3.2 Temperature Limitations of Terminations — 110.14(C)

The Rule Behind Every Conductor-Sizing Question

110.14(C) opens with the governing principle:

The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device.

Read that carefully. The conductor insulation may be rated 90 degrees C, but if it lands on a 75 degrees C lug, the ampacity you are allowed to claim comes from the 75 degrees C column. The weakest link in the chain — lug, breaker, conductor insulation, terminal block — sets the ceiling.

The rest of 110.14(C) also states that conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both. That single sentence creates the two-column workflow that trips up so many candidates.

The 100-Ampere Dividing Line

110.14(C)(1)(a) — Equipment provisions for circuits rated 100 A or less, or marked for 14 AWG through 1 AWG conductors:

1. Conductors rated 60 degrees C; or
2. Conductors with a higher temperature rating, provided the ampacity is determined based on the 60 degrees C ampacity of the conductor size used; or
3. Conductors with a higher temperature rating if the equipment is listed and identified for use with such conductors; or
4. For motors with design letters B, C, or D, conductors rated 75 degrees C or higher, provided the ampacity does not exceed the 75 degrees C ampacity.

110.14(C)(1)(b) — Equipment provisions for circuits rated over 100 A, or marked for conductors larger than 1 AWG:

1. Conductors rated 75 degrees C; or
2. Conductors with a higher temperature rating, provided the ampacity does not exceed the 75 degrees C ampacity of the conductor size used, or up to their temperature rating if the equipment is listed and identified for use with such conductors.

110.14(C)(2) Separate Connector Provisions. Separately installed pressure connectors shall be used with conductors at the ampacities not exceeding the ampacity at the listed and identified temperature rating of the connector.

In practice most modern breakers and panelboards 100 A and below are marked 60/75 degrees C, which permits the 75 degrees C column. But the marking is what permits it — not the conductor's insulation rating. On an exam, if the question does not say the equipment is marked for 75 degrees C and the circuit is 100 A or less, use the 60 degrees C column.

The Two-Column Workflow

This is the procedure that answers nearly every ampacity question:

Step 1  Start from the column matching the CONDUCTOR's insulation rating
        (usually 90 C for THHN/THWN-2/XHHW-2)
Step 2  Apply ambient temperature CORRECTION  -- Table 310.15(B)(1)
Step 3  Apply conductor-count ADJUSTMENT      -- Table 310.15(C)(1)
Step 4  Compare the corrected/adjusted result against the TERMINATION column
        ampacity from Step 0 (60 C or 75 C per 110.14(C))
Step 5  The FINAL allowable ampacity is the LOWER of the two

The 90 degrees C column is a calculation starting point, never an allowable operating ampacity for a conductor landing on 75 degrees C terminals.

Worked Example 1 — Terminal Limit Governs

A 70 A continuous load is supplied through a 100 A circuit breaker whose terminals are marked 75 degrees C. THHN copper in a raceway, 30 degrees C ambient, three current-carrying conductors.

1. Required conductor ampacity: 70 A x 1.25 = 87.5 A (210.19(A)(1) / 215.2(A)(1) continuous-load rule).
2. Termination column: the circuit is 100 A, so 110.14(C)(1)(a) applies; but the equipment is marked 75 degrees C, so the 75 degrees C column is permitted.
3. Table 310.16, copper: 4 AWG is 85 A at 75 degrees C — less than 87.5 A, so it fails. 3 AWG is 100 A at 75 degrees C — passes.
4. Answer: 3 AWG copper. Note that 4 AWG THHN shows 95 A in the 90 degrees C column, which is more than 87.5 A. Choosing 4 AWG on that basis is the trap: 110.14(C) caps the conductor at its 75 degrees C value of 85 A.

Worked Example 2 — Derating Column vs Limiting Column

Six current-carrying 8 AWG THHN copper conductors run in a single EMT through a 40 degrees C ambient. Terminals are rated 75 degrees C.

1. Start at 90 degrees C: 8 AWG THHN = 55 A.
2. Ambient correction, Table 310.15(B)(1), 36–40 degrees C, 90 degrees C column: 0.91. 55 x 0.91 = 50.05 A.
3. Adjustment, Table 310.15(C)(1), 4–6 conductors: 80 percent. 50.05 x 0.80 = 40.04 A.
4. Termination check: 8 AWG at 75 degrees C = 50 A. The corrected and adjusted value of 40.04 A is lower, so 40 A is the allowable ampacity.

Here the derating governs. In Example 1 the terminal governed. You must compute both and take the lower — that is the whole discipline.

Worked Example 3 — Small Conductors and 240.4(D)

A 12 AWG copper THHN conductor, three in a raceway at 30 degrees C, terminating on 75 degrees C lugs.

• 90 degrees C column: 30 A. 75 degrees C column: 25 A. 60 degrees C column: 20 A.
• The lugs are marked 75 degrees C, so 110.14(C) permits the 75 degrees C column. No correction or adjustment applies, so the allowable ampacity is 25 A.
• But 240.4(D)(5) independently limits the overcurrent device on 12 AWG copper to 20 A after any correction and adjustment.

The distinction matters: 25 A is the conductor's ampacity; 20 A is the maximum overcurrent device. They are different numbers set by different rules, and a question that asks for one while you supply the other is a lost mark. 240.4(D) is not a temperature rule and is not part of 110.14(C).

Change one fact and the answer moves. If those same lugs were not marked for 75 degrees C, 110.14(C)(1)(a) would force the 60 degrees C column, the ampacity would fall to 20 A, and the maximum device would still be 20 A. The two values coincide at 60 degrees C and separate everywhere above it.

Common Traps

• Treating the 90 degrees C column as an allowable ampacity. It is a starting point for derating only.
• Assuming a 100 A panel permits the 75 degrees C column without checking for the marking.
• Applying 240.4(D)'s 15/20/30 A small-conductor limits as if they were ampacities.
• Forgetting that 110.5 makes Code conductor sizes copper by default when the metal is not specified.
• Overlooking that the lowest-rated element might be a terminal block inside equipment, not the breaker.