You're right to be questioning this. Let's break down why the inspector is likely arguing for the 70-amp breaker and clarify the relevant NEC code sections.

Understanding the Terms



MCA (Minimum Circuit Ampacity)

This is the minimum ampacity the circuit conductors must have to safely carry the unit's continuous load. Your wire needs to be rated for at least this value.

MOP (Maximum Overcurrent Protection)

This is the maximum size breaker or fuse the manufacturer allows to protect the unit. It's not necessarily what must be installed, but it's the upper limit.

Your Reasoning

Your logic is sound: You sized the #6 wire to handle the MCA of 54.6 amps, and you chose a 60-amp breaker, which falls between the MCA and MOP. This is often acceptable.

Why the Inspector Likely Failed You (and where to look in the NEC)

The inspector is likely focusing on

NEC 440.22(A), which addresses the overcurrent protection for air-conditioning and refrigeration equipment. Here's the relevant part of that section:> "A hermetic refrigerant motor-compressor shall be provided with overcurrent protection that is sufficient to carry the starting load and running load."Here's how it's interpreted



NEC 440.22(A) states

Where the motor-compressor is marked with MOP, the overcurrent protection shall not exceed the value marked. If there is no marked MOP, the overcurrent protection is not limited by the maximum overcurrent protection value in NEC Article 430. The key takeaway is this:

The MOP is the absolute maximum breaker size allowed. If you go over the MOP, you're in violation. It's a hard limit.

How the NEC addresses overcurrent



NEC 440.22(B) states

Overcurrent protection shall be permitted to be increased to comply with motor starting current where the rating is not sufficient for the starting current of the motor.

Why the inspector could be wrong



NEC 240.4(B) allows you to go up to the next standard size breaker. So technically you can round the next breaker size to the 70-amp.

How you could argue the point, if safe and permissible


Manufacturer's Instructions are King

Always start with the manufacturer's installation instructions. If the instructions explicitly allow for a smaller breaker than the MOP, and the instructions are UL listed, you have a strong case.2.

Ampacity of wire

#6 copper wire with THHN insulation is typically rated for 55 amps at 60°C, 65 amps at 75°C, and 75 amps at 90°C. This is where your argument may fall apart. The wire ampacity needs to be suitable for the breaker size and application. You must consult NEC Table 310.16 and the terminal temperature rating of the equipment. If the terminals are only rated for 60C, you are limited to the 55-amp rating. A 70-amp breaker on #6 wire would not be code-compliant in this scenario.

Important Considerations



Consult the Engineer

If you are unsure of the correct breaker and wire size, consult with the engineer of record.

Safety First

Over-fusing is dangerous. Always prioritize safety and proper protection of the equipment and wiring. Undersized wiring is equally dangerous.*

Local Amendments

Check your local building codes for any amendments or interpretations that might affect the application of the NEC.

In summary

The inspector is likely focusing on the MOP and the potential for nuisance tripping if a smaller breaker is used. To resolve this, you need to:1.

Review the manufacturer's installation instructions. Look for any flexibility in breaker sizing.2. Confirm the wire ampacity is correct for the breaker size. If you can prove the #6 wire is rated high enough, you have a stronger case.3. Consider using a slow-blow breaker

A time-delay breaker (like a HACR type) might allow the motor to start without tripping.4.

If none of the above works, you may need to upgrade the wiring to #4. This would allow you to use the 70-amp breaker and meet the code requirements.It's always best to have a respectful and open conversation with the inspector to understand their reasoning and explore possible solutions. Good luck! Flag for review