Electrical - Wire THHN/THWN
THHN wire is typically dual rated to also meet the THWN spec. This means it can be used in wet location (eg a conduit in the ground outside) as well as in dry locations. SouthWire (Largest Electrical wire manufacturer in the USA) decided to put both ratings on all their THHN building wire. The solid conductor variant works well and is the least expensive.
Available from Home Depot or Lowes for about $65 for a 500 foot roll. You will need to buy different colors, ie Black, White, Green, and probably also Red.
White: http://www.homedepot.com/p/Southwire-500-ft-12-Solid-THHN-White-Cable-11588157/202316468
Yellow: http://www.homedepot.com/p/Southwire-500-ft-12-1-Solid-THHN-Wire-Yellow-11592358/203408336
Gray: http://www.homedepot.com/p/Southwire-500-ft-12-Solid-THHN-Wire-Gray-22870001/202316253