Hi - Can someone explain how to do the following: I have an outlet box with 4 wires - two black hotwires and two white neutral wires and a ground wire. My problem - how can I figure out which pair of black/white are the load and which pair are the line? Thanks....

I plan to add a new dedicated circuit to supply a new outlet in the living room. I am considering running type MC cable from the main breaker panel, along the unfinished basement ceiling, and then above into the outlet located in a wall of the living room. The cable will have to run at right angles to the direction of the joists in the basement ceiling. I don't want to bore holes in the joists for running the cable. I would rather run the cable across the bottom of the joists. If I was using Romex/NM cable, I understand I would have to attach it along a running board for protection of the cable. Question: If I use MC cable across the bottom of the joists, is it still required/recommended to use a running board to protect the cable? I checked my copy of Wiring Simplified (2002 NEC edition) but it didn't seem to say whether this applied to MC cable. thanks, Brent