While I want to say that two locknuts are required on at each end of the nipple, I can't find a code rule that requires this. There are some rules in Chapter 5 that require double locknuts, but not in the general use chapters. Our power company requires grounding bushings at both ends, but the code rule in 250.92 does not really say that. This is one that I would run by the AHJ before making the installation. Don
