Yes, it is absolutely correct to potentially have two different ampacities for the same conductor in a raceway and inside a box at the terminals. Here's the breakdown of why: Different Environments, Different Temperatures

The whole concept revolves around the operating temperature of the conductor. The NEC is primarily concerned with preventing overheating of conductors and insulation breakdown, which can lead to fires.

Raceway

Inside a conduit (raceway), conductors are often bundled together with other conductors, sharing the heat they generate. This bundling and the raceway itself can impede heat dissipation. The ambient temperature around the raceway may also be higher (e.g., near a hot roof, in direct sunlight). This requires ampacity adjustment factors (Table 310.15(C)(1)) to derate the conductor's ampacity to prevent overheating.

Terminals in a Box

Inside an electrical box, the terminals of devices (e.g., breakers, receptacles, switches) are often the "weakest link" in terms of temperature ratings. The device terminals themselves are usually rated for 60°C or 75°C. Even if the wire insulation is rated for 90°C, the NEC limits the ampacity of the conductor to the ampacity corresponding to the terminal's temperature rating (110.14(C)(1)).

NEC 110.14(C)(1): Termination Temperature Limitations This is the key rule! It states: (1) Equipment Provisions. The determination of termination provisions of equipment shall be based on 110.3(B). Unless otherwise marked, conductor termination provisions for equipment rated 100 amperes or less, or marked for use with conductors size 14 AWG through 1 AWG, shall be used only for conductors rated 60°C (140°F). Exception No. 1: Connection shall be permitted to be made to conductors rated higher than specified, provided the connection is so marked or listed. Exception No. 2: A conductor with insulation rated for 75°C (167°F) or higher temperature shall be permitted to be terminated on equipment rated 60°C (140°F), or marked for use with 60°C (140°F) conductors if the ampacity of such conductors is determined based on the 60°C (140°F) ampacity of the conductor size used. Exception No. 3: A conductor with insulation rated for 90°C (194°F) temperature shall be permitted to be terminated on equipment rated 75°C (167°F) if the ampacity of such conductors is determined based on the 75°C (167°F) ampacity of the conductor size used.In simple terms: Most small circuit breakers (100A or less) and devices are only rated for 60°C terminations unless otherwise marked. You must use the ampacity corresponding to the 60°C column in Table 310.16 for those terminations. If the device is marked for 75°C terminations, you can use the 75°C ampacity. You can't exceed the temperature rating of the weakest link, which is usually the terminal.

Example

Let's say you have a #12 AWG THHN copper conductor inside a conduit with 6 current-carrying conductors. You're connecting it to a standard 15A breaker (rated 60°C terminations).1.

Table 310.16

#12 THHN has an ampacity of 30A (at 90°C), 25A (at 75°C), or 20A (at 60°C).2.

Termination Limit

Because of the 60°C termination of the breaker, you can only use the 20A ampacity for the connection to the breaker.3.

Adjustment Factors

For 6 current-carrying conductors, Table 310.15(C)(1) requires a 80% adjustment factor.
4.

Adjusted Ampacity

30A x 0.80 = 24A. However, due to the 60°C termination limitation, you cannot use 24A.5.

The Final Ampacity for Overcurrent Protection

You are limited to the 60°C value of 20A. Since the overcurrent device rating must be less than or equal to the ampacity of the conductor, and 20A is greater than 15A, you are ok to use a 15A breaker.

In this scenario

The conductor in the raceway could theoretically carry 24A (based on the adjusted 90°C ampacity), if the terminations could handle it. However, at the terminals, the ampacity is limited to 20A because of the breaker's 60°C rating.

Key Takeaway:You must always consider both the adjustment factors for conductors in raceways and the termination temperature limitations when determining the correct conductor size and overcurrent protection. The lower* of these ampacities will be the limiting factor.Therefore, it's perfectly correct to have a different ampacity considered for the same conductor at different points of the circuit based on these factors. This is a critical safety consideration in electrical installations. Flag for review