Terminal Temperature Rating: The Ampacity Cap
Terminal temperature rating is the listed temperature limit of the equipment connection at the conductor termination, and it caps the conductor ampacity that can be used at that point.
The wire insulation and the equipment terminal are two different temperature problems. The insulation rating decides which ampacity column can be used for correction and adjustment, while the lowest-rated terminal at either end sets the ceiling on the final usable ampacity.
Column versus cap
The conductor insulation selects the starting column for ambient and conductor-count math. The equipment terminal rating sets a ceiling on the result.
A 90 C conductor on 75 C lugs can use the 90 C column for derating, but the final ampacity cannot exceed the 75 C table value for that conductor size.
Why the lower end governs
Heat at a termination is local. If one end is rated lower than the other, that lower connection is the weak point for ampacity.
The calculator treats the entered terminal rating as the limiting rating for the whole run.
How it shows up in results
A factor breakdown may show plenty of corrected ampacity left in the insulation column, then name the terminal cap as the governing limiter.
That is not a contradiction. It means the conductor can thermally survive its insulation, but the equipment connection cannot use that higher temperature rating.
Where the Term Decides Something
Four points in the work where the figure is the answer to a question, not vocabulary.
Panel and breaker lugs
The lug marking on the breaker, panel or equipment decides whether the final conductor ampacity can be capped at 60 C or 75 C.
Small equipment circuits
Equipment rated 100 A or less often uses the 60 C assumption unless it is marked otherwise, which can bind before derating does.
Feeder sizing
A feeder with 90 C conductors may derate from the 90 C column, but the final usable value still cannot exceed the limiting lug temperature.
Aluminum substitutions
Aluminum feeders need listed aluminum terminations and the correct temperature cap. A material swap is not complete from ampacity alone.
Which Figure to Use
Reading the term correctly in practice.
Read both ends
Use the lower terminal rating at the supply end or load end. One 60 C termination makes the conductor a 60 C-capped installation.
Do derating before the cap
Ambient and conductor-count factors are applied to the insulation-column ampacity. The terminal rating caps the corrected result afterward.
Do not assume 75 C
Catalog familiarity is not a marking. Field work needs the actual equipment label, installation instructions or AHJ-accepted documentation.
Keep insulation rating visible
A higher insulation rating is still useful because it can provide derating headroom before the terminal cap is applied.
FAQs
Can I use the 90 C column with 75 C terminals?
Which terminal temperature rating do I use?
Does terminal temperature rating change voltage drop?
Why does a larger conductor sometimes solve a terminal cap?
Calculated against the NEC 2023 edition using 110.14(C), 310.16. Confirm the edition adopted by your AHJ, which may differ and may carry local amendments.
Not covered by this tool
- Equipment without readable terminal markings or installation instructions.
- Medium-voltage, motor, HVAC and listed assemblies with their own sizing rules.
- Field conditions where AHJ documentation requires a different terminal assumption.
Data last verified