Colorado Master Electrician Exam Preparation

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

1. A set of six 600 kcmil THHN ungrounded supply conductors are connected in parallel for the secondary of a transformer. What would be the calculated minimum Grounded Conductor required for this installation?
   • Correct Answer: NEC – Table 250.102(C)(1) Note 1, Chapter 9 Table 8.
     1. Per Table 250.102(C)(1), and using Note 1, calculate the equivalent area: 600 kcmil x 6 conductors = 3600 kcmil.
     2. Then calculate 12.5% of 3600 kcmil: 3600 kcmil x 0.125 = 450 kcmil.
     3. Per Table 8 of Chapter 9; up to a standard size of 500 kcmil.
2. Photovoltaic systems operating at what DC voltage or greater between any two conductors shall be protected by a listed PV arc fault circuit interrupter or other system components listed to provide equivalent protection?
   • Correct Answer: NEC – 690.11 “operating at a PV voltage of 80 volts or more”.
3. Given that a three-phase transformer will supply a load side voltage of 208-volts that draws 385 amperes of current, what would be the MINIMUM size transformer required for this service installation?
   • Correct Answer: Using Ohm’s Law calculate: P = I x E x 1.732 = 385 A x 208 V x 1.732 = 138,699 VA, then size up to 150 kVA .