Let's break this down. The core issue is whether your existing 1200A breaker is adequate and code-compliant for a chiller with a 700A MOCP (Minimum Overcurrent Protection) requirement.

The short answer is likely YES, you can use the existing 1200A breaker IF a properly sized fused disconnect switch is installed.

Here's the reasoning, code references, and considerations

1. Understanding MOCP:
MOCP is the minimum overcurrent protection required for the chiller. The manufacturer determines this based on the characteristics of the chiller (starting current, running current, fault current withstand, etc.). This is not the maximum.2. Key NEC Sections



NEC 430.52: Motor Circuit Overcurrent Protection. (Note: While chillers have other components, they are driven by a motor and this section largely governs the sizing.) NEC 440.22: Overcurrent Protection. This is specifically for air-conditioning and refrigeration equipment. It references back to article 430. NEC 440.12: Single Motor-Compressor. Provides guidance on MOCP marking.3. Analysis and Code Interpretation



Overcurrent Protection

The NEC does allow for overcurrent protection to be higher than the calculated full load current. NEC 430.52 sets limits. For a typical inverse time breaker (like you likely have), the limit is often 250% of the motor's full-load current (FLA), unless the overcurrent device has too small of an AIC rating.

The Role of the Fused Disconnect

The fused disconnect acts as the primary overcurrent protection device for the chiller. The 700A fuses are sized to protect the chiller as specified by the manufacturer's MOCP.

Series Ratings

The existing 1200A breaker is unlikely to have a higher interrupt rating that the 700A fuses and disconnect so it will provide fault protection.

4. Why the 1200A Breaker Can Be Acceptable (with the fused disconnect)



Coordination

The 1200A breaker now serves as a backup. It's primarily there to protect the feeder conductors and the electrical distribution system from larger faults. The 700A fuses within the disconnect will trip first on overloads specific to the chiller. This provides selective coordination - essential in many applications to minimize downtime.

Feeder Protection

Two sets of 500 kcmil conductors are typically sufficient for 700A of continuous load. The 1200A breaker protects those conductors from overload.

Code Compliance

By installing the 700A fused disconnect at or near the chiller, you are satisfying the MOCP requirement. The 1200A breaker upstream doesn't negate that.

5. Important Considerations and Steps



Manufacturer's Instructions

Always follow the chiller manufacturer's installation instructions. They may have specific requirements or exceptions.

Fused Disconnect Rating

Ensure the fused disconnect switch is rated for at least 700A and is suitable for the voltage and horsepower of the chiller.

Fuse Type

Use the correct type of fuse as specified by the chiller manufacturer (e.g., time-delay, Class RK1, etc.).

Conductor Sizing

Verify that the 500 kcmil conductors are properly sized for the continuous load of the chiller (which may be less than the MOCP value, but use at least 125% of the calculated continuous load). Use NEC Table 310.16 (ampacities) and apply appropriate correction factors for ambient temperature and number of conductors in a raceway.

Short-Circuit Current Rating (SCCR)

Ensure the chiller and disconnect switch have an SCCR (Short-Circuit Current Rating) equal to or greater than the available fault current at that location in the electrical system. This is critical for safety.

AIC Rating

Verify that all components (breakers and disconnect switch) have an adequate AIC (Ampere Interrupting Capacity) rating to handle the available fault current.

Labeling

Properly label the disconnect switch with the MOCP rating and the chiller's nameplate information.

Local Codes and AHJ

Always consult with your local electrical inspector (AHJ - Authority Having Jurisdiction). They have the final say on code compliance.

Example Scenario

Let's say the chiller nameplate states: MOCP: 700A RLA (Rated Load Amps): 450AIn this case, the 700A fuses in the disconnect are correct for overcurrent protection. The two sets of 500 kcmil conductors likely meet the requirement for 450A continuous load with the 1200A breaker protecting the feeder.

In Summary:

Using the existing 1200A breaker is likely acceptable provided you install a properly sized (700A) fused disconnect switch at or near the chiller. Ensure proper fuse type, conductor sizing, SCCR rating, and follow all manufacturer's instructions and local codes. Consulting with a qualified electrician is strongly recommended to ensure code compliance and safe installation. Flag for review