For underground service conductors supplying power to limited loads of a single branch circuit, the minimum copper size is:

• A. 10 AWG
• B. 12 AWG
• C. 14 AWG
• D. 8 AWG

Answer: (B)

The key idea is sizing conductors to match the overcurrent protection for the circuit. Underground service conductors used to feed a single branch circuit with limited loads must have enough ampacity to carry the current without overheating, based on the protecting device. If the circuit uses a 20-amp protection device, the conductors must be able to carry at least 20 amps. Copper twelve-gauge conductors have an ampacity around 20 amps (depending on insulation and temperature rating), making them the smallest copper size that meets the 20-amp requirement. Twelve gauge is therefore the minimum that satisfies the protection level while avoiding unnecessary larger conductors. Thirteen gauge or smaller would not meet the 20-amp rating, and larger sizes are simply more material than needed for this limited-load single-branch-circuit scenario. The underground aspect mainly affects installation rules, not the fundamental ampacity sizing.