The circuit breaker is the last line of defense between a wiring fault and a building fire. A breaker sized too large allows conductors to overheat before tripping; a breaker sized too small causes nuisance tripping that disrupts operations and frustrates occupants. NEC Article 240 governs overcurrent protection sizing, establishing a precise relationship between breaker ratings, conductor ampacity, and load characteristics. The fundamental rule is simple: the breaker rating must not exceed the ampacity of the conductors it protects (NEC 240.4), with carefully defined exceptions for motor circuits, transformer secondaries, and tap conductors.

Standard breaker sizes follow NEC 240.6(A): 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. The 'next standard size up' rule (NEC 240.4(B)) permits using the next higher standard rating when the calculated load falls between standard sizes — but only for non-motor loads and only for conductors rated 800A or less. For example, a 38A non-continuous load on 8 AWG copper (50A at 75°C) requires a 40A breaker — not a 35A.

Continuous loads — defined as loads expected to operate for 3 hours or more — require special treatment. The overcurrent device must be rated at not less than 125% of the continuous load plus 100% of the non-continuous load (NEC 210.20(A)). A commercial lighting panel with 80A continuous lighting load requires a breaker rated at least 80 × 1.25 = 100A. This 125% factor ensures the breaker's internal bimetallic strip or electronic trip mechanism does not overheat during sustained operation. Some 100%-rated breakers (Square D HG, Eaton FD 100%) are specifically listed for continuous loading at their full rated current, eliminating the 125% requirement.

Residential panel sizing involves a formulaic approach. A typical US dwelling unit serves a 200A main panel containing: 20A (kitchen, bathroom, laundry, garage GFCI/AFCI circuits), 15A (general lighting), 30A (electric dryer), 40-50A (electric range), and 30-60A (air conditioning or heat pump). NEC 220.82 provides the optional calculation for dwelling units: 100% of the first 10 kVA plus 40% of the remainder, which often results in a significantly lower service size than the standard calculation. A 2,500 sq ft home with electric range, dryer, A/C, and water heater might calculate to only 125A using the optional method — comfortably served by a 150A or 200A service.

Commercial and industrial facilities require a different breaker sizing philosophy. Main breakers must be rated for the available fault current at the service entrance (SCCR — Short-Circuit Current Rating), which can range from 10,000A at a residential panel to 200,000A at an industrial switchboard. Breakers with insufficient interrupting capacity will fail catastrophically during a fault — violating NEC 110.9 and creating an extreme arc-flash hazard. Series-rated systems use a tested combination of a larger upstream breaker and a smaller downstream breaker to achieve adequate fault rating at reduced cost.

Tap conductor protection is governed by NEC 240.21, which provides several exceptions to the general rule that breakers must be at the supply end of conductors. The 10-foot tap rule (240.21(B)(1)) permits unprotected conductors up to 10 feet if the ampacity is at least 10% of the upstream breaker rating, the conductors are enclosed in a raceway, and they terminate in a single overcurrent device. The 25-foot tap rule (240.21(B)(2)) requires ampacity of at least one-third of the upstream device rating. These rules are critical for designing Panel-to-sub-panel feeder taps, transformer secondary connections, and generator paralleling switchgear.