The overcurrent protective device — the circuit breaker or fuse — is the component several NEC sizing rules key off rather than a calculation of its own. Selecting it belongs to NEC 240.4, including the small-conductor caps of 240.4(D): no more than 15 A protection on 14 AWG copper, 20 A on 12 AWG, 30 A on 10 AWG, regardless of what an ampacity table might suggest. For continuous loads — those expected to run at maximum current for three hours or more — NEC 210.19(A) and 215.2(A) require conductors and standard breakers to be sized at 125% of the continuous load plus 100% of the non-continuous, so a 40 A continuous load needs a conductor good for 50 A.
The device rating then propagates into the grounding system. The equipment grounding conductor is sized from the OCPD rating via Table 250.122, with in-between ratings rounding up to the next table row (a 175 A device uses the 200 A row, 6 AWG copper) — the linkage exists because the EGC must carry fault current long enough to trip that very device. Ampacity, overcurrent protection, and voltage drop remain three separate checks on the same circuit: a conductor sized for ampacity still needs its breaker chosen per 240.4, and long runs are often governed by voltage drop before either.