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Terminal Temperature Ratings 110.14(C)

THHN is a 90 C insulation. That does not make the circuit a 90 C circuit. The lowest terminal rating at either end caps the usable ampacity after derating — and assuming 75 C on 60 C equipment is one of the most common sizing errors.
Code basis
110.14(C)
Common ratings
60 C and 75 C
Role
Cap after derating
Frequent error
Assuming 75 C always

The Procedure

5 steps, in the order the code applies them.

  1. Step 1

    Identify both terminations

    Read the breaker or panel lug rating and the equipment lug rating. The lower of the two ends governs.

  2. Step 2

    Choose the insulation lookup column

    Use the conductor insulation temperature for ambient and bundling arithmetic — often 90 C for THHN.

  3. Step 3

    Apply derating multipliers

    Correct for ambient and adjust for current-carrying conductor count from that insulation column.

  4. Step 4

    Cap at the termination column

    The final usable ampacity may not exceed the table value in the 60 C or 75 C column that matches the limiting lug.

  5. Step 5

    Re-select the conductor if the cap binds

    If the capped value is below design current, upsize and repeat. Do not ignore the cap because derating headroom remains in the 90 C column.

Column and terminal are not the same thing

The insulation rating decides the lookup column used for correction and adjustment.

The equipment terminal rating caps the final answer. It does not decide the starting column unless the conductor insulation is also that low.

Small equipment caution

Many field errors come from assuming 75 C lugs where the equipment must be treated at 60 C.

The safe input is the lower marked rating at either end of the conductor.

Decision Guide

The choices that change the answer, and how to make them.

  • 100 A and less often means 60 C unless marked

    Unless equipment is marked for a higher temperature, terminations on equipment rated 100 A or less are commonly treated as 60 C.

  • Column and terminal are different inputs

    Insulation chooses the starting column; terminals choose the ceiling. Collapsing them into one choice undersizes or oversizes depending on which you picked.

  • Field-verify the label

    Panel schedules and catalog pages lie less often than assumptions, but the equipment marking still wins when they disagree.

FAQs

Can I use the 90 C column with 75 C breakers?
Yes for the derating arithmetic when the insulation is 90 C. The final result is still capped at the 75 C column value for those terminations.
Why does 110.14(C) reduce my wire ampacity?
Because the connection point may not be rated for the full insulation temperature. The rule prevents using a 90 C conductor ampacity on a 60 C or 75 C lug.
When should I assume 60 C terminations?
When the equipment is rated 100 A or less and is not marked for a higher temperature, or whenever either end is marked 60 C.

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

  • Motor, air-conditioning and welder circuits, which are sized under their own articles.
  • Conductors in free air and photovoltaic circuits.
  • Parallel conductor sets, taps and busbars.
  • Local amendments, which can be stricter than the published code.

Data last verified