Wow, that isn't even legal in the Hillbilly Electrical Code!
Yes I am well aware this is SOP in the finishing business, and doing it to the letter of the law will cost you business to those who do not. But geez, you're swerving out of your way to do it much more dangerously than you have to, so caring about grounds seems a little ... tone-deaf.
The refinisher who works in my complex does the same thing - tearing open a Federal Pacific panelboard (wow, talk about playing with fire!!!), and knocks the whole complex's power out.
Wow, just wow.
First, I know you're used to plugging into 30A circuits. But you're doing that for the 240V; that doesn't mean your sander *actually* needs 30A. (it probably doesn't). Go look at the sander's nameplate or instructions. First the instructions override - whatever they say goes. Life will be easier if you don't need 30A.
- If the nameplate watt rating is < 2880 watts, you can use 15A circuits.
- If the nameplate watt rating is < 3840 watts, you can use 20A circuits.
Make sure nothing else on the circuit is on. If it constantly trips that breaker, you can go up 1 breaker size because of a special rule relating to large motors.
Once you're straight on that, you have more options because there are a lot more 15/20A circuits than 30A.
Now, you should not be clipping onto "hot" bus bars, EVER. They are completely unbreakered (and your inline "breakers" don't cut it - what protects the gator clips and wiring?) If you clip onto unused bus stabs, you can seriously damage them, which means a breaker put there later will start an arcing fire.
Surely there are breakers in that panel. Find a 2-pole (factory handle-tied) breaker the right size for your load, and add your wires to it. (double-tapping is legal on some breakers, but even if it's not, this is 95% safer than what you're doing now).
If you can't find a 2-pole handle-tied breaker, look for 2 singles on opposite poles (240V between them). Pick ones the homeowner isn't likely to be using. Kitchen receps are a good go-to.
Since you're attaching to branch circuit breakers, you only need to turn off that/those breakers, not the whole panel.
If you did that, and ran your cord out a proper strain relief, that would actually border on *correct*!
If you permanently installed a correct socket in a junction box with the correct wire, that would be Code legal. I realize such elegance is probably not realistic in the hustle-and-bustle finishing business, but just pointing out, you're that close
to totally legal.
Lastly, keep in mind that since you are not a licensed electrician, it is illegal for you to open up panels or do any work, except in a house you own *and* occupy. But there is "legal" and then there is "safe"
, and I would much rather you be as safe as possible, even if it is illegal, rather than say "effit since it's illegal let's do nothing to improve safety".
The official legal way for you to do this is have a generator in the truck, end of subject.
Well, now that you've gotten a free lecture
Put the ground where the grounds are in the panel. If you see a bar with a bunch of grounds, that's fine, even if there's also a bunch of neutrals. If you see no grounds at all, attach to the metal frame of the panel e.g. through a panel cover screw temporarily repurposed as a ground screw.
If you see a bar that's *only* neutrals, do not put a ground there - your buddy is wrong. Neutral is not ground.