Journeyman Electrician Practice Exam

What is the minimum size of 75ºC copper branch-circuit conductors needed for a continuous-duty, 208-volt, 3-phase, 30 horsepower motor with a nameplate ampere rating of 80 amperes?

1. 2 AWG

2. 3 AWG

3. 4 AWG

4. 1 AWG

The correct answer is: 2 AWG

To determine the minimum size of 75ºC copper branch-circuit conductors required for a continuous-duty motor, it’s essential to consider the National Electrical Code (NEC) guidelines, particularly how it relates to the continuous load calculation. For a motor with a rated amperage, the NEC typically requires that conductors supplying continuous loads be sized to handle 125% of the nameplate rating. For an 80-ampere motor, when applying the 125% rule, the calculation is as follows: 80 A (nameplate rating) x 1.25 = 100 A Now, to find the appropriate conductor size, one should refer to the NEC tables which specify the ampacity of conductors based on insulation type and conductor size. For 75ºC copper conductors, 2 AWG can carry approximately 95 amperes, which is close to the required 100 amperes but not sufficient given the calculated load. However, when considering continuous duty, the conductors must be able to handle the load without overheating, which reinforces the need for sizing that includes a safety margin. The minimum size of conductor that can handle the demands of this motor at 100 amperes would actually be 1 AWG, which