Question

A raceway contains nine #12 THHN conductors from a 120/240v, a single- phase panelboard. The conductors supply three 120/240v, three-wire circuits made up of L1, L2 and a grounded conductor (neural). What is the allowable ampacity for each of the #12 conductors?

Answer 1

The allowable ampacity for each #12 conductor in a raceway supplying three 120/240V three-wire circuits is 21 amperes after applying the NEC derating factor due to having more than three conductors bundled together.

Step-by-step explanation:

The question asks about the allowable ampacity for each of the #12 conductors in a raceway that is part of a 120/240V single-phase system supplying three 120/240V three-wire circuits. According to the National Electrical Code (NEC), the standard ampacity for #12 THHN conductors is 30 amperes. However, when multiple current-carrying conductors are bundled together in a raceway, derating factors must be considered. Since there are nine #12 conductors in the raceway, we must apply derating. Typically, for more than three current-carrying conductors in a raceway, the allowable ampacity must be reduced to 70% of the standard ampacity. This means each #12 THHN conductor in this case can carry 0.70 × 30A which equals 21 amperes. Thus, the allowable ampacity for each conductor would be 21 amperes.