You've hit on a key point of confusion that many electricians face with hermetic motor compressors and wiring. Here's a breakdown

Understanding the Rules (Simplified):

1. MCA (Minimum Circuit Ampacity)

This is the

absolute minimum current carrying capacity the circuit conductors must have. It accounts for the motor's rated current and any non-continuous loads (like crankcase heaters). It's calculated using specific rules in NEC Article 440.2. Overcurrent Protection (OCPD)

This protects the motor and conductors from short circuits and excessive overloads. The sizing rules for OCPD for hermetic motor compressors are also in

NEC Article 440 and allow for higher percentages than standard motor circuits because of the unique starting characteristics.3. Conductor Ampacity (Temperature Ratings)

You must choose conductors with an ampacity

equal to or greater than the MCA, but limited by the lowest rated termination point. This is where the confusion often arises.

Why the 60°C Consideration Matters



Terminations

Equipment terminals are often only rated for 60°C or 75°C. You MUST use the

lowest rated temperature rating of the termination point, conductor, or ampacity adjustment factor. Conductor Ampacity

Table 310.16 provides ampacities for conductors based on insulation type and temperature rating. Common insulation types are THHN (dry locations, 90°C), THW (wet or dry locations, 75°C), and NM-B (60°C).

LFMC

Liquid-tight flexible metal conduit (LFMC) is often used for the final connection to the condensing unit. While the conductors inside LFMC might be rated for 75°C or 90°C,

the equipment terminals on the condensing unit or the disconnect switch may only be rated for 60°C.Addressing Your Specific Example



MCA = 31.4 amps Running Amps = 25.1 amps 10 AWG (75°C) = 35 amps (Looks good on the surface) 10 AWG (60°C) = 30 amps (This is where the problem arises)The Correct Application of the Code

In the example you provided, you are absolutely correct that 10 AWG at 60 degrees C would not be sufficient to meet the MCA of 31.4 amps.

Here's the breakdown of what should be considered and why


Termination Temperature Rating

The MOST critical factor. If the terminals on the condensing unit or the disconnect are only rated for 60°C, you

must use the 60°C ampacity of your conductor, regardless of the conductor's insulation rating (THHN, THW, etc.).2. In the Example

Even though you are using LFMC, it is only rated to 60 degrees C. Therefore, your wire (10 AWG) is only rated at 30 amps. Since the MCA is 31.4 amps, you will have to go to a larger wire gauge.
3.

Solution

If using 60°C terminals and 10 AWG THHN, the ampacity is limited to 30 amps. You need to use

8 AWG THHN (60 degrees C = 40 Amps) to meet the MCA requirement.

Key Takeaways



Always check the termination temperature rating of the equipment. It's often printed on the equipment or in the manufacturer's instructions. Use the lowest rated temperature for terminations, conductors, and ambient temperature adjustments. Don't assume you can use the 90°C ampacity of THHN unless all terminations and equipment are specifically rated for 90°C and you are applying the correction factors for the environment.In Conclusion

You are right to question the assertion that you can use 10 AWG in LFMC with a 60°C rating when the MCA is 31.4 amps. You cannot because the ampacity of 10 AWG at 60°C is only 30 amps, which is below the MCA.

Disclaimer: This explanation is for educational purposes and should not be considered a substitute for consulting the NEC or a qualified electrical professional. Always consult the latest edition of the NEC and local codes before performing any electrical work. When in doubt, consult a licensed electrician. Flag for review