Overcurrent Protection: Device Sizing, Conductors, and Common NEC Mistakes

Regulation: NEC Articles 210, 240

Keywords: overcurrent protection NEC, breaker sizing NEC 240, conductor ampacity protection, next size up rule NEC, fuse vs breaker NEC, overcurrent device ampere rating, NEC 240.4 protection rules, standard breaker sizes NEC

Core Concepts

• Basic Rule: Overcurrent protection must not exceed the ampacity of the conductors it protects (NEC 240.4).
• Next Size Up Rule: If calculated ampacity does not match a standard size, you may go to the next standard size up — only if conductor ampacity is 800A or less and next size does not exceed 800A.
• Standard Breaker Sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400A per NEC 240.6.
• Tap Rules: Conductors tapped from a feeder may have reduced ampacity if tap rules are met (10-ft tap, 25-ft tap, transformer secondary tap). Each has specific conditions.

Exam Traps

• The next-size-up rule does NOT apply to flexible cords, fixture wire, or certain special loads — check NEC 240.4(D).
• A 20A receptacle circuit requires 12 AWG minimum — 14 AWG on a 20A breaker is a violation.
• Tap conductors are not protected at their ampacity at the point of tap — that is the entire point of tap rules.
• A 15A breaker on 14 AWG is correct. A 20A breaker on 14 AWG is a violation — the next-size-up rule does not allow skipping two standard sizes.
• Overcurrent protection protects conductors, not loads — load protection (motor overload) is a separate requirement.

Memory Aid

OCP ≤ conductor ampacity. Exception: next size up if ≤800A and no exact match. Never upsize for flexible cord or small-gauge wire.