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DC Electrician practice questions

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Question 1Electrical Theory and Code Knowledge

According to the NEC, what is the maximum permitted voltage drop for a branch circuit plus feeder combined, for a reasonable efficiency of operation?

  1. 5 percent Correct answer.
  2. 2 percent
  3. 10 percent
  4. 3 percent

Explanation

NEC 210.19(A) Informational Note No. 4 and 215.2(A)(1) Informational Note No. 2 recommend that the total voltage drop of feeder plus branch circuit not exceed 5 percent for reasonable efficiency of operation. The 3 percent figure applies to a branch circuit alone or a feeder alone, not the combined total.

Question 2Electrical Theory and Code Knowledge

In a series circuit containing three resistors, which quantity is the same through every resistor?

  1. Voltage
  2. Resistance
  3. Current Correct answer.
  4. Power

Explanation

A series circuit provides only one path for current, so the same current flows through every element regardless of individual resistance values. Voltage divides in proportion to resistance, and power differs with each resistor's voltage drop.

Question 3Electrical Theory and Code Knowledge

Which conductor property is the primary reason aluminum is permitted for feeder and service conductors but requires special termination precautions compared with copper?

  1. Higher cost
  2. Greater tendency to oxidize and creep under pressure Correct answer.
  3. Lower ampacity per circular mil
  4. Lower melting point

Explanation

Aluminum forms an oxide film that is a poor conductor and it creeps under sustained termination pressure, which can loosen connections and cause overheating. These characteristics are why terminations must be listed for aluminum and often use oxide-inhibiting compound.

Question 4Electrical Theory and Code Knowledge

A 120-volt branch circuit supplies a continuous load of 12 amperes. Per NEC 210.19(A)(1), what is the minimum conductor ampacity required before any adjustment factors are applied?

  1. 18 amperes
  2. 20 amperes
  3. 15 amperes Correct answer.
  4. 12 amperes

Explanation

NEC 210.19(A)(1) requires branch-circuit conductors to have an ampacity of at least 125 percent of the continuous load plus 100 percent of the noncontinuous load. Here 12 A times 1.25 equals 15 A, so the minimum conductor ampacity is 15 amperes.

Question 5Electrical Theory and Code Knowledge

What is the resistance of a conductor if 2 amperes flow when 24 volts is applied across it?

  1. 0.083 ohm
  2. 24 ohms
  3. 12 ohms Correct answer.
  4. 48 ohms

Explanation

Ohm's law states that resistance equals voltage divided by current, R = V/I. Dividing 24 volts by 2 amperes gives 12 ohms.

Question 6Electrical Theory and Code Knowledge

A 240-volt single-phase load draws 30 amperes. What is the apparent power in kilovolt-amperes?

  1. 3.6 kVA
  2. 8.0 kVA
  3. 72 kVA
  4. 7.2 kVA Correct answer.

Explanation

Apparent power for a single-phase circuit equals volts times amperes: 240 V times 30 A equals 7,200 volt-amperes, or 7.2 kVA. Kilovolt-amperes are used for load calculations because they do not depend on power factor.

Question 7Electrical Theory and Code Knowledge

Which NEC article governs the general requirements for grounding and bonding, including grounding electrode systems?

  1. Article 250 Correct answer.
  2. Article 210
  3. Article 430
  4. Article 310

Explanation

Article 250 of the NEC contains the general requirements for grounding and bonding, covering system grounding, grounding electrodes, equipment grounding conductors, and bonding. The other articles address branch circuits, conductors, and motors respectively.

Question 8Electrical Theory and Code Knowledge

A 20-ampere branch circuit supplies a single receptacle outlet. What is the minimum receptacle rating permitted?

  1. Any rating equal to or greater than 15 amperes
  2. 20 amperes Correct answer.
  3. 15 amperes
  4. 30 amperes

Explanation

NEC 210.21(B)(1) requires that a single receptacle on an individual branch circuit have an ampere rating not less than that of the branch circuit. A 20-ampere circuit therefore needs a 20-ampere receptacle.

Question 9Electrical Theory and Code Knowledge

What is the standard ampere rating of the smallest overcurrent device generally permitted to protect a 14 AWG copper branch-circuit conductor?

  1. 10 amperes
  2. 15 amperes Correct answer.
  3. 20 amperes
  4. 25 amperes

Explanation

Table 310.16 gives 14 AWG copper, at 60 degrees Celsius, an ampacity of 15 amperes, and NEC 240.4(D) limits 14 AWG copper to a 15-ampere overcurrent device for small conductors. This is why 14 AWG is used on 15-ampere branch circuits.

Question 10Electrical Theory and Code Knowledge

Which type of overcurrent protective device is designed to protect against overloads by responding to heat generated by current flow?

  1. Thermal-magnetic circuit breaker Correct answer.
  2. Arc-fault circuit interrupter
  3. Ground-fault circuit interrupter
  4. Surge protective device

Explanation

A thermal-magnetic circuit breaker uses a bimetal element that bends as heat rises with sustained overload current, tripping the breaker. Its magnetic element responds rapidly to short-circuit currents.

Question 11Electrical Theory and Code Knowledge

In a parallel circuit, what happens to the total resistance as additional branches are added?

  1. It becomes zero
  2. It decreases Correct answer.
  3. It stays the same
  4. It increases

Explanation

Adding parallel branches provides more paths for current, so the equivalent resistance always decreases and the total current increases for a fixed source voltage. The reciprocal of total resistance equals the sum of the reciprocals of the branch resistances.

Question 12Electrical Theory and Code Knowledge

A conductor carries 30 amperes and has a resistance of 0.5 ohm. What power is dissipated as heat in the conductor?

  1. 900 watts
  2. 60 watts
  3. 15 watts
  4. 450 watts Correct answer.

Explanation

Power dissipated in a resistive element equals current squared times resistance: 30 squared is 900, and 900 times 0.5 equals 450 watts. This is why undersized conductors overheat.

Question 13Electrical Theory and Code Knowledge

What is the minimum working clearance requirement that must be maintained in front of electrical equipment operating at 120/240 volts to ground?

  1. 30 inches
  2. 36 inches Correct answer.
  3. 42 inches
  4. 24 inches

Explanation

NEC Table 110.26(A)(1) requires a minimum of 36 inches of working space in front of electrical equipment operating at 150 volts or less to ground. Larger clearances apply at higher voltages and with exposed live parts on both sides.

Question 14Electrical Theory and Code Knowledge

A dwelling unit has a 3,000 volt-ampere general lighting load. Using the NEC standard method, what is the first 3,000 volt-amperes of demand for general lighting in a dwelling?

  1. 2,400 volt-amperes
  2. 900 volt-amperes
  3. 3,000 volt-amperes Correct answer.
  4. 1,500 volt-amperes

Explanation

Table 220.42 requires the first 3,000 volt-amperes of general lighting demand in a dwelling unit to be calculated at 100 percent. Demand factors reduce only the portion of the load above 3,000 volt-amperes.

Question 15Electrical Theory and Code Knowledge

What is the purpose of bonding in an electrical system?

  1. To reduce voltage drop on feeders
  2. To limit the total current in a circuit
  3. To create a low-impedance path for fault current Correct answer.
  4. To balance loads among phases

Explanation

Bonding joins metal parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct any fault current safely. This low-impedance path allows overcurrent devices to clear faults quickly.

Question 16Electrical Theory and Code Knowledge

A motor nameplate shows a full-load current of 26 amperes at 230 volts. When sizing the branch-circuit short-circuit and ground-fault protective device for a standard motor, which value is used as the basis?

  1. The no-load current
  2. The locked-rotor current
  3. The value from the NEC motor tables Correct answer.
  4. The nameplate full-load current

Explanation

NEC 430.6(A)(1) requires motor full-load current for conductor and short-circuit protection sizing to be taken from Tables 430.247 through 430.250, not from the nameplate. Nameplate current is used only for overload protection sizing.

Question 17Electrical Theory and Code Knowledge

What is the ampacity of 12 AWG copper conductors with 90 degrees Celsius insulation when used in a 60 degrees Celsius termination environment?

  1. 20 amperes Correct answer.
  2. 25 amperes
  3. 35 amperes
  4. 30 amperes

Explanation

NEC 110.14(C) requires that conductor ampacity be limited to the temperature rating of the termination, not the insulation, unless the equipment is listed otherwise. Table 310.16 gives 12 AWG copper 20 amperes at 60 degrees Celsius, so 20 amperes is the usable ampacity.

Question 18Electrical Theory and Code Knowledge

Which NEC article covers the requirements for arc-fault circuit interrupter protection in dwelling units?

  1. Article 240
  2. Article 220
  3. Article 230
  4. Article 210 Correct answer.

Explanation

AFCI requirements for dwelling units appear in NEC 210.12, within Article 210, which governs branch circuits. The other articles cover load calculations, services, and overcurrent protection.

Question 19Electrical Theory and Code Knowledge

A 480-volt, three-phase circuit uses the formula I = P / (1.732 x E) to find current. If a balanced load of 60 kVA is connected, what is the line current?

  1. 36 amperes
  2. 72 amperes Correct answer.
  3. 125 amperes
  4. 50 amperes

Explanation

For a balanced three-phase circuit, current equals apparent power divided by the square root of three times line voltage. Here 60,000 VA divided by (1.732 times 480 V), or 831.4, gives approximately 72 amperes.

Question 20Electrical Theory and Code Knowledge

What is the primary function of the equipment grounding conductor in a grounded electrical system?

  1. To carry normal operating current
  2. To provide a low-impedance path for fault current back to the source Correct answer.
  3. To reduce the voltage drop on the neutral
  4. To provide a neutral connection for 120-volt loads

Explanation

The equipment grounding conductor bonds non-current-carrying metal parts to the system grounding point, creating a low-impedance path that lets fault current return to the source and trip the overcurrent device. It does not normally carry current.

Question 21Electrical Theory and Code Knowledge

A continuous load of 40 amperes is supplied by a feeder. What is the minimum feeder conductor ampacity required before adjustment and correction factors?

  1. 40 amperes
  2. 45 amperes
  3. 60 amperes
  4. 50 amperes Correct answer.

Explanation

NEC 215.2(A)(1) requires feeder conductors to have an ampacity of at least 125 percent of the continuous load plus 100 percent of the noncontinuous load. Forty amperes times 1.25 equals 50 amperes.

Question 22Electrical Theory and Code Knowledge

Which condition requires GFCI protection for receptacles in a dwelling unit under the NEC?

  1. Receptacles in a living room
  2. Receptacles in a bedroom
  3. Receptacles in a hallway
  4. Receptacles in a garage Correct answer.

Explanation

NEC 210.8(A) requires GFCI protection for receptacles in locations such as garages, outdoors, bathrooms, kitchens, and basements where personnel are more likely to contact ground. Bedrooms require AFCI protection, not GFCI, under 210.12.

Question 23Electrical Theory and Code Knowledge

A 25-ohm resistor and a 5-ohm resistor are connected in parallel across a 100-volt source. What is the total current drawn from the source?

  1. 30 amperes
  2. 20 amperes
  3. 24 amperes Correct answer.
  4. 4 amperes

Explanation

In a parallel circuit each branch sees the full source voltage, so branch currents are 100/25 = 4 amperes and 100/5 = 20 amperes. The total current is the sum, 24 amperes.

Question 24Electrical Theory and Code Knowledge

What is the minimum size of the grounding electrode conductor for a service supplied by 3/0 AWG copper service-entrance conductors?

  1. 4 AWG copper Correct answer.
  2. 1/0 AWG copper
  3. 2 AWG copper
  4. 6 AWG copper

Explanation

NEC Table 250.66 requires a 4 AWG copper grounding electrode conductor for service conductors sized 2 AWG through 3/0 AWG copper. Larger service conductors require larger grounding electrode conductors.

Question 25Electrical Theory and Code Knowledge

Which unit expresses the rate at which electrical energy is consumed?

  1. Coulomb
  2. Watt Correct answer.
  3. Ohm
  4. Volt

Explanation

The watt is the unit of electrical power, equal to one joule per second, and represents the rate of energy consumption. The coulomb measures charge, the volt measures potential difference, and the ohm measures resistance.

Question 26Electrical Theory and Code Knowledge

Which NEC article contains the requirements for wiring methods and materials, including conductor installation rules for general building wiring?

  1. Article 250
  2. Article 210
  3. Article 300 Correct answer.
  4. Article 430

Explanation

Article 300 covers general requirements for wiring methods and materials, including conductors entering raceways, supports, and protection from physical damage. Article 250 addresses grounding and bonding, Article 210 covers branch circuits, and Article 430 covers motors.

Question 27Electrical Theory and Code Knowledge

Which of the following best describes the relationship between current, voltage, and resistance in a resistive DC circuit?

  1. Current equals voltage divided by resistance Correct answer.
  2. Current equals resistance divided by voltage
  3. Current equals power divided by resistance
  4. Current equals voltage multiplied by resistance

Explanation

Ohm's law states that current in amperes equals voltage in volts divided by resistance in ohms. This relationship holds for resistive circuits and is the basis for most circuit calculations.

Question 28Electrical Theory and Code Knowledge

In a series circuit containing three resistors, which quantity is the same through every component?

  1. Voltage
  2. Current Correct answer.
  3. Power
  4. Resistance

Explanation

A series circuit has only one path for current, so the same current flows through every element. Voltage divides among the resistors in proportion to their resistance, and power differs with each resistor.

Question 29Electrical Theory and Code Knowledge

Which material property makes copper a preferred conductor material for building wiring?

  1. High resistivity combined with high melting point
  2. Low thermal conductivity combined with high hardness
  3. High magnetic permeability combined with low cost
  4. Low resistivity combined with good ductility Correct answer.

Explanation

Copper has low resistivity, so it carries current with limited voltage drop and heat, and it is ductile enough to be bent and pulled through raceways. These properties make it a standard building-wire conductor material.

Question 30Electrical Theory and Code Knowledge

What does the abbreviation AWG represent when sizing electrical conductors?

  1. Alternating Watt Gauge
  2. Ampere Wiring Grade
  3. Approved Wire Group
  4. American Wire Gauge Correct answer.

Explanation

AWG stands for American Wire Gauge, the standardized system used to designate conductor sizes in North America. Larger AWG numbers correspond to smaller conductor diameters.

Question 31Electrical Theory and Code Knowledge

Which device is intended to open a circuit automatically when current exceeds its rating for a specified time?

  1. Junction box
  2. Receptacle
  3. Switch
  4. Circuit breaker Correct answer.

Explanation

A circuit breaker is an overcurrent protective device that opens automatically when current exceeds its rating, protecting conductors from overheating. Receptacles, switches, and boxes are not overcurrent devices.

Question 32Electrical Theory and Code Knowledge

What is the unit of electrical resistance?

  1. Ohm Correct answer.
  2. Volt
  3. Watt
  4. Ampere

Explanation

Resistance is measured in ohms, represented by the Greek letter omega. The ampere measures current, the volt measures potential difference, and the watt measures power.

Question 33Electrical Theory and Code Knowledge

Which effect describes the heating of a conductor caused by current flowing through its resistance?

  1. Capacitive coupling
  2. Skin effect
  3. Inductive reactance
  4. I squared R loss Correct answer.

Explanation

Power dissipated as heat in a conductor equals the square of the current multiplied by resistance, commonly written as I squared R. This heating is a principal reason conductors are sized for ampacity.

Question 34Electrical Theory and Code Knowledge

A 120-volt DC circuit has a resistance of 24 ohms. What current flows in the circuit?

  1. 0.2 ampere
  2. 20 amperes
  3. 5 amperes Correct answer.
  4. 2,880 amperes

Explanation

Using Ohm's law, current equals voltage divided by resistance, so 120 volts divided by 24 ohms equals 5 amperes. The result is confirmed because 5 amperes times 24 ohms returns 120 volts.

Question 35Electrical Theory and Code Knowledge

A 12-ohm resistor and a 4-ohm resistor are connected in series across a 48-volt source. What is the voltage across the 12-ohm resistor?

  1. 36 volts Correct answer.
  2. 12 volts
  3. 24 volts
  4. 48 volts

Explanation

Total series resistance is 16 ohms, so current is 48 divided by 16, or 3 amperes. The voltage across the 12-ohm resistor is 3 amperes times 12 ohms, which equals 36 volts.

Question 36Electrical Theory and Code Knowledge

Which NEC article provides the requirements for services, including service-entrance conductors and service equipment?

  1. Article 240
  2. Article 230 Correct answer.
  3. Article 220
  4. Article 310

Explanation

Article 230 covers service requirements, including service-entrance conductors, service equipment, and service disconnecting means. The other articles address branch-circuit and feeder calculations, overcurrent protection, and conductor ampacities.

Question 37Electrical Theory and Code Knowledge

A conductor has an ampacity of 65 amperes before adjustment. It is installed in an ambient temperature that requires a 0.82 correction factor. What is the adjusted ampacity?

  1. 53.3 amperes Correct answer.
  2. 32.5 amperes
  3. 65 amperes
  4. 79.3 amperes

Explanation

The adjusted ampacity is found by multiplying the base ampacity by the correction factor: 65 times 0.82 equals 53.3 amperes. Ambient temperature correction reduces the allowable current because heat dissipation is reduced.

Question 38Electrical Theory and Code Knowledge

What is the primary purpose of a grounding electrode system at a service?

  1. To provide a low-resistance path for ground-fault current to open a breaker
  2. To reduce the ampacity required for service conductors
  3. To carry normal load current back to the source
  4. To connect the electrical system to earth to limit voltage surges and stabilize voltage to ground Correct answer.

Explanation

The grounding electrode system connects the grounded conductor to earth, which limits voltage surges from lightning and line faults and stabilizes voltage to ground. It is not intended to carry normal load current or to clear ground faults.

Question 39Electrical Theory and Code Knowledge

Which of these conductors is permitted to be used as an equipment grounding conductor in a flexible cord assembly?

  1. The insulated conductor designated as the grounding conductor in the cord Correct answer.
  2. The conductor with the smallest cross-sectional area in the cord
  3. The neutral conductor of the cord
  4. Any conductor in the cord that is not carrying load current

Explanation

In a flexible cord, the equipment grounding conductor is the specific conductor identified for that purpose, typically with green insulation or a distinctive finish. It must be connected to the equipment grounding terminal and not used to carry load current.

Question 40Electrical Theory and Code Knowledge

A 240-volt, single-phase branch circuit supplies a 9-kilowatt fixed appliance. What is the minimum branch-circuit conductor ampacity required before any adjustment factors?

  1. 37.5 amperes Correct answer.
  2. 75 amperes
  3. 21.6 amperes
  4. 18.75 amperes

Explanation

For a single-phase load, current equals power divided by voltage: 9,000 watts divided by 240 volts equals 37.5 amperes. Branch-circuit conductors must have an ampacity at least equal to this calculated load.

Question 41Electrical Theory and Code Knowledge

What is the minimum number of 20-ampere small-appliance branch circuits required for a dwelling unit kitchen?

  1. Four
  2. One
  3. Two Correct answer.
  4. Three

Explanation

The NEC requires at least two 20-ampere small-appliance branch circuits to serve receptacle outlets in the kitchen, pantry, and similar areas. These circuits supply countertop receptacles and related small-appliance loads.

Question 42Electrical Theory and Code Knowledge

A 3-phase, 480-volt motor draws 42 amperes. Using the formula P = 1.732 x E x I, what is the approximate apparent power?

  1. 34.9 kVA Correct answer.
  2. 20.2 kVA
  3. 60.5 kVA
  4. 10.1 kVA

Explanation

Apparent power equals 1.732 times voltage times current, so 1.732 times 480 volts times 42 amperes equals approximately 34,900 volt-amperes, or 34.9 kVA. The 1.732 factor accounts for the square root of three in a balanced three-phase system.

Question 43Electrical Theory and Code Knowledge

Which of the following describes the function of a bonding jumper in an electrical installation?

  1. It reduces the voltage of the circuit
  2. It carries the full load current of the circuit
  3. It provides overcurrent protection for the circuit
  4. It ensures electrical continuity between metal parts that must be connected together Correct answer.

Explanation

A bonding jumper is a conductor that ensures electrical continuity between metal parts that are required to be electrically connected. It equalizes potential between parts so that fault current has a low-impedance path back to the source.

Question 44Electrical Theory and Code Knowledge

What is the purpose of a disconnecting means at a service entrance?

  1. To convert alternating current to direct current
  2. To provide overcurrent protection for branch circuits
  3. To meter the energy consumed
  4. To isolate the service equipment from the supply for maintenance and emergency control Correct answer.

Explanation

The service disconnecting means allows the service equipment to be isolated from the supply so that maintenance and emergency work can be performed safely. It does not itself provide branch-circuit overcurrent protection or metering.

Question 45Electrical Theory and Code Knowledge

Which of the following describes the effect of a short circuit in a branch circuit?

  1. A high-resistance path causes current to drop below normal levels
  2. The voltage at the source increases to compensate
  3. The circuit continues to operate normally because the load limits current
  4. A low-resistance path causes very high current that should operate the overcurrent device quickly Correct answer.

Explanation

A short circuit creates a low-resistance path between conductors, so current rises far above normal and the overcurrent device should open quickly. The heat and magnetic forces from such current can damage conductors if protection does not respond.

Question 46Electrical Theory and Code Knowledge

A 480-volt, three-phase feeder carries 150 amperes. If the conductors have a total resistance of 0.05 ohm per conductor, what is the approximate voltage drop per phase conductor?

  1. 7.5 volts Correct answer.
  2. 0.33 volt
  3. 22.5 volts
  4. 15 volts

Explanation

Voltage drop in a conductor equals current times resistance, so 150 amperes times 0.05 ohm equals 7.5 volts. For a single-phase calculation the line-to-line drop would be twice this value, but the drop across one conductor is 7.5 volts.

Question 47Electrical Theory and Code Knowledge

A feeder supplies a continuous load of 50 amperes and a noncontinuous load of 40 amperes. What is the minimum feeder overcurrent device rating required, assuming the device is rated for 100 percent of its rating?

  1. 90 amperes
  2. 80 amperes
  3. 125 amperes
  4. 102.5 amperes Correct answer.

Explanation

Continuous loads are calculated at 125 percent, so 50 times 1.25 equals 62.5 amperes, plus the 40-ampere noncontinuous load, giving 102.5 amperes. The overcurrent device must be at least this calculated value before standard-size rounding.

Question 48Electrical Theory and Code Knowledge

A grounded electrical system has a ground fault in equipment. Which path is intended to carry the fault current back to the source so the overcurrent device opens?

  1. The earth through the grounding electrode
  2. The metallic water piping alone
  3. The equipment grounding conductor and the system bonding jumper Correct answer.
  4. The neutral conductor of an unrelated circuit

Explanation

Fault current returns to the source through the low-impedance equipment grounding conductor and the system bonding jumper, which allows the overcurrent device to open. Earth itself is not a reliable low-impedance return path for fault current.

Question 49Electrical Theory and Code Knowledge

Why must conductors be derated when more than three current-carrying conductors are installed in a single raceway?

  1. Because the voltage of the circuit increases with more conductors
  2. Because reduced heat dissipation raises conductor operating temperature Correct answer.
  3. Because the neutral conductor always counts as a current-carrying conductor
  4. Because the resistance of each conductor decreases with bundling

Explanation

When conductors are bundled in a raceway, heat from each conductor is less able to dissipate, so conductor temperature rises for a given current. Derating limits the current to keep insulation within its temperature rating.

Question 50Electrical Theory and Code Knowledge

A 120-volt circuit has a load of 1,440 watts. What is the current, and what size copper conductor is typically the minimum for a 20-ampere branch circuit?

  1. 14.4 amperes; 14 AWG copper
  2. 1.2 amperes; 10 AWG copper
  3. 10 amperes; 14 AWG copper
  4. 12 amperes; 12 AWG copper Correct answer.

Explanation

Current equals 1,440 watts divided by 120 volts, or 12 amperes. A 20-ampere branch circuit generally requires at least 12 AWG copper conductors, which have an ampacity of 20 amperes at 60 degrees Celsius.