NEC 240.4(D) — The Small Conductor Rule (14, 12, 10 AWG breaker limits)

NEC 240.4(D) is the rule that decides what breaker is allowed on a small conductor. It applies to 14, 12, and 10 AWG conductors and overrides whatever ampacity Table 310.16 might otherwise allow. The values are:

• 14 AWG copper: 15 A maximum overcurrent protection
• 12 AWG copper: 20 A maximum
• 10 AWG copper: 30 A maximum
• 12 AWG aluminum: 15 A maximum
• 10 AWG aluminum: 25 A maximum

For 8 AWG and larger, the regular ampacity tables (310.16 column for the appropriate temperature rating) govern.

Why the rule exists: small conductors don't have enough thermal mass to safely handle higher currents for the time it takes a thermal-magnetic breaker to trip on a moderate overload. The rule is a safety floor — it prevents the most common branch-circuit fire path (overloaded undersized conductors that don't trigger the breaker fast enough).

Practical implications:

• The classic "swap a 15A breaker for a 20A to stop nuisance trips" move on a 14 AWG circuit is a code violation that creates real fire risk.
• A 30A breaker on 10 AWG is at the limit. Many electricians prefer to upsize to 8 AWG and use a 30A or 40A breaker for sustained loads near the limit (water heaters, dryers).
• EV chargers on 10 AWG circuits are limited to a 30A breaker, which means a 24A continuous charger (24 × 1.25 = 30) — adequate for some Level 2 EVSEs but not all.

The free Malfettone Panel Schedule Builder at /tools/panel-schedule checks every row against 240.4(D) live as you build the schedule, flagging any combination that violates the rule.