Okay, let's break this down and clarify the NEC rules regarding conductor ampacity and overcurrent protection for heat pumps. You're not entirely wrong, but you're missing a key aspect of heat pump installations.

Here's a breakdown of the situation and why the inspector is calling it out



Heat Pump Nameplate Data

You have a heat pump with:

MCA (Minimum Circuit Ampacity)

21 amps. This is the minimum amount of current the circuit must be capable of handling continuously.

MOCP (Maximum Overcurrent Protective Device)

25 amps. This is the maximum size breaker or fuse allowed to protect the circuit.

Your Installation

\#12 THHN conductor. 25 amp breaker.

The Inspector's Reasoning

He's citing the footnotes to Table 310.16 (and similar tables in the NEC), which state (paraphrased): \#14 AWG: Max 15 amp overcurrent protection. \#12 AWG: Max 20 amp overcurrent protection. \#10 AWG: Max 30 amp overcurrent protection. He argues that since you're using a 25 amp breaker, and the footnote says #12 should only be protected by a 20 amp breaker, you're violating the code.

Where the Inspector is Correct (Partially)



The footnotes to Table 310.16 do exist and generally apply. For standard circuits supplying receptacles, lights, etc., these footnotes are very important. You can't just run #14 wire and put it on a 20 amp breaker just because it theoretically can handle that much current.

Where You're Not Entirely Wrong (and Why This Matters)



Heat pumps (and other motor-compressor appliances) have special rules. The NEC allows you to exceed the normal ampacity limitations of conductors when the equipment is specifically designed for, and marked for, a higher overcurrent protection device. NEC 440: Air-Conditioning and Refrigeration Equipment. This is the section of the NEC that governs heat pump installations. Specifically: 440.22 Overcurrent Protection

This section allows the use of a breaker or fuse larger than the ampacity of the conductor in some cases, as long as it's in accordance with the nameplate MOCP rating.

440.12(A)

Specifically States (paraphrased) - "The equipment shall be marked with the size and type of branch circuit overcurrent device required."

440.33 Single Motor Compressor. This section explains how to determine MOCP based on inrush current and other factors. Manufacturers already do this and put the MOCP on the nameplate! The nameplate data is key. The manufacturer has already determined the MOCP based on the motor's characteristics. They have to follow strict testing requirements to get that MOCP listing.

Why Your Installation Might Be Code-Compliant (But Needs Verification)

Given that your heat pump's nameplate explicitly states a

Maximum Overcurrent Protective Device (MOCP) of 25 amps, you are potentially allowed to use the 25 amp breaker even though #12 THHN is normally only rated for 20 amps. The NEC permits the MOCP to exceed the conductor ampacity in specific circumstances, such as for motor-compressor loads.

What You Need To Do To Verify Compliance and Respond to the Correction


Quote the NEC

Point out that you are following NEC Article 440, specifically

440.22, 440.12(A) and 440.33. Explain that these sections allow you to use an overcurrent device larger than the conductor ampacity when the equipment nameplate specifies it.2. Provide Documentation

If possible, provide the inspector with a copy of the heat pump's nameplate showing the MCA and MOCP. If you have the installation manual, include that as well. Highlight the MOCP rating.3.

Double-Check Your Work



Conductor Type

You stated you used THHN. Make sure it is installed in conduit or cable assemblies suitable for the environment. If it's exposed and in a damp location, that might change the ampacity derating factors.*

Derating Factors

While unlikely with a relatively short run and 25 amps on #12, double-check that you haven't exceeded the conductor's ampacity due to ambient temperature correction factors or conduit fill derating factors. This is usually not a problem, but it's good to be thorough.

In Summary

You are likely correct, but the inspector is focusing on a simplified interpretation of Table 310.16. Heat pumps are a special case, and NEC Article 440 governs their installation. The nameplate MOCP is what you need to follow. Politely and respectfully explain this to the inspector, providing the NEC references and the nameplate data. If you can't convince the inspector, you can always request a second opinion from a different inspector or consult with a qualified electrical engineer.

Important Disclaimer: I am an AI chatbot and cannot provide official code interpretations. This information is for educational purposes only. Always consult with a qualified electrician and your local authority having jurisdiction (AHJ) for code compliance. It is up to the AHJ to make the final determination of compliance. Flag for review