Master Electrician (NEC) — All Questions

Electrical equipment associated with a fountain, including submersible pumps and luminaires, must be protected by:

• a.an isolation transformer only
• b.an arc-fault circuit interrupter
• c.a ground-fault circuit interrupter (GFCI)✓
• d.a surge arrester

2023 NEC 680.51(A): fountain luminaires and other electrical equipment associated with the fountain must be GFCI protected.

In agricultural buildings, the site-isolating disconnecting means at the distribution point must:

• a.be a service disconnect at each building only
• b.provide a means to disconnect all site conductors and be identified as the site-isolating device✓
• c.be limited to 60 A maximum
• d.be a snap switch inside the barn

2023 NEC 547.9/547.25: the site-isolating device at the agricultural distribution point disconnects the site conductors and must be pole-mounted or otherwise identified as such.

A feeder supplies a 50-HP (65 A), a 30-HP (40 A), and a 20-HP (27 A) motor, all 460-V three-phase. What minimum feeder conductor ampacity is required?

• a.156 A
• b.140 A
• c.132 A
• d.148.25 A✓

2023 NEC 430.24: 125% of the largest motor FLC plus the sum of the others: (65 x 1.25) + 40 + 27 = 81.25 + 67 = 148.25 A.

A motor marked with a temperature rise of 40 C (and no service factor) has a nameplate full-load current of 52 A. What is the maximum overload protection?

• a.52 A
• b.59.8 A
• c.65 A✓
• d.60 A

2023 NEC 430.32(A)(1): motors marked with a temperature rise not over 40 C may set overload at 125% of nameplate FLA: 52 x 1.25 = 65 A.

In a Class II, Division 1 (combustible dust) location, enclosures for arcing equipment must be:

• a.identified (listed) as dust-ignitionproof for the location✓
• b.explosionproof for Class I only
• c.weatherproof rated
• d.ordinary NEMA 1 enclosures

2023 NEC 502.115/502.6: Class II, Division 1 arcing equipment must be in enclosures identified as dust-ignitionproof; Class I explosionproof enclosures are not necessarily suitable for dust.

An indoor dry-type transformer rated 45 kVA with standard (Class 105) insulation must maintain what minimum separation from combustible material unless otherwise protected?

• a.18 in
• b.3 in
• c.12 in✓
• d.6 in

2023 NEC 450.21(A): dry-type transformers 112.5 kVA and under must be separated at least 12 in from combustible material unless the unit has a listed fire-resistant barrier or Class 155 (or higher) insulation.

Which value from a motor nameplate must be used with the tables to size branch-circuit short-circuit protection and conductors, per 430.6(A)?

• a.the nameplate full-load amperes
• b.the service-factor amps
• c.the locked-rotor current
• d.the table full-load current, not the nameplate current rating✓

2023 NEC 430.6(A)(1): for general motor applications, the full-load current values in Tables 430.247-430.250 (not the nameplate FLA) are used to size conductors and short-circuit/ground-fault protection. Overload protection uses the nameplate FLA.

The disconnecting means for air-conditioning or refrigerating equipment with a motor-compressor must be located:

• a.only at the main service panel
• b.inside a locked mechanical room only
• c.at least 25 ft away
• d.within sight from and readily accessible from the equipment✓

2023 NEC 440.14: the disconnect must be within sight from, and readily accessible from, the air-conditioning or refrigerating equipment.

How many overload (running-protection) units are required for a three-phase alternating-current motor?

• a.one
• b.three✓
• c.four
• d.two

2023 NEC Table 430.37: a three-phase AC motor requires three overload units, one in each phase.

In patient care spaces, grounding of receptacles and fixed electrical equipment must be accomplished by:

• a.an isolated ground only
• b.the metal raceway alone as the only ground path
• c.the grounded (neutral) conductor
• d.an insulated copper equipment grounding conductor plus a metal raceway or cable armor (redundant ground)✓

2023 NEC 517.13: patient care spaces require redundant grounding - an insulated copper EGC AND a metallic raceway or cable that itself qualifies as a grounding path.

The disconnecting means for a motor must have an ampere rating of at least what percentage of the motor full-load current?

• a.115%✓
• b.125%
• c.100%
• d.110%

2023 NEC 430.110(A): the disconnecting means for a motor circuit must be rated at least 115% of the motor full-load current.

A load draws 60 kW of real power and 100 kVA of apparent power. What is the reactive power?

• a.60 kVAR
• b.40 kVAR
• c.80 kVAR✓
• d.100 kVAR

Reactive power = sqrt(kVA^2 - kW^2) = sqrt(100^2 - 60^2) = sqrt(6,400) = 80 kVAR.

A load has a current lagging the voltage by a phase angle of 30 degrees. What is its power factor?

• a.0.50
• b.0.87✓
• c.0.71
• d.0.94

Power factor = cos(theta) = cos(30 degrees) = 0.866, about 0.87.

What is the real power of a three-phase load at 480 V, 40 A, and 0.90 power factor?

• a.19,200 W
• b.24,000 W
• c.29,930 W✓
• d.33,254 W

P = 1.732 x V x I x PF = 1.732 x 480 x 40 x 0.90 = 29,930 W.

What is the real power of a single-phase load drawing 25 A at 240 V with a 0.85 power factor?

• a.6,000 W
• b.5,100 W✓
• c.7,200 W
• d.4,335 W

P = V x I x PF = 240 x 25 x 0.85 = 5,100 W.

In a balanced wye (star) three-phase system, the line-to-line voltage equals:

• a.2 times the line-to-neutral voltage
• b.0.577 times the line-to-neutral voltage
• c.1.732 times the line-to-neutral voltage✓
• d.the line-to-neutral voltage

In a wye system the line-to-line voltage is the square root of 3 (1.732) times the phase (line-to-neutral) voltage.

In a 240/120-V, three-phase, four-wire high-leg delta system, what is the voltage from the high (stinger) leg to the neutral?

• a.240 V
• b.277 V
• c.208 V✓
• d.120 V

The high leg to neutral in a 240-V delta is 240 x (sqrt(3)/2) = 208 V; that phase must never supply line-to-neutral loads.

An 80-kVA transformer serves a single-phase load at 240 V. What is the full-load current?

• a.333 A✓
• b.577 A
• c.240 A
• d.192 A

Single-phase current = VA / V = 80,000 / 240 = 333 A.

A 100-kW load at 0.70 power factor is corrected to 0.90 (tan of the 0.70 angle = 1.020, tan of the 0.90 angle = 0.484). How many kVAR of capacitance are required?

• a.20 kVAR
• b.48 kVAR
• c.100 kVAR
• d.53.6 kVAR✓

Qc = kW x (tan(theta1) - tan(theta2)) = 100 x (1.020 - 0.484) = 100 x 0.536 = 53.6 kVAR.

A transformer steps 480 V down to 240 V. What is its turns (voltage) ratio?

• a.8:1
• b.1:2
• c.2:1✓
• d.4:1

Turns ratio = primary voltage / secondary voltage = 480 / 240 = 2:1.

A transformer has a 10:1 turns ratio and a 5-A primary current. What is the ideal secondary current?

• a.5 A
• b.50 A✓
• c.15 A
• d.0.5 A

Current is inversely proportional to the turns ratio: Is = Ip x (Np/Ns) = 5 x 10 = 50 A.

A purely resistive load connected to 120 V draws 10 A. What is its resistance?

• a.130 ohms
• b.12 ohms✓
• c.1,200 ohms
• d.0.083 ohms

By Ohm's law, R = V / I = 120 / 10 = 12 ohms.

A resistive heating element rated 240 V has a resistance of 12 ohms. What power does it dissipate?

• a.28,800 W
• b.2,880 W
• c.20 W
• d.4,800 W✓

P = V^2 / R = 240^2 / 12 = 57,600 / 12 = 4,800 W.

A single-phase load draws 50 A at 240 V. What is the apparent power?

• a.12 kVA✓
• b.24 kVA
• c.6 kVA
• d.12 kW

Apparent power = V x I = 240 x 50 = 12,000 VA = 12 kVA (kW would require the power factor).

A motor delivers 25 HP of mechanical output. Ignoring losses, how many watts of output is that?

• a.25,000 W
• b.746 W
• c.33,540 W
• d.18,650 W✓

1 HP = 746 W, so 25 HP = 25 x 746 = 18,650 W of mechanical output.

A motor delivers 20 HP of mechanical output at 92% efficiency. What is its electrical input power?

• a.14,920 W
• b.13,726 W
• c.20,000 W
• d.16,217 W✓

Output = 20 x 746 = 14,920 W. Input = output / efficiency = 14,920 / 0.92 = 16,217 W.

A feeder conductor carries 20 A and has a total resistance of 0.5 ohm. What is the power lost as heat in the conductor?

• a.2,000 W
• b.200 W✓
• c.10 W
• d.40 W

Line loss = I^2 x R = 20^2 x 0.5 = 400 x 0.5 = 200 W.

What is the apparent power of an 80-kW load operating at a 0.80 power factor?

• a.80 kVA
• b.64 kVA
• c.125 kVA
• d.100 kVA✓

Apparent power = real power / power factor = 80 / 0.80 = 100 kVA.

For a 120-V rms sinusoidal supply, what is the approximate peak voltage?

• a.120 V
• b.170 V✓
• c.240 V
• d.85 V

Peak = rms x 1.414: 120 x 1.414 = 169.7 V, about 170 V.

In a three-phase, four-wire wye system with heavy nonlinear (electronic) load, why can the neutral current exceed the phase current?

• a.the neutral carries the vector sum of balanced fundamentals
• b.reactive current cancels in the neutral
• c.third-harmonic (triplen) currents are in phase across all three legs and add in the neutral✓
• d.the neutral is undersized by code

Triplen harmonics, principally the 3rd, are in phase across all three phases and add arithmetically in the neutral rather than canceling, so the neutral can carry more than a phase conductor.

A balanced three-phase panel has phase loads of A = 48 A, B = 36 A, C = 24 A. What is the ideal per-phase current if the loads were perfectly balanced?

• a.36 A✓
• b.24 A
• c.40 A
• d.48 A

Total = 48 + 36 + 24 = 108 A; perfectly balanced = 108 / 3 = 36 A per phase.

What three-phase transformer kVA rating corresponds to a full-load current of 200 A at 240 V?

• a.48 kVA
• b.100 kVA
• c.83 kVA✓
• d.144 kVA

kVA = (1.732 x V x I) / 1000 = (1.732 x 240 x 200) / 1000 = 83.1 kVA.

A 60-Hz sinusoidal waveform has a period of approximately:

• a.1.0 s
• b.8.3 ms
• c.60 ms
• d.16.7 ms✓

Period = 1 / frequency = 1 / 60 = 0.0167 s = 16.7 ms.

For equipment operating at 151-600 V to ground under Condition 2, what is the minimum depth of working space in front of the equipment?

• a.5 ft
• b.3.5 ft✓
• c.4 ft
• d.3 ft

2023 NEC Table 110.26(A)(1): for over 150 to 600 V, Condition 2 (grounded parts on the opposite side) requires 3.5 ft of clear working depth.

For equipment operating at 0-150 V to ground, what is the minimum depth of working space regardless of condition?

• a.3 ft✓
• b.3.5 ft
• c.2.5 ft
• d.4 ft

2023 NEC Table 110.26(A)(1): at 0-150 V to ground the working depth is 3 ft for Conditions 1, 2, and 3.

Personnel doors for electrical equipment rated at what current or more must open in the direction of egress and be equipped with panic hardware?

• a.800 A✓
• b.1,000 A
• c.1,200 A
• d.600 A

2023 NEC 110.26(C)(3): where equipment rated 800 A or more has a personnel door within 7.6 m (25 ft) of the working space, the door must open in the direction of egress and have listed panic hardware.

Under 2023 NEC 110.22, each disconnecting means must be:

• a.legibly marked to indicate its purpose unless located and arranged so the purpose is evident✓
• b.installed only by the utility
• c.rated at least 100 A
• d.painted red

2023 NEC 110.22(A): each disconnecting means must be legibly marked to indicate its purpose unless located and arranged so the purpose is evident.

Under 2023 NEC 110.14(D), terminations that are marked with a tightening torque must be:

• a.tightened until the conductor deforms
• b.tightened to the specified value using a calibrated torque tool✓
• c.left finger-tight for expansion
• d.hand-tightened only

2023 NEC 110.14(D): where a tightening torque is indicated on equipment or in instructions, a calibrated torque tool must be used to achieve the specified value.

One of the primary purposes of system grounding under 2023 NEC 250.4(A)(1) is to:

• a.limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines✓
• b.carry normal load current
• c.eliminate the need for overcurrent protection
• d.reduce conductor resistance

2023 NEC 250.4(A)(1): system grounding limits the voltage imposed by lightning, line surges, and unintentional contact with higher-voltage lines and stabilizes voltage to earth during normal operation.

Under 2023 NEC 408.4(A), every circuit and circuit modification in a panelboard must be:

• a.numbered only, with no description
• b.marked only if over 100 A
• c.legibly identified as to its clear, evident, and specific purpose in a circuit directory✓
• d.identified only for motor circuits

2023 NEC 408.4(A): each circuit must be legibly identified as to its clear, evident, and specific purpose on a directory located at the panelboard.

In the 2023 NEC, the word 'shall' indicates a requirement that is:

• a.mandatory✓
• b.permissive
• c.advisory only
• d.a recommendation

2023 NEC 90.5(A): mandatory rules use the word 'shall'; permissive rules use 'shall be permitted'; explanatory material is informational.

Under 2023 NEC 110.12, electrical equipment must be installed in a manner that is:

• a.energized before inspection
• b.at the installer's sole discretion
• c.concealed from view
• d.neat and workmanlike✓

2023 NEC 110.12: electrical equipment must be installed in a neat and workmanlike manner, with unused openings closed and internal parts kept free of foreign material.

Under 2023 NEC 110.2, conductors and equipment covered by the Code are acceptable only if:

• a.approved by the authority having jurisdiction✓
• b.rated over 600 V
• c.installed by a master electrician
• d.manufactured domestically

2023 NEC 110.2: conductors and equipment required or permitted by the Code are acceptable only if approved (accepted as suitable) by the authority having jurisdiction.

Under 2023 NEC 110.11, equipment must be selected considering:

• a.the utility's meter type
• b.only the ambient temperature
• c.the installer's warranty period
• d.any deteriorating agents or environmental conditions present at the installation✓

2023 NEC 110.11: equipment must be identified for the environment; presence of moisture, corrosive vapors, dust, or other deteriorating agents must be considered in selection and installation.

Service equipment (other than one- and two-family dwellings) rated 1,200 A or more must be legibly field marked with:

• a.the installer's license number
• b.the maximum available fault current and the date of the calculation✓
• c.the panel's short-circuit color code
• d.the phase rotation only

2023 NEC 110.24(A): service equipment must be field marked with the maximum available fault current, including the date of the calculation, so the interrupting/short-circuit ratings can be verified.

In the NEC definitions, a 'continuous load' is one where the maximum current is expected to continue for:

• a.30 minutes or more
• b.8 hours or more
• c.1 hour or more
• d.3 hours or more✓

2023 NEC Article 100: a continuous load is one where the maximum current continues for 3 hours or more, which triggers the 125% conductor and device sizing rules.

Under 2023 NEC 110.26, the working space in front of energized electrical equipment must:

• a.be reduced to 24 in for panelboards
• b.be kept clear and not be used for storage✓
• c.double as an egress corridor only
• d.be usable for storage when de-energized

2023 NEC 110.26(B): the required working space must be kept clear and must not be used for storage; when normally enclosed live parts are exposed for service, the space must be guarded.

For equipment operating at 151-600 V to ground under Condition 1, what is the minimum depth of working space?

• a.3 ft✓
• b.4 ft
• c.2.5 ft
• d.3.5 ft

2023 NEC Table 110.26(A)(1): at over 150 to 600 V, Condition 1 (no grounded or live parts on the opposite side) requires 3 ft of working depth.

Because jurisdictions adopt different Code editions and amendments, a master electrician sizing a service should always:

• a.assume the newest published NEC applies everywhere
• b.use whichever edition is most economical
• c.verify the NEC edition and any local amendments the jurisdiction has adopted✓
• d.follow the manufacturer's catalog instead of the Code

The NEC is adopted (often with amendments) on a state/local basis, and Article 220 was renumbered in the 2023 edition; always verify the edition and local amendments your jurisdiction has adopted before finalizing a calculation.

Under 2023 NEC 110.14(C), the temperature rating of a conductor used for ampacity at a termination is governed by:

• a.the ambient temperature only
• b.the lowest temperature rating of any connected termination, conductor, or device✓
• c.the 90 C column in all cases
• d.the highest-rated component in the circuit

2023 NEC 110.14(C): the conductor ampacity at a termination is based on the lowest temperature rating of any connected termination, conductor, or device, so the 90 C column is generally usable only for derating, not for final termination sizing.