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MASTER NEC 2023

Master Electrician Practice Test - Motors, Transformers & Industrial

Master electrician practice test on motor branch circuits, overload protection, feeder sizing, transformer overcurrent, separately derived systems, arc welders, industrial machinery, generators, and A/C equipment.

TOPICS COVERED

Motors Transformers Grounding HVAC Circuits Welders

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Frequently Asked Questions

Yes — completely free with no signup required. All answers include detailed explanations.

Requirements vary by state but typically include 4–8 years of documented electrical work experience plus a valid Journeyman license.

Advanced NEC code interpretation, load calculations, service entrance design, motor controls, generator systems, and electrical system design.

The Master exam focuses more on design, calculations, and planning rather than installation details. Load calculation and service sizing questions are weighted heavily.

How to Use This Practice Test

This Master Electrician Practice Test - Motors, Transformers & Industrial has 26 questions with detailed answers and NEC references. Take the full test, review every wrong answer, and look up the NEC article it references before moving on.

Tip: Focus on multi-step calculations. Write each step on scratch paper — one math error means a wrong answer.

Topics covered: Load calculations, transformer sizing, three-phase circuits, service entrance, and special occupancies.
Passing score: 70–75% (most states). Master exams are calculation-heavy — practice the math, not just the lookup.
Questions: 26 multiple-choice, one correct answer each

Questions

45 min

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70%

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Master Electrician Practice Test - Motors, Transformers & Industrial — Question List

Question 1: A 460V, 3-phase motor runs at 28 amps full load. What is the smallest conductor ampacity you can use for its branch circuit?

28A — match the motor FLA exactly
32A — 115% of FLA
35A — 125% of FLA
42A — 150% of FLA

NEC Reference: NEC 430.22

Question 2: You are picking a breaker for a 230V, single-phase motor with 20A full-load current. Using an inverse-time breaker and Table 430.52, what is the biggest breaker you can use?

NEC Reference: NEC Table 430.52

Question 3: A motor nameplate shows FLA = 40A and service factor = 1.15. What is the highest trip setting allowed on the overload relay?

40A — match the FLA exactly
46A — 115% of FLA
50A — 125% of FLA
56A — 140% of FLA

NEC Reference: NEC 430.32(A)(1)

Question 4: Three motors share a feeder at 460V, 3-phase — 20HP (27A FLA), 15HP (21A FLA), and 10HP (14A FLA). What is the minimum ampacity for the feeder conductors?

62A — sum of all three motor FLAs
69A — 125% of largest plus sum of the rest
78A — 125% applied to the total FLA
88A — 125% of all three motors added together

NEC Reference: NEC 430.24

Question 5: When the motor nameplate is not available, which NEC table gives the full-load current for 3-phase AC induction motors?

Table 310.16
Table 430.248
Table 430.250
Table 430.52

NEC Reference: NEC Table 430.250

Question 6: A motor is in the basement and its disconnect is on the first floor where you cannot see the motor. What does the NEC allow in this situation?

A second disconnect must be installed at the motor
The disconnect must be capable of being locked in the open position
A warning label on the motor is sufficient
Nothing extra is needed as long as the disconnect is identified

NEC Reference: NEC 430.102(B)

Question 7: When picking a motor controller, what is the minimum current it must be rated to handle?

Full-load current only
Locked-rotor current only
Both full-load current and locked-rotor current
The available short-circuit current from the utility

NEC Reference: NEC 430.110(A)

Question 8: A 45 kVA, 480–208Y/120V transformer has both primary and secondary overcurrent protection. The primary current works out to about 93.75A. What is the maximum size primary OCPD?

100A
125A
225A
250A

NEC Reference: NEC 450.3(B)

Question 9: Same 45 kVA transformer (primary current = 93.75A), but now you use primary-only protection with no secondary OCPD. What is the maximum primary OCPD size?

100A
125A
150A
250A

NEC Reference: NEC 450.3(B)

Question 10: You install a 75 kVA, 480–208Y/120V transformer in a building. The secondary creates a separately derived system. Where should the main bonding jumper (system bond) be made?

At the transformer secondary terminals or at the first disconnect — but only one location
At the transformer and again at every subpanel fed from it
At the main service entrance of the building
No bonding is required on the secondary of a separately derived system

NEC Reference: NEC 250.30(A)(1)

Question 11: A motor with 25A FLA will not start on the 250% inverse-time breaker allowed by Table 430.52. Per 430.52, what is the absolute maximum breaker size you are allowed to install?

60A — next standard above the 250% value
80A — 300% of FLA
90A — next standard below the 400% ceiling
100A — 400% of FLA (the hard maximum)

NEC Reference: NEC 430.52(C)(1) Exception No. 2

Question 12: A central A/C condensing unit nameplate shows a branch-circuit selection current of 24A. What is the maximum overcurrent device allowed on its branch circuit?

NEC Reference: NEC 440.22(A)

Question 13: The motor nameplate shown has the following data: 460V, 3-phase, 25HP, FLA = 34A, SF = 1.0. What is the minimum conductor ampacity for the branch circuit feeding this motor?

Motor nameplate showing 460V, 3-phase, 25HP, FLA 34A, SF 1.0

34A — use the nameplate FLA directly
39A — 115% of FLA
42.5A — 125% of FLA
51A — 150% of FLA

NEC Reference: NEC 430.22

Question 14: The secondary conductors from a wound-rotor motor to its controller are used for continuous-duty operation. They must be sized at what percentage of the motor rated secondary current?

100%
115%
125%
150%

NEC Reference: NEC 430.23(A)

Question 15: A motor nameplate shows FLA = 40A and temperature rise = 50°C. There is no service factor listed. What is the maximum overload relay setting allowed?

40A — use FLA with no multiplier
46A — 115% of FLA
50A — 125% of FLA
56A — 140% of FLA

NEC Reference: NEC 430.32(A)(1)

Question 16: A nonautomatic arc welder has a 60% duty cycle and primary current of 50A. Table 630.11(A) gives a multiplier of 0.78 for 60% duty cycle. What is the minimum branch circuit conductor ampacity?

33A — using a 0.65 factor
39A — 50A × 0.78
44A — 50A × 0.89 (80% factor)
50A — no duty cycle reduction

NEC Reference: NEC 630.11(A)

Question 17: The MCC diagram shows three motors: 50HP (65A FLA), 25HP (34A FLA), and 15HP (21A FLA), all at 460V, 3-phase. What is the minimum feeder conductor ampacity to this MCC?

Motor control center diagram showing three motors at 460V with FLA values labeled

120A — sum of all three FLAs with no multiplier
136A — 125% of largest plus sum of the rest
150A — 125% applied to the full sum
175A — 125% of each motor added separately

NEC Reference: NEC 430.24

Question 18: Which of the following is covered by Article 670 (Industrial Machinery)?

A residential HVAC unit
A CNC machining center with multiple motors on a single frame
A 200A commercial panel
A crane in a warehouse

NEC Reference: NEC Article 670

Question 19: An industrial machine nameplate says: 240V, 3-phase, supply current = 60A. What is the minimum conductor ampacity for the supply circuit to this machine?

60A — match the nameplate supply current
69A — 115% of supply current
75A — 125% of supply current
80A — next standard size above 60A

NEC Reference: NEC 670.4(B)

Question 20: Cranes and hoists are covered by which NEC article?

Article 430
Article 530
Article 610
Article 670

NEC Reference: NEC Article 610

Question 21: Per 445.13, the conductors running from a generator output terminals to the first overcurrent device must be sized at what percentage of the generator nameplate current rating?

100%
115%
125%
150%

NEC Reference: NEC 445.13

Question 22: The panel diagram shows a 200A main breaker and a 200A bus rating. A solar inverter will be connected at the bottom of the panel using the 120% rule in 705.12(B). What is the maximum size breaker for the solar inverter backfeed?

Residential panel diagram showing 200A main breaker and 200A bus rating with solar backfeed location at bottom

NEC Reference: NEC 705.12(B)(3)(2)

Question 23: A motor nameplate is stamped "Thermally Protected." What does this mean for external overload protection?

The motor still needs an external overload relay regardless
The motor has a built-in protector — no separate overload relay is required
Only GFCI protection is required instead of an overload relay
The thermal protection only covers short circuits, not overloads

NEC Reference: NEC 430.32(A)(2)(a)

Question 24: When sizing conductors for an air conditioning condensing unit, which current value do you use?

The FLA value from Table 430.250 based on the motor horsepower
The rated-load current or branch-circuit selection current from the equipment nameplate
The locked-rotor current divided by 6
The compressor horsepower × 1.5

NEC Reference: NEC 440.6

Question 25: A rooftop A/C unit disconnect is installed in the equipment room below — you cannot see the unit from the disconnect. What must you do to comply with 440.14?

The disconnect must be lockable in the open position per 110.25
A second disconnect must be installed on the roof next to the unit
Post a warning sign at the panel — no other action needed
Nothing extra is required as long as the disconnect is labeled

NEC Reference: NEC 440.14

Question 26: A feeder serves two motors. The 50HP motor has a 100A branch circuit breaker and the 25HP motor has a 60A branch circuit breaker. The 25HP motor FLA is 34A. What is the maximum feeder overcurrent device size?

100A — match the largest branch circuit breaker
125A — largest breaker plus other motor FLA, capped below 134A
150A — largest breaker plus other motor FLA rounded up
160A — sum of both branch circuit breakers

NEC Reference: NEC 430.62(A)

Key NEC References for This Test

Article	What It Covers
Art. 220	Load Calculations — optional and standard methods for multifamily
Art. 230	Services — service design, sizing, multiple disconnects
Art. 430	Motors — complete branch circuit design (conductor + OCPD + overload + disconnect)
Art. 445	Generators — standby systems, transfer switches
Art. 450	Transformers — sizing, primary/secondary OCPD, installation
Art. 706	Energy Storage Systems — battery/solar storage (major NEC 2026 expansion)

Study Tip: Practice transformer kVA calculations daily. Know the formula: I = kVA × 1000 ÷ (V × 1.732) for three-phase. This comes up on almost every master exam.

Master Electrician Practice Test - Motors, Transformers & Industrial

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Key NEC References for This Test

Master Electrician Practice Test - Motors, Transformers & Industrial

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Master Electrician Practice Test - Motors, Transformers & Industrial — Question List

Question 1: A 460V, 3-phase motor runs at 28 amps full load. What is the smallest conductor ampacity you can use for its branch circuit?

Question 2: You are picking a breaker for a 230V, single-phase motor with 20A full-load current. Using an inverse-time breaker and Table 430.52, what is the biggest breaker you can use?

Question 3: A motor nameplate shows FLA = 40A and service factor = 1.15. What is the highest trip setting allowed on the overload relay?

Question 4: Three motors share a feeder at 460V, 3-phase — 20HP (27A FLA), 15HP (21A FLA), and 10HP (14A FLA). What is the minimum ampacity for the feeder conductors?

Question 5: When the motor nameplate is not available, which NEC table gives the full-load current for 3-phase AC induction motors?

Question 6: A motor is in the basement and its disconnect is on the first floor where you cannot see the motor. What does the NEC allow in this situation?

Question 7: When picking a motor controller, what is the minimum current it must be rated to handle?

Question 8: A 45 kVA, 480–208Y/120V transformer has both primary and secondary overcurrent protection. The primary current works out to about 93.75A. What is the maximum size primary OCPD?

Question 9: Same 45 kVA transformer (primary current = 93.75A), but now you use primary-only protection with no secondary OCPD. What is the maximum primary OCPD size?

Question 10: You install a 75 kVA, 480–208Y/120V transformer in a building. The secondary creates a separately derived system. Where should the main bonding jumper (system bond) be made?

Question 11: A motor with 25A FLA will not start on the 250% inverse-time breaker allowed by Table 430.52. Per 430.52, what is the absolute maximum breaker size you are allowed to install?

Question 12: A central A/C condensing unit nameplate shows a branch-circuit selection current of 24A. What is the maximum overcurrent device allowed on its branch circuit?

Question 13: The motor nameplate shown has the following data: 460V, 3-phase, 25HP, FLA = 34A, SF = 1.0. What is the minimum conductor ampacity for the branch circuit feeding this motor?

Question 14: The secondary conductors from a wound-rotor motor to its controller are used for continuous-duty operation. They must be sized at what percentage of the motor rated secondary current?

Question 15: A motor nameplate shows FLA = 40A and temperature rise = 50°C. There is no service factor listed. What is the maximum overload relay setting allowed?

Question 16: A nonautomatic arc welder has a 60% duty cycle and primary current of 50A. Table 630.11(A) gives a multiplier of 0.78 for 60% duty cycle. What is the minimum branch circuit conductor ampacity?

Question 17: The MCC diagram shows three motors: 50HP (65A FLA), 25HP (34A FLA), and 15HP (21A FLA), all at 460V, 3-phase. What is the minimum feeder conductor ampacity to this MCC?

Question 18: Which of the following is covered by Article 670 (Industrial Machinery)?

Question 19: An industrial machine nameplate says: 240V, 3-phase, supply current = 60A. What is the minimum conductor ampacity for the supply circuit to this machine?

Question 20: Cranes and hoists are covered by which NEC article?

Question 21: Per 445.13, the conductors running from a generator output terminals to the first overcurrent device must be sized at what percentage of the generator nameplate current rating?

Question 22: The panel diagram shows a 200A main breaker and a 200A bus rating. A solar inverter will be connected at the bottom of the panel using the 120% rule in 705.12(B). What is the maximum size breaker for the solar inverter backfeed?

Question 23: A motor nameplate is stamped "Thermally Protected." What does this mean for external overload protection?

Question 24: When sizing conductors for an air conditioning condensing unit, which current value do you use?

Question 25: A rooftop A/C unit disconnect is installed in the equipment room below — you cannot see the unit from the disconnect. What must you do to comply with 440.14?

Question 26: A feeder serves two motors. The 50HP motor has a 100A branch circuit breaker and the 25HP motor has a 60A branch circuit breaker. The 25HP motor FLA is 34A. What is the maximum feeder overcurrent device size?

Key NEC References for This Test

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