Ampacity, or AMPERE CAPACITY, is the rated maximum number of amperes an electrical device can safely convey, switch, limit or otherwise utilize by design without the device being altered, overheated or destroyed.
Ampacity is synonymous with AMPERES, a reasonably complex mathematical measurement of electrical current named after Andre-Marie Ampere, and who's boldly-labeled numerical value limit is most commonly found two places: fuses and circuit breakers. I.e., a socket fuse with an ampere limit value of say, 25 amperes, will have the number "25" in large, bold font printed directly on the fuse face and stamped into the metal conductor tip. In the case of a circuit breaker the number is molded into the end of the breaker throw and is printed on a label adhered to the body of the breaker. Such ampacity may be displayed by the simple number 25, 25 AMP or 25A.
It is important to note that the ampacity rating of a device or conductor is unchanged by the electrical charge or VOLTAGE that is being applied as long as the voltage limit is observed. For instance a 25 ampere circuit breaker rated at 600 volts will continue to limit the current flow to 25 amperes irregardless of whether the device is used on a 120, 240 or 480 volt system. This is an important electrical theory: Think of the circuit breaker as an, "electrical narrow bridge," whereas electrical energy is represented by cars and voltage represents the speed of the cars traversing the bridge.
The ampacity bridge was purposely built narrower than the road (wiring) so as to prevent the road from becoming too congested, thus creating an unsafe condition. Traffic over the bridge is immediately halted when vehicles attempt to cross at a rate faster than acceptable or physically able. When the ampere limiter device limit is momentarily exceeded the limit device melts in the case of a fuse or "trips" in the case of circuit breaker. In most cases the device requires manual replacement or resetting. In power distribution systems circuit breakers are sometimes designed to self-reset a number of times in quick succession before interrupting current flow in the manner where personnel are required to intervene. Self resetting breakers eliminate nuisance tripping when a momentary overload occurs of very short duration whereas damage to the system is not likely and certainly not as detrimental as loosing electrical service all together.