Can I use exposed romex outdoors? Specifically under a deck attached to the joists? If so, how far above the ground does it have to be? What is the rule for when it has to be enclosed in NM conduit?

Last year, knowing I'd be finishing my basement in the future, I had an electrician add a sub-panel to my basement. My wife and I gave the electrician a guess about what kind of things we'd need to power in the basement, then he added the sub-panel. Now I'm in the process of planning the circuits for the basement and I'm wondering if the sub-panel connection will be adequate. It's connected to the main panel by a double-pole 50 amp breaker. I've added up the power requirements of the things we want in the basement, which is more than we originally expected when the sub-panel was installed (big surprise), and I'm coming up with nearly 8,000 watts. I'll only need 120 volt circuits for lighting, home theater setup, several small appliances, etc. (no 240 volt appliances). With the current sub-panel, am I limited to 6,000 watts (50 amp breaker * 120 volts)? If so, I assume the fix is to have the electrician install a higher amp breaker (likely requiring new wiring to the sub-panel). Thanks! Dan