NEC Article 240 — Overcurrent Protection: Beyond the Small-Conductor Rule

Article 240 governs overcurrent protection — the rule of thumb that every conductor in the system must be protected at its ampacity by an overcurrent device (breaker or fuse). The most-cited sub-rules:

Standard breaker amperages (240.6): standard inverse-time breaker ratings are: 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. If your calculated load lands at 138 A, you size up to the next standard rating (150 A) — you can't pick a non-standard size for the breaker.

Location of overcurrent device (240.21): the overcurrent device must be at the POINT where the conductor receives its supply. There are exceptions (the "tap rules") that allow a small unprotected segment between a feeder and a branch circuit, but these are tightly limited:

• 10-foot tap (240.21(B)(1)) — common in panel sub-feeds in commercial work
• 25-foot tap (240.21(B)(2)) — feeder taps with conductor sizing rules
• Outside taps (240.21(B)(5)) — between a service conductor and a remote disconnect

For residential work, you'll rarely use tap rules — every branch circuit gets its own dedicated breaker at the panel.

Fuses vs breakers (240.51 / 240.52): fuses are still allowed and sometimes preferred (better at clearing high fault currents than breakers in some scenarios), but residential work in NJ is essentially 100% breakers now. Fuses show up in older fuse-box panels (which should be replaced), and in service-entrance fused disconnects on commercial.

The small-conductor rule (240.4(D)) is its own deep topic — see our guide on NEC 240.4(D). It's the rule that caps 14 AWG at 15 A, 12 AWG at 20 A, 10 AWG at 30 A regardless of higher ampacity tables.

The free Malfettone Panel Schedule Builder at /tools/panel-schedule checks every breaker amperage against the 240.4(D) small-conductor rule live as you build. For load calc, see /tools/load-calculator.