Are plastic boxes ok to use in commercial buildings? Some do have a knockout for pipe or mc cable to run into.

Non-metallic boxes cannot be used with metallic wiring methods unless meeting one of the two exceptions. 314.3.

Why then is there a 1/2" knockout? for PVC? What would be wrong with plastic boxes, there used in homes?

The non-metalic boxes with standard K.O’s are for ENT (smurf tube) or can be used with RNMC. There is nothing wrong with non-metallic boxes when used with non-metallic wiring methods. You can’t use metal raceways or cable armors with non-metallic boxes since the raceway or armor would not be properly bonded.

Curt

caselec,
Yes true, unless you run metal cable from a bonded metel panel to the plastic box, then its bonded at one end.

electricman2 has the correct response.
I wonder if a twin M/C connector would satisfy the exception to 314.3?