Colorado Journeyman Wireman Exam Preparation

Below are a few questions from your upcoming PSI state exam:

1. What would be the calculated the voltage drop on the #6 AWG copper conductors that serve a 15 HP, 480 V three-phase motor? (Assume the voltage source and the load are 150 feet apart installed in rigid metal conduit.)
   • Correct Answer: NEC – Table 430.250, Chapter 9 Table 8 (Conductor Properties), and Voltage Drop formula. See calculation steps below.
     1. Per Table 430.450 the FLC for the 15 HP 480 V motor is 21 amperes.
     2. Per Chapter 9 Table 8 the #6 AWG Circular mils is 26240.
     3. Use the Voltage Drop Formula, Vd = (1.732 x K x Length x Ampacity) / kcmil, Vd = (1.732 x 12.9 x 150 feet x 21 amps) / 26240 = 2.68 Volts.
2. What is the minimum required feeder tap conductor size for a circuit protected by an 800 ampere OCP device, where the length of the tap conductors is 20 feet, using copper THWN wire?
   • Correct Answer: NEC – 240.21(B)(2)(1) Therefore, calculate; 800 amperes / 3 = 266 amperes, then use Table 310.15(B)(16) 75-degree C (THWN) column to size the feeder tap as 300 kcmil.
3. The ampere rating of a feeder that supplies a continuous lighting load and a motor load shall be how many amperes when the lighting load is equivalent to 39 amperes and the motor load is equivalent to 24 amperes?
   • Correct Answer: 430.22, and 600.5(B) Therefore, calculate the total load by: 39 amperes + 24 amperes = 63 amperes, then multiply by 125%. 63 amperes x 1.25 = 78.75 amperes.