Okay, let's break down how to size a subpanel for 12 condensers. To give you the BEST advice, I need some key information about each condenser. The more detail you provide, the more accurate my response can be.1. Information Needed for Each Condenser



Voltage

(e.g., 208V, 230V, 460V, 480V, etc.) This is critical for selecting the correct panel and breakers.

Phase

(Single-phase or Three-phase). Very important for panel selection.

Minimum Circuit Ampacity (MCA)

This is usually found on the condenser's nameplate. It's the minimum ampacity required for the circuit feeding the unit, taking into account motor starting and other factors.

THIS IS THE MOST IMPORTANT VALUE. Maximum Overcurrent Protection (MOCP) or Maximum Fuse/Breaker Size

Also on the nameplate. This is the maximum size of the breaker or fuse you can use to protect the condenser.

Locked Rotor Amps (LRA)

This is the current the motor draws when it's initially starting. It's needed for motor-rated breaker sizing.

Full Load Amps (FLA)

Amperage rating for a motor running at full power.

2. General Considerations and Calculations



Calculate Total Load (After Diversity/Demand Factors)

You cannot simply add up all the MCAs. Condensers rarely all run at their maximum simultaneously. You need to apply a diversity factor or demand factor. How you determine this depends on the application and local codes.

Example

If you're confident that only 8 out of 12 condensers will ever run at the same time, you would sum the MCAs of those 8, plus add a percentage of the remaining 4 MCAs (example 50% if they are staggered). This percentage can vary.

NEC (National Electrical Code) Article 440

This article specifically covers air-conditioning and refrigeration equipment. You must consult this article for proper sizing. It dictates how to handle motor starting currents, conductor sizing, and overcurrent protection.

125% Rule (for Continuous Loads)

If the condensers are considered "continuous loads" (operate for 3 hours or more at a time), you must size the conductors and overcurrent protection for at least 125% of the continuous load. Often, condensers are considered continuous.

Neutral Load

For single-phase 120/240V loads, the neutral will carry current. For three-phase balanced loads, the neutral current is ideally zero. You need to consider any other loads on the subpanel that will contribute to neutral current (e.g., lighting, receptacles).

Future Expansion

Always leave room in your subpanel for future expansion. It's generally recommended to size the panel larger than you currently need.

Feeder Size and Overcurrent Protection

The subpanel feeder (the wires feeding the subpanel) must be sized to handle the calculated load, including the 125% rule for continuous loads. The main breaker in the subpanel protects the feeder conductors.

3. Subpanel Selection Criteria



Amperage Rating

The subpanel's amperage rating must be equal to or greater than the calculated load (including any oversizing for future expansion).

Voltage

The subpanel's voltage rating must match the voltage of the condensers.

Number of Circuits (Spaces)

Ensure the subpanel has enough spaces for all the condenser breakers, plus any additional circuits you might need (lighting, receptacles, etc.) and room for future circuits.

Phase

The subpanel must be the same phase as the condensers (single-phase or three-phase).

Main Lug Only (MLO) vs. Main Breaker



MLO

A main lug only panel doesn't have a main breaker. It's fed from a breaker in another panel. The feeder to the MLO panel needs its own overcurrent protection at the source.

Main Breaker

A main breaker panel has a built-in main breaker that protects the panel and the feeder.

Enclosure Type

Choose an enclosure suitable for the environment (e.g., NEMA 1 for indoor, NEMA 3R for outdoor).

Short Circuit Current Rating (SCCR)

The SCCR of the panel needs to be greater than the available fault current at its location. This is a safety issue.

4. Breaker Selection



HACR Breakers

Use HACR-rated breakers for air conditioning equipment. These are designed to handle the high inrush currents of motor starting.

Breaker Size

The breaker size for each condenser should be based on the lower of: The MOCP (Maximum Overcurrent Protection) from the condenser nameplate. What the conductor ampacity allows based on the ampacity chart.

5. Example Scenario (Simplified)

Let's say you have 12 condensers, each: 240V, Single-Phase MCA: 20 Amps MOCP: 30 Amps1.

Total MCA

12 condensers 20 Amps/condenser = 240 Amps2.

Diversity/Demand Factor

Let's assume you determine that only 8 condensers will likely run at the same time at full load, and the remaining 4 will run at 50%. Therefore: (8 20) + (4 20 0.5) = 160 + 40 = 200 Amps3.

Continuous Load

Assume continuous load. 200 Amps 1.25 = 250 Amps.
4.

Subpanel Size

You'd need a subpanel with a minimum amperage rating of 250 Amps. A 300 Amp panel would provide more room for future expansion.5.

Breaker Size per Condenser

You'd likely use 2-pole 25 amp or 30 amp HACR breakers for each condenser, (verify conductor ampacity).6.

Feeder Size

The conductors feeding the subpanel need to be rated for at least 250 Amps (based on the 125% rule).7.

Main Breaker (if applicable)

If the subpanel has a main breaker, it should be rated at 250 Amps or higher, sized to protect the feeder conductors.

Important Reminders



Consult the NEC

Absolutely necessary.

Local Codes

Your local electrical codes may be stricter than the NEC.

Qualified Electrician

This type of work must be performed by a qualified and licensed electrician. Improper electrical work is dangerous and can be a fire hazard.

Permitting

Check with your local authorities about required permits.

To help me give you more precise advice, please provide the information for each condenser as described above. Once you provide that, I can help you with more accurate calculations and subpanel selection. Flag for review