I have a bonus room we are trying to get permitted and the power is from a sub panel next to the room that fed a 220 volt circuit for a lathe in the garage and single 120v circuit.
The light circuit was from the original wiring for the upstairs. The panel was fed with #8 three wire. The inspector said I could get away with the three wire if a used a main breaker which I have and added a ground bar. They also require an AFCI breaker . The breaker trips anytime anything is plugged into an outlet. I am not an electrican but my thought is I need a neutral as well as the ground in the sub panel for the AFCI to work properly.

Thanks in advance

The circuit feeding the existing sub panel, is it a straight 240 Vac system or a 120/240 feeder.

If it is a 120/240 feeder, then the existing sub panel will have at least 1 Black, 1 Red, 1 White and 1 ground.

If that is true, then you will need a two pole breaker as the main breaker.

Your sub-panel will also need to have a ground bus-bar and a isolated neutral bus-bar.

If the sub panel only uses black and red conductors, then it is a 240 Vac panel.

This could explain your breaker failures. If you are trying to connect any 120 device to a 240 Vac system, mark it fried and go get new equipment.

The AFCI breaker is made to watch for the electronic signature of arcing. If you are tripping the breaker because of the load, then the breaker is doing exactly what it is supposed to do.

You may have not used line and load neutral correctly. If this is the main breaker in the sub panel then the feed goes to line neutral (pigtail) and load neutral feeds the panel.