You're likely thinking of NEC 690.8(B)(2) and/or 690.9(C)(2). Let's break down what those sections cover and how they relate to your situation.690.8(B)(2) Inverters and AC Modules. This section deals with calculating the output circuit current of inverters and AC modules. Specifically, it aims to determine the maximum output current that the inverter/AC module is likely to produce under normal operating conditions. This is a crucial value for sizing conductors, overcurrent protection, and other components.The relevant part for your question is that the calculated output current can sometimes be lower than the inverter's rated output current if it is current limited.690.9(C)(2) Circuits Overcurrent Protection This section deals with sizing the overcurrent protection device (OCPD) for the output circuit of inverters and AC modules. It states:"Inverter output circuits shall be protected in accordance with 240.4. Where overcurrent protection is provided and the inverter output is limited in current to less than the rating of the transformer on the load side, the inverter output shall be protected on the line side of the transformer with a device rated no more than the output rating of the inverter."How These Sections Relate to Your Question (Inverters & Transformers)

Here's how these sections work together and address the scenario you described:1. Inverter Current Limiting

Many inverters have built-in current limiting capabilities. This means the inverter can be designed so it cannot output a current greater than a specific value, even under overload or short-circuit conditions.2.

Transformer Compatibility

The transformer on the load side of the inverter needs to be rated for the output of the inverter. In some cases, the transformer needs to be "backed up" by an upstream OCPD to protect against overcurrents.3.

OCPD Sizing: The Code allows for a smaller OCPD if the source is current limited: If the inverter is current limited and the current cannot exceed the rating of the transformer on the load side, then the OCPD can be sized according to the output of the inverter.

Example

Let's say you have an inverter with the following characteristics: Rated AC Output: 20 Amps Current Limited to: 20 Amps (The inverter will never output more than 20 Amps, regardless of the load)You are using a transformer rated for 20 Amps on the load side of the inverter. If the inverter is protected by an OCPD on the line side of the transformer rated for 20 Amps, it is protected in compliance with NEC 690.9(C)(2)

Important Considerations and Cautions



Documentation is Key

You must have documentation from the inverter manufacturer clearly stating the current limiting capability and the maximum output current under all operating conditions. This documentation is essential for justifying the reduced OCPD size to an inspector.

Listing and Labeling

Ensure that the inverter, transformer, and any other equipment are properly listed and labeled for their intended use.

Transformer Protection

Always ensure that the transformer is also protected against overload or fault conditions from the load side.

In summary:

The NEC allows for a reduction in OCPD size in specific scenarios where the inverter is demonstrably current-limited to a value less than the transformer's rating on the load side. This exception is based on the principle that the inverter itself prevents overcurrent conditions from occurring. However, rigorous documentation and adherence to the Code's requirements are critical to ensure safety and code compliance.Always consult the latest edition of the NEC and consult with a qualified electrical inspector or engineer for guidance on specific installations. Local jurisdictions may have additional requirements or interpretations. Flag for review