THHN wire is 600 volt 90 deg. centigrade building wire for use in DRY locations only.
It's 90 deg rating allows for a higher ampacity in sizes #8 or larger. Sizes #14,#12, and #10 , use the standard ampacity tables (NEC 310-16)(SEE NOTES) . THHN has a thin layer of insulation (2 MIL) plus a nylon jacket. They also make a THHN , THWN, MTW rated wire which is rated for 90 deg ,dry and wet locations , gas and oil resistent and is allowed for use in machine tool wiring. The size of the wire and the termination method used usually is the source of most problems. Voltage drop across the conductor is a problem with 12 VDC. wiring. A #12 copper conductor may be able to carry 20 amps but at 12VDC the voltage drop may be too large.
Limit the load (amperage) and length of the conductor and you can make it (# 12) work. I always use the 50 % rule I.E. 10 amps maximum load on a 20 amp conductor and limit wire lengths to about 25 ft. max.