You've hit on a very common misunderstanding in the electrical field, and your understanding is correct. Let's break it down

The Core Issue: 110.14(C)(1) and Termination Temperatures
The Rule

110.14(C)(1) is the key. It mandates that, unless the equipment (circuit breaker, disconnect, etc.) is explicitly listed and marked for use with conductors at higher temperatures, you must use the ampacities in Table 310.16 for determining the termination ampacity.

Why? Termination temperature is the critical factor. The equipment is designed and tested to operate safely with conductors carrying no more current than those listed in Table 310.16 at the termination point. Exceeding that current will heat the terminations beyond their design limits, leading to potential failure, fire, and a code violation. The Misunderstanding

People focus on the conductor itself, assuming that if the wire is rated for a higher temperature (e.g., 90°C), they can load it up to its 90°C ampacity. But the issue isn't solely the wire's insulation; it's the termination point and the equipment it's connected to.

Example Breakdown

Let's say you have: #3 AWG THHN copper conductors (rated 90°C) Running in free air Connected to a standard 100A circuit breaker in a panel.1. Table 310.17 (Free Air)

#3 AWG THHN in free air at 90°C is rated for 110A. So far, so good, right?2.

110.14(C)(1) Termination Limit

However, you must check the breaker's listing. Most breakers and equipment up to 100A are only listed for 60°C or 75°C terminations (meaning the wire needs to be rated 60 or 75 degrees at its termination, and can not exceed this rating). Let's assume the breaker is only rated for 75°C terminations.3.

Table 310.16

#3 AWG copper at 75°C is rated for 85A.4.

The Result

You can only load this circuit to 85A, even though the wire in free air could theoretically handle 110A. The termination temperature limitation dictates the maximum allowable current. The breaker could also be derated with an additional temp correction if located outside 30 degrees C/86 degrees F

So What's the Purpose of Multiple Ampacity Tables?

You're right, they serve a purpose, but not in the way many people think.1. Derating and Adjustment

You start with the ampacity from Table 310.16 (based on termination temperature). Then, you use other tables (like 310.15(B)(2)(a) for multiple conductors in a raceway or Table 310.15(B)(3)(a) for ambient temperature correction) to derate or adjust the ampacity downward. You would only use other tables to increase if the terminals/breakers are rated at 90 degrees C.2.

Voltage Drop Calculations

The higher ampacities from tables like 310.17 can be useful when performing voltage drop calculations, even if you can't actually use the conductor at that higher ampacity for overcurrent protection. This is because voltage drop is based on the actual current flowing through the conductor, not the ampacity limit.
3.

Specific Equipment

As the code states, some equipment is specifically listed and marked for use with conductors at higher temperatures. In these cases, and only these cases, can you use the higher ampacity ratings (from Table 310.17 or other appropriate tables) and the conductors must be sized appropriately to handle the inrush.

In Summary 110.14(C)(1) is paramount. Termination temperature is key. Use Table 310.16 as your starting point for termination ampacity, unless equipment is specifically marked otherwise. Use other tables primarily for derating or adjustment downward. Don't assume higher wire temperature ratings automatically mean you can load the wire to its full potential; the equipment* dictates the final allowable current.You are absolutely correct in pointing out this common misunderstanding! It's a crucial aspect of safe and compliant electrical installations. Flag for review