Journeyman Electrician Practice Exam

What is the ampacity of a size 1/0 AWG THW copper current-carrying conductor in a raceway with three other conductors at 86°F?

1. 100 amperes

2. 120 amperes

3. 140 amperes

4. 160 amperes

The correct answer is: 120 amperes

To determine the ampacity of a size 1/0 AWG THW (Thermoplastic Heat and Water-resistant) copper conductor, it’s important to refer to the National Electrical Code (NEC) ampacity tables, specifically considering the conditions under which the conductor operates—such as the number of conductors in a raceway and their temperature rating. In this scenario, with four conductors in a raceway at a temperature of 86°F, the ampacity of a single 1/0 AWG THW copper conductor is typically listed in Table 310.16 of the NEC. For conductors rated at an ambient temperature of 86°F, the standard ampacity is often at least 140 amperes. However, since there are three other conductors in the raceway, it's crucial to apply the adjustment factors provided in the NEC for the number of conductors, which typically reduces the ampacity to prevent overheating. Specifically, when more than three conductors are present in a raceway, the ampacity is usually reduced by a percentage. Based on the NEC adjustment for having four conductors, you would take the initial rating and adjust it accordingly. The adjusted ampacity should ultimately lead to a rating of 120 amper