You're on the right track. While generally you want the OCPD to be at or below the feeder ampacity, the NEC does allow for an OCPD to be larger than the feeder ampacity if specific conditions are met and an overload protection device is provided.Here's the breakdown, referencing the relevant NEC sections

The General Rule (and why your intuition is right):
NEC 240.4(B): Protection of Conductors

Conductors must be protected against overcurrent in accordance with their ampacities as specified in 310.15, subject to the conditions in 240.4(C) through (G). This essentially means, the OCPD should be sized at or below the conductor ampacity.

The Exception: Overload Protection is Key NEC 240.4(E): Overload Protection. Where overload protection is provided in accordance with 240.4(E)(1) through (E)(4), the conductor shall be protected for overload. The short-circuit and ground-fault protection shall be permitted to be provided by the branch-circuit overcurrent protective device.

This section opens the door to the possibility of a larger OCPD than the feeder ampacity provided that overload protection is separately handled. Let's look at some scenarios: (E)(1)

Motor branch-circuit conductors. Motor branch-circuit conductors shall be protected against overcurrent in accordance with Parts III and IV of Article 430.

(E)(2)

Air-conditioning and refrigeration equipment branch-circuit conductors. Air-conditioning and refrigeration equipment branch-circuit conductors shall be protected against overcurrent in accordance with Parts III and VI of Article 440.

(E)(3)

Transformer secondary conductors. Transformer secondary conductors shall be protected against overcurrent in accordance with 450.3 or as provided in 240.
5.

(E)(4)

Feeder taps. Feeder taps shall be protected against overcurrent in accordance with 240.21.

Why is this allowed?

The core reason is to allow for short-circuit and ground-fault protection without nuisance tripping due to temporary overloads or inrush currents that a smaller OCPD would trip on. Think of a motor starting. The inrush current is several times the running current, but that inrush is short-lived. If the OCPD was sized exactly to the feeder ampacity, it would likely trip every time the motor started.

Example: Motor Circuits

This is the most common scenario where this is applied. Let's say you have a motor that draws 20 amps continuously. You need to select the conductor size and OCPD.1. Conductor Sizing (Article 430)

You'd typically size the conductors at 125% of the motor's full-load current (FLA) which would make it 20A 1.25 = 25A. You'd choose conductors rated for at least 25 amps (e.g., #10 AWG THHN copper, assuming typical conditions).
2.

Motor Overload Protection (Article 430, Part III)

The motor must have overload protection. This is typically provided by:

Thermal Overload Relays

These are specifically designed to trip on sustained overcurrent (overload), but not on short-circuits or ground faults.

Integral Motor Protection

Many modern motors have built-in thermal protection that will trip if the motor overheats.3.

Short-Circuit and Ground-Fault Protection (Article 430, Part IV)

This is where the larger OCPD comes in. NEC Article 430 allows for an inverse-time breaker to be sized up to 250% of the motor's FLA (in certain cases, even higher depending on the motor's design letter code). Therefore, in the case of a 20A FLA motor, a 50A breaker could be used. This OCPD protects the circuit from short circuits and ground faults, but not overload.

Key Takeaways



Overload vs. Short-Circuit/Ground-Fault

It's crucial to distinguish between overload protection and short-circuit/ground-fault protection. Overload protection is for sustained overcurrent that can damage the equipment. Short-circuit/ground-fault protection is for immediate and severe overcurrent that can cause fire or injury.

Dedicated Overload Device

The key is the presence of a dedicated overload device that is sized to protect the equipment connected to the feeder.

NEC 240.4(E) is NOT a blanket permission

You can't just slap any oversized OCPD on a feeder. You must comply with the specific rules and requirements of the relevant articles (430, 440, 450, 240).

Careful Coordination

You need to carefully coordinate the OCPD and the overload device so that the overload device trips before the feeder conductors are damaged, and the OCPD trips quickly in case of a short-circuit or ground fault.

Documentation

Properly document the design and calculations. Inspectors will want to see how you complied with the code.

In Conclusion

Yes, the NEC permits the OCPD to be larger than the feeder ampacity in specific situations when overload protection is separately provided. This is common in motor circuits and some transformer applications. However, it requires careful design, calculations, and adherence to the relevant NEC articles. Don't attempt this without a thorough understanding of the code requirements.

Disclaimer: I am an AI chatbot and cannot provide professional engineering or electrical design advice. Consult with a qualified electrical engineer and refer to the latest edition of the National Electrical Code (NEC) for specific applications and local regulations. Flag for review