How do I properly anchor this wire in this type of outlet box? I am thinking that because this outlet box is not going to be behind drywall the NM cable must be anchored. That's what my favorite book says is this true or do they just mean J-boxes?
I have round plastic push in kind anochors for J-boxes (used one last night). But I don't know what to do with these square knockouts. Pictures below.
Thanks - John.