Master Electrician Practice Exam

What is the ampacity of size 1/0 AWG THWN/THHN dual-rated copper conductors in RMC installed underground?

1. 100 amperes

2. 125 amperes

3. 150 amperes

4. 175 amperes

The correct answer is: 150 amperes

The ampacity for 1/0 AWG THWN/THHN dual-rated copper conductors in rigid metal conduit (RMC) installed underground can be determined by referring to the National Electrical Code (NEC) tables that specify conductor ratings under different conditions, including ambient temperature, insulation type, and installation method. For 1/0 AWG THHN or THWN conductors, when installed in RMC underground, the ampacity is generally classified based on the derating factors included in the NEC. Specifically, THWN/THHN conductors are rated to handle higher temperatures and are suitable for wet locations—which is essential for underground installations. When considering the typical conditions for underground installations (such as grouped conductors), and applying the appropriate adjustments, the ampacity for 1/0 AWG is usually determined to be approximately 150 amperes. The adjustment can vary slightly based on specifics such as the number of conductors run together, but under typical installations without excessive derating, 1/0 AWG copper conductors in RMC typically sustain an ampacity of 150 amperes. This demonstrates the suitability of 1/0 AWG conductors for significant electrical loads while ensuring safety and compliance with electrical codes