What is circuit breakerin construction?

AIC (ampere interrupting capacity) is the maximum fault current a breaker can safely interrupt, commonly 10, 22, or 65 kAIC in commercial work. The engineer's fault current study sets the requirement, and the breaker rating has to meet or exceed the available fault current at its location. Higher AIC means a meaningfully higher price, so pricing a whole job at standard interrupting ratings when the one-line calls for higher ones leaves the estimate badly low.

It depends on how the gear is bought. Panelboards are often quoted as a factory-assembled package with the breaker fill from the schedule included, but interiors, enclosures, and breakers can also be bought separately, and circuits added during construction are field-installed. The takeoff should state which convention it used, because pricing breakers inside the panel assembly and again as loose material is one of the most common ways an electrical estimate double counts.

Both interrupt fault current, but a breaker is a mechanical device that trips and resets, while a fuse melts and must be replaced. Commercial branch circuits are almost always breaker-protected because resetting is cheaper than restocking fuses, but fused disconnects still appear at equipment connections, often because the equipment nameplate requires fuse protection. The takeoff prices those as fused switches with their fuses, a separate line from breakers.