You are correct in your assessment and calculations. It's unusual, but perfectly valid under the NEC, and here's why it's happening and what you can't do: NEC 250.122: Equipment Grounding Conductor Size Based on Overcurrent Device Rating. This is the controlling section. The size of the EGC is determined by the rating of the overcurrent protective device (OCPD) protecting the circuit, not the size of the conductors. You've correctly identified that a 30A OCPD requires a #10 AWG EGC. Why it's happening in your case

Your #12 conductors are allowed because of motor rules, specifically related to the use of inverse-time circuit breakers for motor protection. NEC 430 allows you to size the OCPD much larger than the conductor's ampacity to allow for motor starting inrush current. The EGC, however, must be sized based on the potential fault current that the OCPD is designed to interrupt. Since the circuit breaker is rated at 30A, you need an EGC sized based on a 30A circuit.

No Reduction Allowed Here

There's no provision in the NEC to reduce the size of the EGC in this particular scenario. 250.122(B) has reduction rules, but they are for parallel conductors and/or situations where the circuit conductors are adjusted for voltage drop. Neither of these situations exist.

Common Misconceptions

A common error is to assume the EGC size is based on the conductor ampacity. This is incorrect. It's based on the OCPD rating protecting the circuit.

What to do

Run a #10 AWG EGC with your #12 conductors. This is compliant with the NEC.

In summary, you are correctly applying the NEC. While it might seem counterintuitive, the code requires a larger EGC in this situation to ensure adequate ground fault current carrying capacity, even if the normal load current is relatively low and the phase conductors are smaller.Additional Considerations (Not Required, But Recommended)



Terminations

When using a larger EGC with smaller conductors, ensure the terminals in the motor junction box and at the breaker panel are appropriately sized for both the #12 conductors and the #10 EGC. You may need to use a connector that allows for different size wires.

Color Coding

While not strictly code, be sure to properly color-code the EGC (green or green with a yellow stripe) to avoid confusion during future maintenance or troubleshooting.

Documentation

It can be helpful to document why you have a larger EGC than phase conductors, especially if it's unusual in your facility. This can prevent confusion down the road.

Example:

Let's say you're running this circuit in conduit. Your materials list would include: #12 AWG THHN/THWN (2 conductors - Hot & Neutral) #10 AWG THHN/THWN (1 conductor - EGC)* Appropriate size conduit for 3 #12 & 1 #10 conductors.You must follow the above, there are no alternatives in the NEC to reduce the EGC size in this scenario. Flag for review