You've hit on a very important distinction and a common area of confusion. Let's break it down

Understanding Branch Circuits vs. Feeders
Branch Circuit

The circuit conductors after the last overcurrent protection device (OCPD) that supplies the load. In this case, that's the disconnect switch.

Feeder

All circuit conductors before the final OCPD that supplies the load. In this case, that's the panel to the disconnect switch.

Applying MCA and MOP

You are correct in saying that MCA and MOP apply to the conductors and overcurrent protection after the disconnect. Disconnect Conductors (Branch Circuit)

These must be sized based on the MCA on the A/C unit nameplate. They must have an ampacity of at least 125% of the MCA.

Disconnect OCPD (Branch Circuit)

The size of this breaker or fuse is governed by the MOP on the A/C nameplate, but can be rounded up to the next standard size OCPD.

Sizing the Feeder (Panel to Disconnect)

This is where it changes, and you are right to question this. The feeder (panel to disconnect) is NOT sized based on the MCA and MOP. It is sized based on the load calculation using the methods outlined in the NEC (National Electrical Code). You need to do a proper load calculation for the AC unit as part of the total building load.Here's how the feeder sizing works:1. Determine the Calculated Load

You start with the MCA of the AC unit. However, this MCA needs to be included in the building's total load calculation. If it's a dwelling, it goes into the dwelling unit load calculation. If it's a commercial building, it goes into the commercial load calculation.2.

Consider Diversity Factors/Demand Factors

Depending on the occupancy type and other loads, you might be able to apply demand factors to reduce the calculated load. This means that the entire building will not be operating at it's full load at the same time.3.

Feeder Conductor Sizing

Once you have the calculated load for the feeder, you select conductors with an ampacity equal to or greater than this calculated load. Refer to NEC tables (310.16, etc.) for conductor ampacities, making sure to consider correction factors for ambient temperature and number of conductors in a raceway.
4.

Feeder Overcurrent Protection

The feeder OCPD (in the panel) must be sized to protect the feeder conductors, not necessarily based on the MOP of the A/C unit. Typically, the OCPD rating will be close to, but slightly larger than, the calculated load.

Key Differences and Why it Matters MCA/MOP for Branch Circuit (Disconnect to Unit)

Guarantees the A/C unit receives enough current and is protected from overcurrent based on the manufacturer's specific requirements.

Calculated Load for Feeder (Panel to Disconnect)

Considers the entire building's electrical demand and allows for diversity (loads not all operating simultaneously). This prevents oversizing feeders and service equipment, saving money and space.

Example

Let's say an A/C unit has an MCA of 20 amps and a MOP of 30 amps. Branch Circuit (Disconnect to Unit)

Conductors: 20 amps 1.25 = 25 amps. Use #10 AWG copper THHN (typically good for 30 amps) OCPD (Disconnect): Use a 30 amp fuse or breaker (based on MOP)

Feeder (Panel to Disconnect)

Let's assume, after a load calculation for the whole house, including this A/C unit and other appliances, the calculated load for the feeder is 80 amps. Conductors: Use #3 AWG copper THHN (typically good for 100 amps) OCPD (Panel): Use an 80 amp breaker

Important Considerations Load Calculations

Master the art of load calculations (NEC Articles 220, 310). This is where many people make mistakes.

Local Amendments

Always check for local electrical code amendments as they can modify NEC requirements.*

Professional Expertise

For complex installations or unclear situations, consult a licensed electrical engineer or master electrician.

In summary, the MCA/MOP values are specific to the A/C unit's branch circuit. The feeder supplying the disconnect is sized based on a proper load calculation that includes the A/C unit's MCA, but considers the overall electrical demand of the building. Flag for review