Okay, let's break down the subpanel sizing for your two condensing units, ensuring we meet code requirements and safety standards.1. Understanding the Load Calculations



RLA (Rated Load Amps)

This is the steady-state current the compressor draws when running.

LRA (Locked Rotor Amps)

This is the very high current drawn briefly when the compressor starts. It's a surge.

MCA (Minimum Circuit Ampacity)

This is the minimum calculated ampacity your circuit must be rated for, considering the compressor's normal running current and any other loads on the circuit. This accounts for things like start-up surges and continuous operation.

MOCP (Maximum Overcurrent Protection)

This is the maximum size of the circuit breaker or fuse allowed to protect the equipment.

2. Calculating the Subpanel Load



Individual Unit Calculations

You'll use the MCA to calculate the load.

Largest Motor (Cooling Unit) Contribution

You must take the largest motor's MCA and add 25% of it to the sum of the other loads.

Calculate Total Load

Cooling Unit #1 MCA: 21 amps Cooling Unit #2 MCA: 21 ampsLargest motor = 21 amps25% of 21 is 5.25 amps21 (unit 1) + 21 (unit 2) + 5.25 =

47.25 amps3. Subpanel Breaker and Wire Sizing



Subpanel Breaker Size

You need a breaker that is at least as large as your calculated load of 47.25 amps, but without exceeding the ampacity of the wire. Since breakers are not rated for the decimal point. You need a 50 amp breaker.

Wire Size

The wire must be rated to handle at least the calculated load (47.25 amps) and the breaker amperage (50A). The wire ampacity needs to be based on the NEC (National Electrical Code) guidelines, specifically Table 310.
16. Assuming THHN or THWN copper conductors run in conduit and an ambient temperature of 30°C,

you'll need at least #6 AWG copper wire. Always verify wire ampacity based on the specific wire type and installation conditions.

Ground Wire

The size of the ground wire will be determined by the size of the ungrounded conductors, again referring to the NEC. For #6 AWG copper conductors, you'll need a #10 AWG copper grounding wire.

4. Sizing the Main Panel Breaker Feeding the Subpanel

The breaker in your main panel that feeds the subpanel must match the ampacity of your subpanel's main breaker (50 amps). You'll use a 50 amp double-pole breaker in the main panel. The wire between the main panel and the subpanel also needs to be at least #6 AWG copper (or the equivalent ampacity in aluminum, sized according to NEC Table 310.16, adjusted for temperature and conduit fill). Remember to run four conductors: two hots, one neutral, and one ground.

Summary and Key Considerations



Subpanel Main Breaker

50 amp double-pole.

Subpanel Wire (to condensing units)

Minimum #12 AWG copper per unit (20 amp single pole breaker)

Wire from Main to Subpanel

Minimum #6 AWG copper (verify based on conditions) (four conductors: two hots, one neutral, one ground).

Main Panel Breaker (feeding subpanel)

50 amp double-pole.

Ground Wire from Main to Subpanel

Minimum #10 AWG copper.

Individual Unit Protection

Use 20 amp single pole breaker with #12 awg copper wire for each condensing unit.

Important Safety and Code Notes



Local Codes

Always check your local electrical codes and regulations. They may have stricter requirements than the NEC.

Licensed Electrician

This is complex electrical work. It's highly recommended to hire a qualified, licensed electrician to perform this installation. Improper wiring can be dangerous and could lead to fire, injury, or death.

Voltage Drop

At 30 feet, voltage drop is less likely to be a problem, but it's still good practice to check it, especially if the distance were significantly longer. A voltage drop calculation ensures the equipment receives the proper voltage under load.

Conduit

Protect the wires in conduit (e.g., PVC or EMT) for physical protection.

Neutral and Ground Separation

Ensure the neutral and ground wires are properly separated in the subpanel (they should only be bonded together at the main panel).

Torque

Torque all connections to the manufacturer's specifications.

Inspection

Get the work inspected by a qualified electrical inspector after completion.

Aluminum Wire

If you choose aluminum wire, the wire size will need to be larger to achieve the same ampacity as copper. Consult NEC Table 310.16 for appropriate aluminum wire sizes. Also, be sure to use connectors and terminations rated for aluminum wire (AL/CU).

Why Not Simply Add the MOCP values?While the MOCP seems like a straightforward solution, it's intended to protect the equipment (compressor motors) from overcurrent damage. The MCA is what you use to determine the circuit loading and size the wires to prevent overheating and fire hazards. The MCA already includes a safety factor for the motor's starting surge. Adding the MOCP values directly would likely result in an oversized circuit. The correct method ensures the circuit can safely handle the continuous load and start-up surges.By following these steps and consulting with a qualified electrician, you can ensure a safe and code-compliant subpanel installation for your condensing units. Remember safety first! Flag for review