Good morning and welcome to the forum! This is a great question and it highlights a common area of confusion for many electricians.Here's a breakdown to address your concerns, aiming to keep it as simple as possible

The Core Rule: 110.14(C) - Termination Temperature
You are absolutely correct. NEC 110.14(C) dictates that conductors must be selected and installed so their ampacity, after any adjustment or correction factors, doesn't exceed the temperature rating of the lowest rated termination point. This typically refers to the terminals on circuit breakers, switches, and other equipment. Commonly, equipment is rated for 75°C terminations. That means you generally must use the 75°C column in the ampacity tables to determine the minimum conductor size.

Why the Apparent Discrepancy?

Here's where things get nuanced, and why you see the "common" practices that seem to contradict the rule:1. Oversizing (Not the Root Reason, but Relevant)

Sometimes, electricians and inspectors will require larger conductors than the minimum required by the code. This could be for:

Voltage drop

Longer runs of wire can experience voltage drop. A larger conductor reduces resistance and minimizes voltage drop.

Future capacity

If there's a reasonable expectation of future expansion or increased load, larger conductors can save money later.

Local Jurisdictional Preferences

Some areas might have more stringent requirements that exceed the minimums in the NEC.2.

The Allowable Exception

While 110.14(C)(1) generally mandates 75°C ampacity, there's language that allows you to use the higher temperature ratings (90°C, etc.)

for ampacity adjustment and correction factors ONLY. This is the key. You can determine the minimum conductor size using the 75°C column. Then, if you have to apply derating factors (like ambient temperature corrections or conduit fill adjustments - which are common), you can use the higher temperature rating columns (90°C) to calculate the ampacity reduction. The higher starting point gives you more headroom. However, you still terminate on equipment rated at 75°C. You cannot use the 90C column for your initial conductor sizing if the terminals are rated for 75C.3. Feeder and Service Calculations

The NEC allows for demand factors, diversity factors, and other reductions to the calculated load. This means that the actual load on the service might be significantly less than the sum of all the individual circuit breakers.

Example

A dwelling unit's service calculation (Article 220) can often result in a calculated load much lower than 200 amps, even though there are many branch circuits. Because the calculated load is lower, a smaller conductor is permitted, even if that conductor's ampacity (at 75°C) is lower than the service rating.

Agricultural Buildings (Non-Dwelling)

Article 547 covers agricultural buildings. While dwelling load calculations are quite conservative, agricultural loads can be very heavy, especially with motors, heating, and lighting. It's crucial to perform a thorough load calculation to ensure that the service conductors are adequately sized.

Simplified Explanation1. Start with the Load Calculation

Perform a careful load calculation according to the NEC. Determine the total calculated demand on the service.2.

Check Termination Ratings

Confirm the temperature ratings of the service equipment (main breaker, panelboard terminals, etc.). Assume 75°C unless explicitly marked otherwise.3.

Select Conductor (Initially) at 75°C

Based on the calculated load and the 75°C column, choose a conductor size.4.

Apply Derating Factors (if necessary)

If you need to derate for ambient temperature, conduit fill, or other reasons, then you can temporarily use the higher temperature columns (90°C) to calculate the adjusted ampacity. Ensure the adjusted ampacity is still greater than or equal to the calculated load.5.

Ensure Termination Ampacity is Met

Regardless of your calculations, the conductor's ampacity at 75°C (after all derating) must be at least equal to the continuous load plus 125% of the largest motor load, but cannot exceed the termination ampacity.

Practical Examples 200 Amp Service (Dwelling)

Due to the dwelling load calculations, a calculated load of 150-175 amps is not uncommon. Therefore, 4/0 Aluminum (rated 180 amps at 75°C) might be acceptable, especially if there's no need for significant derating. However, if the calculated load is closer to 200 amps, or there are derating factors, you'd need to increase the conductor size.

100 Amp Service (Dwelling)

Similarly, for a 100 amp service, the calculated load might be significantly lower, allowing for #2 Copper (rated 95 amps at 75°C).

Important Considerations for Agricultural Buildings Agricultural buildings often have heavy and continuous loads. Motors are common, and motor starting currents must be considered. Dust and moisture can affect conductor ampacity and equipment ratings. Perform a very detailed load calculation that includes all equipment, motors, and lighting. Consider future expansion possibilities. Consult with an experienced electrician familiar with agricultural installations.

In Summary Always start with 110.14(C). Know your termination temperature ratings. Use the 75°C column for initial conductor selection. Use the higher temperature columns only for ampacity adjustments and corrections. Base conductor size on the calculated load and termination ratings, not just the service amperage. When in doubt, upsize the conductors.I hope this clarifies the issue. It's a complex topic, and code interpretation can vary. If you are unsure always consult with a qualified electrician or electrical engineer. Flag for review