You're right to be cautious and investigate further. You're also thinking about it correctly. Here's a breakdown of the considerations and best practices

Understanding the Nameplate and the Code:
MOPD (Maximum Overcurrent Protective Device)

This is the maximum size breaker or fuse allowed to protect the outdoor unit. It's there to protect the equipment from fault currents and shouldn't be sized smaller than the MCA.

MCA (Minimum Circuit Ampacity)

This is the minimum ampacity the wiring must have to safely carry the continuous load of the outdoor unit. It should include the loads of the indoor units powered through it. This is the critical number for sizing your conductors.

NEC 440.4(B) - Ampacity and Rating of Disconnecting Means

This section is crucial for air-conditioning and refrigeration equipment. It essentially confirms the manufacturer's MCA must include all components connected and powered from that unit.

Why the MCA is Likely Correct (and What to Double-Check)

The manufacturer should have factored in the load of the indoor units that are powered from the outdoor unit when calculating the MCA. However, you're right to verify:1.

Manufacturer's Instructions are King

Always follow the manufacturer's installation instructions. They are the ultimate authority. If there is any contradiction between general NEC rules and the manufacturer, default to the instructions.2.

Combination Scenarios

Good manufacturers take into account that some combinations of heads have more current draw than others, and they have likely sized the unit for this purpose. You can try to estimate the total load for all combinations and compare it to the MCA.3.

Documentation is Key

Make sure you have the model numbers of all indoor and outdoor units. Search for the installation manuals for each, not just the spec sheets. Look for any specific notes about wiring or load calculations.4.

Component Load

Look for installation manuals of indoor units to see if it provides MCA that is not mentioned on the outdoor unit label.

Steps to Take


Call the Manufacturer

This is the best step. Provide them with the model numbers of all units (outdoor and all three indoor units). Ask: "Does the outdoor unit's MCA (18.1 Amps) include the power consumption of all three indoor heads when they are operating at maximum capacity?" "Is there any specific combination of indoor heads that would require a larger conductor size than what the MCA indicates?" "Can you provide documentation supporting the MCA calculation?" "Is the outdoor unit designed to supply power to the indoor units?"2.

Calculate the Load Yourself (as a Check)

Find the rated current (RLA - Rated Load Amps) for each indoor unit. This is not the startup current. Find the compressor's RLA for the outdoor unit. Find the fan motor's FLA for the outdoor unit. Check for any other loads specified by the manufacturer. Total the loads. This should not exceed the MCA.3.

Consider a Larger Conductor

If you are concerned about the calculated load and the MCA being too close to the 80% continuous load limit of a 20A circuit on 12 AWG, or if the manufacturer advises it, consider using 10 AWG conductors. This would give you more headroom.

Important Considerations with NM Cable (Romex)



Temperature Rating

NM cable is typically rated for 60°C. Make sure your terminations (breakers, disconnects, unit terminals) are also rated for 60°C. Some equipment requires 75°C rated terminations.

Derating

If the conductors are bundled for more than 24 inches or run through insulation, derating may be required. This would lower the ampacity of the conductors.

In summary: While the manufacturer's MCA is likely correct, verifying with them and doing a sanity check calculation will provide peace of mind and ensure a safe and code-compliant installation. Don't cut corners on conductor size if there's any doubt. Flag for review