Master Electrician

1. Question 1 · Batteries

   A 48 V battery bank must run a 1,200 W inverter load for 4 hours. The battery manufacturer allows only a 50% depth of discharge. Ignoring inverter losses, what is the minimum rated capacity of the bank?
   • A.400 Ah
   • B.100 Ah
   • C.200 Ah
   • D.50 Ah
   Show answer and explanation

   Answer: C. 200 Ah

   Load current is 1,200 W ÷ 48 V = 25 A, and 25 A × 4 h = 100 Ah must be drawn. Because only half of the rated capacity may be used, the bank must be rated 100 Ah ÷ 0.50 = 200 Ah. 100 Ah ignores the depth-of-discharge limit; 50 Ah multiplies by 0.50 instead of dividing.

2. Question 2 · Motors

   The same 25 hp, 460 V motor (table FLC 34 A) is protected by an inverse-time circuit breaker. What is the largest breaker permitted by Table 430.52 and its next-size-up allowance?
   • A.100 A
   • B.90 A
   • C.70 A
   • D.80 A
   Show answer and explanation

   Answer: B. 90 A

   Inverse-time breakers are sized at up to 250% of table FLC: 34 × 2.5 = 85 A. 85 A is not a standard size, so 430.52(C)(1) Exception No. 1 permits the next higher standard rating, 90 A. Going two sizes up to 100 A is not permitted without the starting exception.

3. Question 3 · Switches

   Under NEC 404.8(A), what is the maximum height of the center of a switch or circuit breaker operating handle in its highest position, above the floor or working platform?
   • A.6 ft 0 in
   • B.7 ft 0 in
   • C.5 ft 6 in
   • D.6 ft 7 in
   Show answer and explanation

   Answer: D. 6 ft 7 in

   Switches and circuit breakers used as switches must be installed so that the center of the operating handle, at its highest position, is not more than 6 ft 7 in (2.0 m) above the floor or working platform, with exceptions such as busway switches.

4. Question 4 · Motors

   A dwelling feeder supplies an 8 kW range demand load. Under NEC 220.61(B), what neutral load is used for the range?
   • A.8,000 W
   • B.6,400 W
   • C.5,600 W
   • D.4,000 W
   Show answer and explanation

   Answer: C. 5,600 W

   The feeder or service neutral load for household ranges, ovens and dryers may be calculated at 70% of the demand determined for the ungrounded conductors: 8,000 × 0.70 = 5,600 W.

5. Question 5 · National Electrical Code

   A service uses 500 kcmil copper conductors. The grounding electrode conductor runs only to a ground rod and connects to no other electrode. What is the largest size it is required to be?
   • A.4 AWG copper
   • B.6 AWG copper
   • C.2 AWG copper
   • D.1/0 AWG copper
   Show answer and explanation

   Answer: B. 6 AWG copper

   Although Table 250.66 would require 1/0 AWG for 500 kcmil, 250.66(A) states that the part of a GEC that is the sole connection to a rod, pipe or plate electrode need not be larger than 6 AWG copper. A concrete-encased electrode connection need not be larger than 4 AWG copper.

6. Question 6 · Batteries

   An energy storage design needs a 48 V, 300 Ah battery bank built from 12 V, 100 Ah batteries. How many batteries are required, and how are they arranged?
   • A.8 batteries: two parallel strings of four in series
   • B.12 batteries: three parallel strings of four in series
   • C.16 batteries: four parallel strings of four in series
   • D.4 batteries: one string of four in series
   Show answer and explanation

   Answer: B. 12 batteries: three parallel strings of four in series

   Four 12 V batteries in series give 48 V at 100 Ah. Three such strings in parallel give 300 Ah, so 3 × 4 = 12 batteries. Series connections add voltage, and parallel strings add capacity.

7. Question 7 · Motors

   A 10 hp, 460 V, three-phase motor (table FLC 14 A) is protected by nontime-delay fuses. What is the largest fuse permitted, including the next-size-up allowance?
   • A.50 A
   • B.40 A
   • C.45 A
   • D.25 A
   Show answer and explanation

   Answer: C. 45 A

   Table 430.52 allows nontime-delay fuses at up to 300% of table FLC: 14 × 3 = 42 A. 42 A is not standard, so the next higher standard size, 45 A, is permitted. 25 A is the 175% value for dual-element fuses.

8. Question 8 · Switches

   A contactor will be used for plugging and inching (jogging) a squirrel-cage motor. Which IEC utilization category fits this duty?
   • A.AC-4
   • B.AC-15
   • C.AC-3
   • D.AC-1
   Show answer and explanation

   Answer: A. AC-4

   AC-3 covers starting squirrel-cage motors and switching them off while running. AC-4 covers the more severe duty of plugging, reversing and inching, where the contactor breaks locked-rotor current. AC-1 is for resistive loads, and AC-15 for control of electromagnetic loads.

9. Question 9 · Motors

   A 50 kW, 480 V three-phase load has its power factor corrected from 0.70 to 0.95. Its line current falls from about 86 A to about:
   • A.82 A
   • B.72 A
   • C.60 A
   • D.63 A
   Show answer and explanation

   Answer: D. 63 A

   I = P ÷ (1.732 × V × PF). At 0.70: 50,000 ÷ (1.732 × 480 × 0.70) = 86 A. At 0.95: 50,000 ÷ (1.732 × 480 × 0.95) = 63 A. Correcting power factor cuts current, I²R losses and voltage drop.

10. Question 10 · National Electrical Code

    A building has a metal underground water pipe, a concrete-encased electrode and a ground rod. Under NEC 250.50, how must they be treated?
    • A.Only the electrode with the lowest measured resistance is used
    • B.Each must serve a separate panelboard
    • C.All must be bonded together into one grounding electrode system
    • D.Only the water pipe is used if it is 10 ft long
    Show answer and explanation

    Answer: C. All must be bonded together into one grounding electrode system

    All grounding electrodes present at each building must be bonded together to form the grounding electrode system. A metal water pipe must also be supplemented by an additional electrode, because it could later be replaced with plastic.

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