Overcurrent Protection and Conductors
Conductor ampacity and protection
Article 240 sets overcurrent protection rules, while Article 310 gives conductor ampacities. The ampacity must be adjusted for ambient temperature and for the number of current-carrying conductors in a raceway or cable. Termination temperature ratings under 110.14(C) often control the final usable ampacity. Small conductors have absolute limits: 14 AWG is generally limited to 15 A, 12 AWG to 20 A, and 10 AWG to 30 A under 240.4(D). The next-size-up rule in 240.4(B) permits a larger standard breaker only when the conductor ampacity does not correspond to a standard rating and the conditions of that rule are met.
Traps
Candidates often use the 90°C insulation rating as the circuit ampacity without checking terminations. Another trap is applying the next-size-up rule to a small conductor that is already limited by 240.4(D). Tap conductors under 240.21 have strict length and protection rules and are not general-purpose feeders. Continuous loads generally require 125% sizing for the overcurrent device and conductors.
Worked example
A 12 AWG THHN conductor has a 90°C ampacity of 30 A. In a 40°C ambient, apply a 0.91 correction factor. With four current-carrying conductors in one raceway, apply a 0.80 adjustment factor: 30 x 0.91 x 0.80 = 21.8 A. The 75°C terminal rating may be 25 A, but 12 AWG is limited to a 20 A overcurrent device. Use a 20 A breaker.