What are the ampacity ratings for THHN wire?

THHN is rated 90 degrees C in dry and damp locations, and nearly all of it sold today is dual rated THHN/THWN-2, which adds a 90 degree C wet rating. That 90 degree C column in Table 310.16 is almost never the number you put on a load calculation. Section 110.14(C) limits the circuit ampacity to the lowest temperature rating of any connected termination, and equipment terminations in the field are marked 60 degrees C or 75 degrees C. The 90 degree C column exists so you have headroom for ambient correction and conductor bundling adjustment, not so you can load the conductor to it.

Copper THHN values from Table 310.16

Size, then 60 degree C, 75 degree C, 90 degree C. 14 AWG: 15, 20, 25. 12 AWG: 20, 25, 30. 10 AWG: 30, 35, 40. 8 AWG: 40, 50, 55. 6 AWG: 55, 65, 75. 4 AWG: 70, 85, 95. 3 AWG: 85, 100, 115. 2 AWG: 95, 115, 130. 1 AWG: 110, 130, 145. 1/0: 125, 150, 170. 2/0: 145, 175, 195. 3/0: 165, 200, 225. 4/0: 195, 230, 260. 250 kcmil: 215, 255, 290. 350 kcmil: 260, 310, 350. 500 kcmil: 320, 380, 430.

The 110.14(C) termination rule

For circuits rated 100 amperes or less, or conductors 14 AWG through 1 AWG, use the 60 degree C column unless the equipment is listed and identified for 75 degrees C. Most modern breakers and panelboards carry a 75 degree C or a 60/75 dual marking, and when both ends are marked 75 degrees C you may use that column. For circuits over 100 amperes or conductors larger than 1 AWG, use the 75 degree C column unless the equipment is listed for higher. Motor circuits get their own treatment in 110.14(C)(1)(a)(4), where the conductor may be sized from the 75 degree C column when supplying a motor marked with design letters B, C, or D.

Correction and adjustment come off the 90 degree C number

Section 310.15(B)(1) applies the ambient temperature correction factors from Table 310.15(B)(1) when the ambient is other than 30 degrees C. Section 310.15(C)(1) applies the adjustment factors when more than three current-carrying conductors share a raceway or cable for more than 24 inches. Apply both to the 90 degree C ampacity, then compare the result to the termination limit from 110.14(C) and take the lower of the two.

Do not forget 240.4(D)

Regardless of what the ampacity table says, 240.4(D) caps overcurrent protection at 15 amperes for 14 AWG copper, 20 amperes for 12 AWG copper, and 30 amperes for 10 AWG copper, with the exceptions in 240.4(E) through (G) for taps, motors, and similar. A 12 AWG THHN conductor is good for 30 amperes on paper and still gets a 20 ampere breaker on a general branch circuit.

Field takeaway

Size from the 90 degree C column for correction and adjustment math, land on the 75 degree C column for the final answer when the equipment allows it, and apply 240.4(D) on the small stuff.