Should metal or plastic electrical boxes be used when i put wiring in a room addition ?
I would check the rest of the boxes in the house and the wiring.Sometimes BX metal sheathed wire is used and metal boxes and that serves as the ground.I think plastic boxes are easier,just make sure that the wiring you use includes a ground wire and your panel is grounded properly to a water pipe or a stake into the ground with a heavy gauge copper wire.you may want to check building codes.
