According to NEC Article 240.4(D)(5), 12 AWG copper shall be protected against overcurrent by a device with a maximum ampacity of

• A. 15 amps
• B. 20 amps
• C. 25 amps
• D. 30 amps

Answer: (B)

The main idea is matching overcurrent protection to the conductor’s ability to carry current. For a 12 AWG copper conductor, the NEC limits the protective device to not exceed about 20 amperes. That means the device you use to protect this conductor should be rated 20 A or less, with 20 A being the largest standard size that safely covers typical installations.

Why this matters: if you used a higher-rated device, like 25 A or 30 A, the conductor could be overloaded during normal operation or a fault without tripping, causing overheating and potential damage or fire. A 15 A device would still protect the wire, but it isn’t the maximum allowed—the standard, practical maximum for 12 AWG copper under these rules is 20 A.