Overcurrent Protection and Short-Circuit Current Ratings
Core concepts
Overcurrent protective devices must have an ampere rating that protects the conductor, an interrupting rating at least equal to the available fault current, and a short-circuit current rating adequate for the equipment it protects.
Standard device ratings (240.6) run 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200 A. The "next size up" rule in 240.4(B) allows the next higher standard device when the conductor ampacity does not match a standard rating, provided the device does not exceed 800 A, the ampacity does not correspond to a standard rating, and the circuit is not a multioutlet receptacle circuit supplying cord-and-plug loads. Small conductors have hard caps in 240.4(D): 14 AWG = 15 A, 12 AWG = 20 A, 10 AWG = 30 A.
Feeder and branch-circuit taps are permitted under 240.21. The common rules are the 10 ft. tap (ampacity not less than one-third the device rating, in raceway, no taps off the tap) and the 25 ft. tap (ampacity not less than one-third, with additional physical protection requirements).
Traps
- Applying the next-size-up rule to a 14 AWG lighting circuit.
- Confusing interrupting rating with SCCR of the assembly.
- Treating a series-rated combination as a fully rated system.
Worked example
A 75°C-rated 4 AWG THWN copper conductor has an ampacity of 85 A. A 90 A standard breaker is permitted under 240.4(B) if all conditions are met - a 90 A device is the next standard size above 85 A.