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.2,400 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 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.Because the safeties are in parallel with the gas valve, the valve still receives a reduced voltage and opens partially.
   • B.A fault code is always set when a safety opens, so the absence of a code rules the rollout switch out.
   • C.In a series string one open device stops current through the entire rung, so the gas valve is simply never energized; the technician measures across each control in turn to find the one dropping the full control voltage.✓ Answer
   • D.The open rollout switch shorts the low-voltage circuit, so the transformer fuse should be replaced first.

   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, but at half voltage and half speed.
   • B.The remaining motor now sees twice the applied voltage and overheats.
   • C.The remaining motor keeps running at full supply voltage, and the total current drawn from the supply drops.✓ Answer
   • D.Both motors stop, because current in a parallel circuit is the same in every branch.

   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 has high resistance — a good closed contact drops close to zero volts, while an open device in a circuit that still has a load drops the full source voltage.✓ Answer
   • B.The switch is closed and passing current normally; a good closed contact drops the full source voltage.
   • C.The meter is on the wrong function; a closed switch cannot be checked with the power on.
   • D.The load downstream is shorted, which is the only condition that can put source voltage 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 before any rung can operate.
   • B.The equipment grounding conductor and the chassis bonding path.
   • C.The physical left and right walls of the electrical cabinet, so components can be located by touch.
   • D.The power supply to the circuit — for example L1 and L2, or the hot and common sides of the control transformer.✓ 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 intentionally bonded to the housing for safety.
   • B.An open winding, because an open winding always reads zero ohms to the housing.
   • C.A shorted run capacitor, which always reads to ground through its case.
   • D.A grounded winding: the winding insulation has failed to the housing, giving fault current a path to ground that trips the breaker, and the compressor must be replaced.✓ 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; the contacts it closes belong to the load (power) circuit that carries the compressor's line current.✓ Answer
   • B.The control and load circuits are the same circuit; the contactor simply steps the voltage down for the compressor.
   • C.The 24 volt coil and the compressor are both part of the load circuit, since both carry working current.
   • D.The contacts carry the 24 volt control current while the coil carries the compressor's 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, whose bimetal contacts are warmed by a small heater so each stage closes after a delay; staggering the stages keeps the entire element load and its inrush from being applied at one instant.✓ Answer
   • B.A contactor, because its coil takes several seconds to build a magnetic field strong enough to pull in.
   • C.A high-limit switch, which opens and recloses repeatedly to bring in each element in turn.
   • D.A potential relay, because it drops the elements out as back-EMF rises.

   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 DC.
    • B.Yes, as long as the DC voltage matches the AC primary rating stamped on the transformer.
    • C.No, because DC would reverse the polarity of the secondary output.
    • D.No — a transformer transfers energy by mutual induction, which requires a continuously changing current; steady DC produces no secondary output and can overheat the primary winding.✓ 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.The neutral is carrying a partial short to ground, which halves one of the readings.
    • B.The utility transformer secondary is center-tapped and that center tap is the neutral, so a voltage reading is only meaningful when the two conductors it was taken between are named — leg to leg, or leg to neutral.✓ Answer
    • C.The two hot legs are supplied at different frequencies.
    • D.One leg is alternating current and the other is direct current.

    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 an inductive load, so its current lags the voltage and the power factor is less than one — true power in watts is therefore lower than the apparent power in volt-amps.✓ Answer
    • B.The motor is a resistive load, and resistive loads always show fewer watts than volt-amps.
    • C.The clamp meter must be defective, because volts multiplied by amps always equals watts.
    • D.The missing volt-amps are lost as heat in the supply conductors, which the power meter does not include.

    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, the same way a PSC motor is reversed.
    • C.Reverse the two low-voltage thermostat wires so the contactor pulls in on the opposite phase.
    • 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.The motor runs normally at reduced torque with no measurable change in current.
    • B.It keeps turning but draws sharply higher current on the two remaining legs, and once it stops it will not restart; the overload or a phase-loss monitor should take it off line.✓ Answer
    • C.The motor stops immediately 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 feeding the contactor coil.
    • B.A seized bearing, since a humming motor always indicates a mechanical failure.
    • C.Reversed line leads, which prevent a single-phase motor from starting.
    • D.A failed (open) run capacitor: without it the start winding receives no phase-shifted current, so the motor cannot develop a rotating field or starting torque and stalls at locked-rotor current.✓ 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 the start capacitor mechanically as the rotor comes up to speed.
    • B.The start capacitor is rated for momentary duty only; a potential (voltage) relay senses the rising back-EMF of the start winding and opens its normally closed contacts to drop the start capacitor out, while the run capacitor stays in circuit.✓ Answer
    • C.The start capacitor stays in the circuit continuously, and the run capacitor drops out once the motor reaches speed.
    • D.Both capacitors remain in the circuit continuously; the potential relay only protects them from 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 the start winding.
    • B.Terminal 3 — the highest reading (2 to 3) is start to run, so terminal 1 must be common; from common, the higher of the two remaining readings identifies the start winding, which has more turns of smaller wire and therefore higher resistance.✓ Answer
    • C.Terminal 1, because common is always the terminal with the single highest reading.
    • D.It cannot be determined without energizing the compressor and measuring amperage on 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 any other check is made.
    • B.A constant-airflow ECM holds its programmed airflow by raising rpm and watt draw as external static pressure rises, so the higher watts point to increased system restriction rather than a failing motor.✓ Answer
    • C.The supply voltage has dropped, and a lower supply voltage always makes an ECM run faster.
    • D.The motor's permanent magnets have weakened, which always causes higher 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.Because the supply voltage rises whenever a motor stalls, and higher voltage means higher current.
    • B.Because the resistance of a stalled winding rises sharply as it heats, and higher resistance draws more current.
    • C.Because the run capacitor discharges continuously into the winding while the rotor is stopped.
    • D.A turning rotor generates counter-EMF that opposes the applied voltage and limits current; with the rotor stopped no counter-EMF is produced, so only the low resistance of the winding itself limits current — this is locked-rotor 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 →