Section 240.4/Protection of Conductors
NEC 240.4 requires conductors to be protected against overcurrent in accordance with their ampacity, with absolute limits for small conductors.
Conductors must be protected against overcurrent in accordance with their ampacities. For 14 AWG copper, overcurrent protection must not exceed 15 amps; for 12 AWG, 20 amps; for 10 AWG, 30 amps. These limits apply even when the ampacity from Table 310.16 might be higher due to conductor insulation temperature rating.
Why this section exists
Without proper overcurrent protection, conductors can overheat and ignite surrounding materials. The overcurrent device must trip before the conductor reaches unsafe temperatures. The small conductor limits (14, 12, 10 AWG) provide absolute caps that prevent designers from using higher-temperature-rated insulation to justify larger breakers on small wires.
What plan reviewers look for
Plan reviewers verify that every conductor on the panel schedule is protected by an overcurrent device equal to or less than its ampacity. They specifically check the small conductor limits and verify that the next-standard-size-up rule in 240.4(B) is applied correctly when used.
Common violations
Related NEC requirements
Table 310.16 provides conductor ampacity values. Section 240.6 lists standard overcurrent device ratings. Section 210.19 covers branch circuit conductor sizing. Section 240.21 covers tap conductor overcurrent protection rules.