You're right to question this. Here's how to interpret that requirement and how it affects your ampacity calculations

Understanding the Requirements
"#10AWG Stranded Copper Wire"

This specifies the conductor size and material for all power load wiring. It means you must use #10 gauge stranded copper wire.

"All Feeder Conductors Rated 90°C"

This specifies the insulation temperature rating for the feeder conductors. They must have an insulation rating of at least 90°C (194°F). This is important because the insulation's temperature rating determines how much heat the conductor can withstand before the insulation degrades.

How to Interpret for Ampacity

The requirement for 90°C rated feeder conductors does not automatically mean you use the 90°C column for ampacity in all cases. Here's a breakdown:1. Starting Point

You
can look up the ampacity of #10 AWG copper wire in the 90°C column of NEC Table 310.16 (or the appropriate table in your local code). This will give you the highest possible ampacity based solely on the wire's temperature rating. For #10 AWG copper, that's typically 40 amps.2. Termination Temperature Limits (Critical!): This is the most important factor and where the 90°C advantage often gets reduced. You MUST consider the temperature ratings of the termination points (breakers, switches, receptacles, etc.). NEC 110.14(C) limits the ampacity of conductors to the lowest rated termination. Common Scenarios



15A or 20A Receptacles/Switches

These are often rated for 60°C or 75°C terminations. If you connect your #10 wire to a 60°C rated device, you MUST use the 60°C ampacity rating (typically 30 amps for #10 copper). If they are rated 75°C, you must use the 75°C ampacity rating (typically 35 amps for #10 copper).

Breakers

Check the breaker's label. Many residential and light commercial breakers are only rated for 60°C or 75°C terminations. You'll be limited by that rating.

90°C Rated Equipment

If all termination points (breakers, panels, devices, etc.) are explicitly rated for 90°C and are tested and identified for use with 90°C conductors, then you can use the full 90°C ampacity rating, but this is rare in typical applications.3.

Other Correction/Derating Factors

After determining the applicable ampacity based on termination temperature, you still need to apply any other correction or derating factors required by the NEC (or your local code) based on:

Ambient Temperature

If the conductors are in a hot environment (above 30°C), you must derate the ampacity.

Number of Conductors in a Raceway/Cable

If you have more than three current-carrying conductors in a raceway or cable, you must derate the ampacity.

Other Specific Conditions

Any other conditions that might affect the conductor's ability to dissipate heat.

Example Scenario

Let's say you're wiring a circuit using #10 AWG copper THHN wire (rated 90°C) to power a string of standard 15-amp receptacles. The breaker and receptacles are rated for 75°C terminations. There are no other derating factors.1.

90°C Ampacity

Table 310.16 says #10 copper THHN is good for 40 amps at 90°C.
2.

Termination Limitation

However, your breaker and receptacles are only rated for 75°C. Table 310.16 says #10 copper is good for 35 amps at 75°C.
3.

Final Ampacity

Therefore, your circuit is limited to 35 amps. Even though the wire can handle more, you can't exceed the termination's rating. Since you're using 15 amp receptacles, the breaker would likely be a 15 or 20 amp breaker, as it is not safe to install 15 amp receptacles on a 35 amp circuit.

Why the 90°C Requirement Might Be in Place

Even if you can't always utilize the full 90°C ampacity, there are a few reasons why this requirement might be specified: Future-Proofing

The project designer might be anticipating future upgrades where equipment with 90°C terminations will be used.

Higher Safety Margin

Even if you're limited by termination temperatures, 90°C insulation generally offers a higher margin of safety and longer lifespan in normal operating conditions.

Specific Conditions

There might be areas in the installation where the ambient temperature is higher, making the 90°C rating more important.

Voltage Drop Mitigation

While not directly related to ampacity, using #10 wire (which is often larger than strictly necessary) can help reduce voltage drop over long runs.

In Summary

1. The 90°C rating on the feeder conductors allows you to potentially use a higher ampacity, but...2.

The termination temperature ratings are the FIRST and MOST IMPORTANT limiting factor. Always comply with NEC 110.14(C).
3. After addressing termination temperature, apply any other applicable correction and derating factors.4. Understand why* the 90°C requirement is in place and design your system accordingly.

Recommendation:

Consult with a qualified electrical engineer or licensed electrician to ensure your design complies with all applicable codes and regulations. They can help you correctly interpret the requirements and select the appropriate wiring and equipment. Flag for review