Re: bonding additional grounding buss
In my opinion the add on terminal bar that is fastened to the enclosure with machine screws is suitable only for use with equipment grounding conductors and not for use with grounded conductors. Most panel manufacturers have both equipment grounding and grounded conductor treminal bars availabe for this type of applicaiton. If there are EGCs on the existing bar, I'd move them to the new grounding bar and land only the grounded conductors on the existing terminal bar. If there is not enough space for all of the grounded conductors on the existing bar I would add an additional neutral bar and land both the grounded and EGCs on this new bar. An additional neutral bar will require a jumper from the original bar. In my opinion the enclosure itself should not be used to carry neutral current.
"It is the first responsibility of every citizen to question authority." B Franklin