circuits 100 A or less, the burden of proof is on you to show the terminal is marked for 75 degrees C if you want to use that column
Exam Tip
The exam loves to combine 110.14(C) with derating. The approach is: (1) look up the 90-degree-C ampacity from 310.16, (2) apply temperature and conduit-fill derating, (3) check that the result does not exceed the ampacity from the column matching the termination temperature rating. If it does, the termination value is your ceiling.
Frequently Asked Questions
Check the equipment labeling or documentation. If no temperature marking exists, assume 60 degrees C for circuits 100 A or less and 75 degrees C for circuits over 100 A.
Because the terminal or lug where the wire connects may not be able to handle 90-degree heat. If the plastic or metal at the connection point degrades at that temperature, you get a loose connection or a fire. The conductor and the termination must agree on a safe operating temperature.
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Related Code Sections
This is an educational summary, not the official code text. The NEC® is a registered trademark and copyright © National Fire Protection Association (NFPA). The CEC is copyright © CSA Group. For official code text, visit nfpa.org or csagroup.org. SparkShift is not affiliated with NFPA or CSA Group.