20 Basic Electrical Practice Questions & Answers

1. 1. A technician measures 240 volts across an electric resistance heating element and clamps 10 amps on the single conductor feeding it. What is the resistance of the element?

   • A.240 ohms
   • B.2400 ohms
   • C.24 ohms✓ Answer
   • D.2.4 ohms

   Ohm's law states E = I x R, so R = E / I. Dividing the 240 volts measured across the element by the 10 amps flowing through it gives 24 ohms. Note that resistance here is calculated from live readings rather than measured with an ohmmeter, because an ohmmeter supplies its own voltage and must never be connected to an energized circuit.

   Source: NATE Core knowledge areas, Using Basic Science — Ohm's law and basic circuit analysisReport a problem with this question

2. 2. A technician needs to know the current draw of a 4,800 watt electric heat element that operates at 240 volts, in order to compare it with a clamp-on reading. What current should the element draw?

   • A.11.5 amps
   • B.2.0 amps
   • C.200 amps
   • D.20 amps✓ Answer

   The power relationship is P = E x I, so I = P / E. Dividing 4,800 watts by 240 volts gives 20 amps. Because a heating element is a purely resistive load, its power factor is one and watts equal volt-amps, so this simple calculation matches the clamp-on reading closely.

   Source: NATE Core knowledge areas, Using Basic Science — power formulas and electrical load typesReport a problem with this question

3. 3. In a furnace control circuit the thermostat contacts, a rollout switch, a high limit and the gas valve are all wired in series on one rung. The rollout switch has opened. The unit does nothing at all and no fault is indicated. Which statement best explains the symptom and the next step?

   • A.The safeties parallel the gas valve, so it still opens on reduced voltage.
   • B.An open safety always sets a fault code, so the rollout switch is cleared.
   • C.One open device stops current in the whole rung; measure across each control.✓ Answer
   • D.The open rollout shorts the low-voltage circuit; replace the transformer fuse.

   Safety controls are deliberately wired in series with the load they protect, so that any one of them opening interrupts current to that load. Series current is the same everywhere in the string, which is why a single open device produces a dead unit rather than a partial operation, and why an open safety often shows up as no operation with no message. The open device is the one that will read the full source voltage across its terminals while every closed device reads near zero.

   Source: NATE Core knowledge areas, Basic Electricity — schematic (ladder) diagrams and series safety controlsReport a problem with this question

4. 4. Two condenser fan motors are wired in parallel across the same 240 volt supply. One motor's winding opens. What happens?

   • A.The remaining motor runs at half voltage and half speed.
   • B.The remaining motor now sees twice the voltage and overheats.
   • C.The remaining motor runs at full voltage; total current drops.✓ Answer
   • D.Both motors stop, since parallel branches share one current.

   In a parallel circuit every branch sees the same applied voltage, and the branch currents add to make the total. Losing one branch therefore removes that branch's current from the total but changes nothing about the voltage across the surviving branch. This is why loads are wired in parallel across the supply while switches and safety controls are wired in series with their own load.

   Source: NATE Core knowledge areas, Using Basic Science — series and parallel circuit analysisReport a problem with this question

5. 5. With the thermostat calling and the equipment powered, a technician places the meter leads across the two terminals of a pressure switch that the schematic shows should be closed at this point in the sequence, and reads the full control voltage. What does this indicate?

   • A.The switch is open or high-resistance; a good contact drops near zero volts.✓ Answer
   • B.The switch is closed and conducting; a good contact drops full source voltage.
   • C.The meter is on the wrong function; a closed switch cannot be checked live.
   • D.The load downstream is shorted, the only way voltage appears across a switch.

   This is the core voltage-drop rule of electrical troubleshooting. A closed contact has almost no resistance, so almost no voltage is dropped across it; an open contact interrupts the circuit and the entire source voltage appears across the break as long as the circuit still contains a load to complete the path. Measurable voltage across a contact that should be closed therefore proves it is open or badly pitted, not that it is working.

   Source: NATE Core knowledge areas, Basic Electricity — digital electrical meters and voltage-drop troubleshootingReport a problem with this question

6. 6. On a ladder (schematic) diagram, what do the two vertical lines drawn down the left and right sides of the page represent?

   • A.Two separate loads that must both be energized for a rung to work.
   • B.The equipment grounding conductor and the chassis bonding path.
   • C.The left and right walls of the cabinet, so parts can be located.
   • D.The power supply for every rung — L1 and L2, or hot and common.✓ Answer

   The two vertical rails of a ladder diagram are the power source for every rung — L1 and L2 on a line-voltage ladder, or the hot and common sides of the control transformer on a low-voltage ladder. Each horizontal rung between them is one complete circuit containing exactly one load at the right-hand rail, with the switches and safety contacts that control it drawn in series to its left. Because the rungs are drawn in order of operation, the ladder is the diagram type best suited to tracing sequence of operation, unlike a pictorial diagram, which shows physical location instead.

   Source: NATE Core knowledge areas, Basic Electricity — schematic (ladder) diagramsReport a problem with this question

7. 7. A compressor trips the breaker instantly on every start attempt. With the disconnect open, the capacitor discharged and the compressor leads removed, the technician reads near zero ohms from one motor terminal to the compressor housing. What does this indicate?

   • A.Nothing abnormal; compressor windings are bonded to the housing.
   • B.An open winding, since an open winding reads zero ohms to ground.
   • C.A shorted run capacitor, which reads to ground through its case.
   • D.A winding is grounded to the housing; replace the compressor.✓ Answer

   A motor winding is supposed to be electrically isolated from the metal housing, so a healthy motor reads extremely high resistance — megohms — from any terminal to the frame. Continuity to the housing means the insulation has broken down and current is finding a path to ground, which is exactly what an overcurrent device or ground-fault protection responds to. Note the distinction the technician is drawing: an open is a break in the winding itself, a short is an unintended path between windings or turns, and a ground is a path from a winding to the frame.

   Source: NATE Core knowledge areas, Basic Electricity — motor testing with digital meters, opens, shorts and groundsReport a problem with this question

8. 8. A 24 volt signal from the thermostat energizes a coil, and the coil pulls in a set of heavy contacts that feed line voltage to the compressor. Which statement describes this arrangement correctly?

   • A.The 24 volt coil circuit is the control circuit; its contacts carry the load.✓ Answer
   • B.Control and load are one circuit; the contactor steps the voltage down.
   • C.The coil and the compressor are both loads, since both carry work current.
   • D.The contacts carry the 24 volt control current; the coil carries line current.

   A contactor or relay is the bridge between two electrically separate circuits: a small control circuit energizes the coil, and the magnetic field the coil produces closes contacts that switch a much larger load circuit. That separation is why a low-voltage thermostat can safely switch a line-voltage compressor, and why a coil and its contacts carry the same identifying letter on a schematic even when they are drawn on rungs far apart. A contactor is simply a heavy-duty relay sized for line-current loads, while an overload is a protective device that opens on excessive current rather than a switching device commanded by a control signal.

   Source: NATE Core knowledge areas, Basic Electricity — control circuits versus load circuits, contactors and relaysReport a problem with this question

9. 9. An electric furnace energizes its heating elements one stage at a time over a period of seconds rather than all at once. Which device does this, and why is it done?

   • A.A sequencer: heated bimetal contacts close in stages, limiting inrush.✓ Answer
   • B.A contactor, whose coil needs several seconds to build its field.
   • C.A high-limit switch that opens and recloses to add elements in turn.
   • D.A potential relay, which cuts elements out as back-EMF builds.

   A sequencer is a time-delay switching device: current through a small heater warms a bimetal element, which bends and closes the contacts after a delay, and the stages are staggered by using several such contact sets. Bringing the resistive heat load on in steps limits the momentary current surge and the mechanical and voltage disturbance that would result from switching the whole load at once. The delay is thermal, not magnetic, which is the key difference between a sequencer and an ordinary relay or contactor.

   Source: NATE Core knowledge areas, Basic Electricity — electric heat sequencers, relays and contactorsReport a problem with this question

10. 10. A control transformer is marked 24 volt secondary, 40 VA. What is the maximum current its secondary can supply?

    • A.About 0.6 amps
    • B.About 960 amps
    • C.About 1.67 amps✓ Answer
    • D.About 40 amps

    VA is volt-amps, the apparent power the transformer can deliver, so the secondary current limit is VA divided by secondary volts: 40 divided by 24 is about 1.67 amps. Adding low-voltage accessories that push the total coil and board current beyond that figure will overheat the winding or open its protective fuse, which is why the VA rating is checked before adding relays, valves or dampers to an existing control circuit.

    Source: NATE Core knowledge areas, Using Basic Science — transformers and control voltageReport a problem with this question

11. 11. A technician asks whether a DC power supply can be used on the primary of a control transformer for a bench test. What is the correct answer?

    • A.Yes; a transformer works equally well on AC or on DC supply.
    • B.Yes, if the DC volts match the AC primary rating on the label.
    • C.No, because steady DC would reverse the secondary's polarity.
    • D.No; mutual induction needs changing current, and DC is steady.✓ Answer

    Mutual induction depends on a magnetic field that is expanding and collapsing, which only an alternating current provides. With steady DC the field becomes static after the first instant, so no voltage is induced in the secondary, while the primary is left with only its own low winding resistance to limit current and will overheat. This is also why a step-down control transformer, whose turns ratio sets the voltage ratio, is strictly an AC device.

    Source: NATE Core knowledge areas, Using Basic Science — magnetism and mutual induction in transformersReport a problem with this question

12. 12. On a single-phase service a technician reads 240 volts between the two hot legs and 120 volts from either hot leg to the neutral. What explains the two different readings?

    • A.A partial short to ground on the neutral is halving one of the readings.
    • B.The utility secondary is center-tapped, and that tap is the neutral.✓ Answer
    • C.The two hot legs are being supplied at two different frequencies.
    • D.One leg carries alternating current and the other carries direct.

    Voltage is a difference of potential between two points, so it is never a property of a single wire — it always has to be stated between named conductors. A center-tapped single-phase secondary puts the full winding voltage across the two ends and half of it from either end to the center tap, which is grounded and used as the neutral. The same principle applies to three-phase systems, where a technician must say whether a reading is line to line or line to neutral before it means anything.

    Source: NATE Core knowledge areas, Using Basic Science — electricity generation and distributionReport a problem with this question

13. 13. A technician clamps 8 amps on a motor running at 240 volts, but a power meter connected to the same motor reports fewer watts than 240 multiplied by 8. What is the explanation?

    • A.A motor is inductive: current lags voltage, so watts are below volt-amps.✓ Answer
    • B.A motor is resistive, and resistive loads show fewer watts than volt-amps.
    • C.The clamp meter is defective, since volts times amps always equals watts.
    • D.The missing volt-amps are heat in the supply wires, which the meter omits.

    Power factor is true power in watts divided by apparent power in volt-amps. In a purely resistive load such as an electric heat strip, current and voltage rise and fall together, power factor is one and watts equal volt-amps. In an inductive load such as a motor winding, transformer or solenoid coil, the collapsing and building magnetic field makes current lag voltage, so the two are out of step and the product of the meter readings overstates the real work being done. Capacitance has the opposite effect, making current lead, which is why capacitance is used to offset lagging motor power factor.

    Source: NATE Core knowledge areas, Using Basic Science — electrical load types and power factorReport a problem with this question

14. 14. After a compressor change-out, a three-phase condenser fan motor is turning backward. What is the correct correction?

    • A.Interchange any two of the three line leads feeding the motor.✓ Answer
    • B.Swap the start and run winding leads, as when reversing a PSC.
    • C.Reverse the two low-voltage thermostat wires at the contactor.
    • D.Add a run capacitor to the third phase to shift its rotation.

    A three-phase motor develops a rotating magnetic field from the fixed 120-degree spacing of its three phases, and the direction of that rotation is set by the order in which the phases arrive. Swapping any two of the three line leads reverses the phase sequence and therefore the direction of rotation. A three-phase motor has no start winding, start capacitor, centrifugal switch or start relay to swap, which is why the single-phase reversal methods do not apply. Verify rotation with the disconnect locked out before and after the change, and re-check current on all three legs after the correction.

    Source: NATE Core knowledge areas, Basic Electricity — three phase motorsReport a problem with this question

15. 15. A three-phase compressor was running normally when one line fuse opened. What behavior should the technician expect?

    • A.It runs normally at reduced torque with no measurable current change.
    • B.It keeps turning, but current on the two remaining legs climbs sharply.✓ Answer
    • C.The motor stops at once and the two remaining legs read zero current.
    • D.The motor speeds up, because it now has less load applied to it.

    Losing one leg is called single-phasing. A motor already turning has enough momentum and residual field to keep running, but it must now produce the same work from two conductors instead of three, so current on those two rises steeply and the windings overheat quickly. A stopped motor cannot start at all, because two legs alone cannot produce the rotating field. This is also why voltage unbalance matters: a small percentage of voltage unbalance produces a much larger percentage of current unbalance, so the technician should measure voltage and current on all three legs rather than only on the leg that looks suspect.

    Source: NATE Core knowledge areas, Basic Electricity — three phase motors and phase lossReport a problem with this question

16. 16. A PSC condenser fan motor hums but will not turn. With power off, the shaft spins freely by hand. The technician discharges and disconnects the capacitor and finds it reads open. When power is restored the motor draws locked-rotor current until the overload opens. What is the most likely cause?

    • A.A shorted thermostat energizing the contactor coil continuously.
    • B.A seized bearing, since a humming motor is always a mechanical fault.
    • C.Reversed line leads, which keep a single-phase motor from starting.
    • D.An open run capacitor: no phase shift, so no starting torque.✓ Answer

    A permanent split capacitor motor keeps its run capacitor permanently in series with the start winding, and that capacitor is what shifts the start-winding current out of phase with the run-winding current to create a rotating field. Lose the capacitor and the motor is left with a single pulsating field, which produces vibration and noise but no starting torque, so it sits stalled and draws locked-rotor current. The free-spinning shaft rules out a mechanical seizure, and the capacitor must always be discharged through a resistor and disconnected before it is measured, never shorted with a screwdriver.

    Source: NATE Core knowledge areas, Basic Electricity — single phase motors and run capacitorsReport a problem with this question

17. 17. A hermetic compressor uses both a start capacitor and a run capacitor. Which statement correctly describes the start capacitor and how it is taken out of the circuit?

    • A.A centrifugal switch inside the sealed compressor removes it at speed.
    • B.A potential relay senses rising back-EMF and opens to drop it out.✓ Answer
    • C.The start capacitor stays in circuit; the run capacitor drops out.
    • D.Both stay in circuit; the potential relay only guards overvoltage.

    A start capacitor has a large capacitance for a big boost of starting torque, but it is built for a few seconds of duty at a time and will fail if left energized. As the rotor accelerates, the start winding generates a rising counter-EMF; the potential relay's coil senses that rising voltage and opens its normally closed contacts, dropping the start capacitor out of the circuit. A run capacitor is a much smaller, continuously rated, oil-filled unit that stays in series with the start winding while the motor runs. A hermetic compressor is sealed, so no centrifugal switch can be used inside it — that is a feature of open split-phase motors.

    Source: NATE Core knowledge areas, Basic Electricity — single phase motors, start capacitors and potential relaysReport a problem with this question

18. 18. With power off, the capacitor discharged and all leads removed, a technician measures across the three terminals of a hermetic compressor: terminals 1 to 2 read 2 ohms, terminals 1 to 3 read 5 ohms, and terminals 2 to 3 read 7 ohms. Which terminal is Start?

    • A.Terminal 2, because the run winding has more resistance than start.
    • B.Terminal 3: the 2-to-3 reading is start to run, so 1 is common.✓ Answer
    • C.Terminal 1, since common shows the single highest reading of all.
    • D.It cannot be told without energizing and clamping each terminal.

    On a three-terminal hermetic compressor the three readings are common to run, common to start, and start to run, and the last of these is the sum of the other two — here 2 plus 5 equals 7, which confirms the readings are consistent. The largest reading is always start to run, so the terminal not involved in it is common: that is terminal 1. From common, the smaller reading is the run winding and the larger is the start winding, because the start winding is wound with more turns of smaller-diameter wire and therefore has the higher resistance. This point is frequently stated backwards in low-quality study material.

    Source: NATE Core knowledge areas, Basic Electricity — single phase motors and hermetic winding identificationReport a problem with this question

19. 19. A constant-airflow ECM (electronically commutated motor) blower is drawing noticeably more watts and turning faster than it did when the system was commissioned. What does this most likely indicate?

    • A.The control module is failing and should be replaced before other checks.
    • B.Higher static pressure; a constant-airflow ECM adds rpm and watts to hold cfm.✓ Answer
    • C.The supply voltage has dropped, and low voltage makes an ECM run faster.
    • D.The motor's permanent magnets have weakened, which always raises the rpm.

    An ECM is a brushless permanent-magnet motor whose onboard module electronically commutates the windings, so it is commanded by a low-voltage signal rather than by line-voltage speed taps. A constant-airflow model is programmed to deliver a target airflow and will increase rpm, torque and watt draw to hold it as resistance in the system increases, so rising watts are a diagnostic clue about the system, not proof that the motor is bad. Because the module drives the windings electronically, an ECM must never be bench-tested by applying line voltage directly to the motor windings.

    Source: NATE Core knowledge areas, Basic Electricity — variable speed motors (ECM)Report a problem with this question

20. 20. A single-phase motor is stalled and drawing several times its normal running amperage. Why is the current so high?

    • A.Supply voltage rises whenever a motor stalls, and that raises current.
    • B.The stalled winding's resistance climbs as it heats, drawing more current.
    • C.The run capacitor discharges into the winding while the rotor is stopped.
    • D.With the rotor stopped there is no counter-EMF left to limit current.✓ Answer

    A running motor is not current-limited by winding resistance alone. As the rotor turns through the stator field it induces a voltage that opposes the applied voltage, called counter-EMF or back-EMF, and it is that opposition that holds running current down. At zero rpm there is no counter-EMF, so the only limit is the very low resistance of the copper winding, and current jumps to locked-rotor value — a multiple of running amps. Locked-rotor current is normal for the instant of starting but must not persist, which is why the overload opens and why low supply voltage, which slows a motor and increases current, is itself a cause of overheating.

    Source: NATE Core knowledge areas, Using Basic Science — counter-EMF, locked rotor and running currentReport a problem with this question

Practice questions based on the published NATE knowledge areas and standard HVACR theory and service practice. NATE is a mark of North American Technician Excellence; this site is not affiliated with or endorsed by NATE. NATE certification is not a licence — HVAC licensing is set by your state and locality. Confirm current exam requirements with NATE and current code requirements with the authority having jurisdiction. About NATE certification →