Yes, that would be allowed if the conductors meet the specific requirements of being service entrance conductors. Here's why: NEC 240.4(B) vs. 230.90 (Service Conductors)

NEC 240.4(B) is the general rule for overcurrent protection of conductors. It requires that conductors be protected at their ampacity. 500kcmil copper conductors are typically rated for around 380A at 75°C. Three sets would have a combined ampacity of 1140A. Thus, 240.4(B) normally prohibits a 1200A overcurrent protective device (OCPD).

Exception for Service Conductors

The NEC provides exceptions for service conductors. Specifically, NEC 230.90 addresses overcurrent protection of service conductors. While it typically requires protection at the ampacity, it also allows for specific exceptions.

NEC 230.90(A) Exception No. 3

This is the key exception that likely applies to your scenario. It allows the overcurrent protection to exceed the ampacity of the service conductors if: The calculated load is less than the ampacity of the service conductors plus the allowable overcurrent protection device rating. Example: Conductor ampacity (3 sets of 500 kcmil) = 1140A OCPD rating = 1200A Total capacity = 1140A + 1200A = 2340A. The calculated load MUST be less than 2340A.

Therefore, the 1200A OCPD is allowed ONLY IF


Calculated Load

You must perform a load calculation (following Article 220) for the building or system being served. This calculation must demonstrate that the total calculated load is less than the ampacity of the conductors plus the rating of the overcurrent device as described in 230.90(A) Exception No. 3.
2.

Service Entrance Conductors

The conductors must truly be service entrance conductors. They must originate at the service point (typically the connection to the utility) and run to the service disconnecting means (the 1200A breaker in your switchboard).

Important Considerations and Cautions



Coordination Study

It's good practice (and may be required by some jurisdictions) to perform a coordination study to ensure that the overcurrent protection devices coordinate properly. This helps minimize nuisance tripping and ensures that the appropriate breaker trips under fault conditions.

AHJ Approval

It's highly recommended that you discuss this design with the local Authority Having Jurisdiction (AHJ) before installation. They have the final say on whether the design complies with the NEC and local amendments. Present your load calculations and conductor information to them.

Labeling

You may want to label the panel or switchboard to clearly indicate that the service conductors are protected under the exception to NEC 230.90(A) Exception No. 3, and state the design total capacity.

Continuous Load

If the load is considered a continuous load (operates at maximum current for 3 hours or more), then the OCPD must be rated at 125% of the continuous load (NEC 210.19 and 210.20). This may affect your allowable calculated load.

In summary

While 240.4(B) would normally prohibit the 1200A OCPD, the exception for service conductors in 230.90(A) Exception No. 3 can allow it, provided* the calculated load is significantly lower than the combined conductor ampacity and breaker rating.

Thoroughly document your load calculations and get approval from the AHJ.Disclaimer: Always consult with a qualified electrical engineer and follow all applicable codes and regulations. Flag for review