Wiring for basement - do I need conduit?
Nope, single wires can't be used unless part of a Chapter 3 wiring method in the NEC that calls for their use, such as conduit.  This is specified in 300.3(A):


  (A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3.