Raco junction boxes are rated/listed for use with 277V circuits even with concentrics.
Ditto the competition, IIRC.
The only time I face issues is with disconnects -- especially NEMA3R for RTU.
I've never been given the opportunity to install bonding locknuts. Out my way, the only ones I've seen are in Code Handbooks.
I can say from witnessing aged rooftop installations that bonding takes a beating over time. Hence, I always run a separate bonding conductor for all circuits that leave the building.
If you've never seen it, what happens to roof top EMT runs is trips and falls. Over the years various HVAC boys manage to bump what ever EMT can possibly be bumped.
As for current installations, root top EMT runs are either contract prohibited or banned by the AHJ. Only short, to the point, EMT is permitted.
BTW, old disconnects/fuse holders are scary. Generally they are too small and too often they do not keep rain or leaves out. And then the labels are all ruined. This is not the type of stuff to handle on a wet roof.
The situation I am writing about deals with a new panel installed before that main service breaker. It feeds a PV system. So the feeder to this new PV panel has no overcurrent protection. Now Would a bonding locknut be suitable for use on a panel with concentric knockouts?
Harold I have modified my position. I am thinking it will be acceptable. The function of the bonding locknut is to bond the raceway and if it is made up tight against the connector lip and the concentric ring with a 250.66 sized jumper, the raceway is bonded, even if the ring breaks out completely.
For some reason I had in it my mind that the concentric locknut would loosen if the eccentric broke. I guess I should not type after midnight
If the concentric KO pops out, the grounding would be lost. The bonded locknut will still be tight against the shoulder of the connector, but the metal to metal grounding will be missing because the KO busted loose. Right?