You stated a # 12 ground. Are they for sure talking about the grounding conductor or is it a grounded conductor.
Just curious. As some have already stated that the emt could possibly be used for the equipment ground.
If it is a grounded conductor it is possible that it could be sized smaller based on the unbalanced load. Do not know what the circuit is being used for?
That is what the code requires but the question is why. If the #12 gnd is good for the #12 ckt, why is it not good if the conductor is changed to #10? The gnd is supposed to have low enough impedance to ensure the breaker trip when a fault occurs. That has not changed.
I can only speculate...Can anyone answer this question?