Never understood why just the drilling/tapping of the box wasn't good enough. I know that sometimes paint might be in pre-drilled holes, but if you drill and tap, there is a conductive path. No different than running the bonding screw in on a panel.
Once you drill and tap, you have bare metal to metal contact. I've done it that way for years and never been called out on it. But having a bare metal space behind the ground bar itself assures good continuity even more.
I've also added ground bars without scrapping any paint on my newly drilled holes and never been questioned. I often take a meter and check continuity between the new bar and the cabinet/can.
With most panels the metal is not thick enough to provide the required two threads in the metal. If you look at the factory holes for grounding and bonding connections, you will find that they have some of the metal "extruded" out the back side in order to provide enough metal for the two threads.
It would make sense that it's good enough but the NEC is specific as to how it needs to be done unless it's a connection using listed ground bar in the hole in a panel tested with that ground bar. A painted Square D panel only requires that you use the mounting screws that came with the bar and install it in the proper location in the panel. Otherwise the paint needs to be removed.
Suppose you put nuts on the backside?
Pretty much the same as bolting a lug to the panel.
That meets the requirements in 250.8, but in a typical panelboard a field drilled and tapped hole does not.
This meets the requirement by not having to remove all of the paint behind the ground bar. I've done the same thing with ground bars in screw cover boxes.
A quick and easy way to provide enough metal for two threads is to drill a very small hole, then enlarge it with a hammer and awl; this pushes the metal toward the rear of the enclosure. Then tap it for the screw or use a self-tapper.
Only if the material is thick enough to engage two threads of the attachment screws.
I was referring to the part where he ran a bonding jumper and was not relying on the screws into the metal.
If you remove the paint behind the bar and nut and bolt the bar you do not need the jumper. You could leave the paint and use the jumper instead.
