Q. Does the NEC specifically state that a generator must be grounded to the earth if its wiring is connected to the premises via a transfer switch that does not open the neutral?
A. According to the grounding requirements of the NEC, the answer is no, since it would serve no purpose to do so. But many generator manufacturers have boilerplate instructions specifying how they want the generator grounded. Because the instructions specifically require grounding, you have to waste your time and money to drive a ground rod [11.3(b)].
It’s unfortunate that many generator manufacturers don’t understand the principles of grounding and the NEC requirements for such equipment. I hope that one day the generator manufacturers would simply state that the generator must be grounded in “accordance with the NEC.”