Electrician's Licensing Practice Exam

What does NEC Chapter 9, Table 5 state regarding wire type and size?

1. Locate wire type without derating

2. Repeat for each conductor and derate by 40%

3. Install the largest wire possible

4. Choose wire based on cost

The correct answer is: Repeat for each conductor and derate by 40%

The correct assertion focuses on the requirements outlined in NEC Chapter 9, Table 5, which pertains to the ampacity of conductors and the necessity of derating when multiple conductors are installed together. Specifically, when multiple conductors are run in parallel or in close proximity, it is crucial to derate their individual ampacities to account for the cumulative heating effect they create. The table assists in determining the appropriate conductor size and de-rating factors to ensure that the wiring can safely handle the load without exceeding temperature limits. This process entails applying the derating factors based on the number of conductors present. When there are more than three conductors, the ampacity must often be significantly reduced — for instance, the National Electrical Code suggests derating by 40% when there are many conductors involved. This practice is essential for maintaining electrical safety and preventing overheating, which could lead to insulation breakdown and increased risk of fire. Thus, using guidelines from NEC Chapter 9, Table 5 ensures that electricians select wire types and sizes that are compliant with safety standards, while also protecting against potential overcurrent situations that can arise from insufficiently sized conductors.