Journeyman Electrician Practice Exam

What is the ampacity of size 8/3 AWG with ground copper NM cable installed in an attic where the ambient temperature reaches 130ºF?

1. 30.0 amperes

2. 35.0 amperes

3. 40.0 amperes

4. 45.0 amperes

The correct answer is: 40.0 amperes

To determine the ampacity of 8/3 AWG copper NM (non-metallic) cable installed in an attic with a high ambient temperature of 130ºF, it is essential to consider both the basic ampacity values set forth in the National Electrical Code (NEC) and adjustments required for temperature. Under normal conditions, the ampacity of 8 AWG copper wire is typically around 40 amperes. However, when the cable is exposed to higher temperatures, such as 130ºF, derating is necessary. The NEC mandates that for every degree above 86ºF, there should be a reduction in the ampacity rating to account for potential overheating. Specifically, at 130ºF, the temperature adjustment would reduce the ampacity from its base rating. Since common practice suggests using a 30% derating factor for NM cables in high temperature environments, we apply this derating to the 40 amperes base rating of the 8 AWG copper. The calculation would involve adjusting the ampacity down to a maximum allowable capacity, which amounts to 40 amperes multiplied by the remaining capacity after temperature adjustments. As a result, grounding the calculations, the adjusted ampacity leads to a final value