NEC Table 240.6(A) lists them: 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, 1600, 2000, 2500, 3000, 4000, 5000 and 6000 amperes. Fuses have five additional standard ratings — 1, 3, 6, 10 and 601 A — that do not apply to breakers. Watch for calculators that hand back 16 A, 32 A or 63 A: those are IEC miniature circuit breaker ratings and appear nowhere in the NEC. Every size this tool returns is selected from the published list, so a non-NEC rating cannot come out of it.

Because the two rules point in opposite directions. NEC 210.20(A) says a general branch-circuit device "shall not be less than" the non-continuous load plus 125% of the continuous load — that is a floor, so you round up to the next standard size. NEC 440.22(A) says an air-conditioning device "shall not exceed" 175% of the compressor's rated-load current — that is a ceiling, so you round down. A 22 A compressor gives a 38.5 A ceiling, and the answer is a 35 A device, not the 40 A a general-purpose calculator would return.

NEC 240.4(B) permits the next standard rating above the conductor's ampacity, but all three of its conditions must hold: the conductors are not part of a branch circuit supplying more than one receptacle for cord-and-plug-connected portable loads; the ampacity does not already correspond to a standard rating; and the next size up does not exceed 800 A. That second condition is arithmetic and this tool checks it for you — a conductor whose ampacity lands exactly on a standard size never gets bumped. The other two are facts about your installation that you have to assert.

NEC 240.4(D) is a hard ceiling that survives every other permission in the section: 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, and 15 A and 25 A for 12 and 10 AWG aluminum. It applies after correction factors, not before, so a favourable ampacity calculation never buys you past it. If the load needs more, the answer is a larger conductor — never a larger device.

Not yet, deliberately. Motor branch-circuit protection under NEC 430.52 needs a percentage that varies by protective-device type, and we hold every rule to a standard of two independent published sources agreeing before we code it. Table 430.52 did not clear that bar — no ungated source reproduces the full matrix, and two published reproductions contradict each other on the wound-rotor row. The full-load-current tables (430.248 and 430.250) did verify cleanly, so this is one table short rather than the whole article, and we would rather leave the mode out than ship a percentage we cannot stand behind.