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Every HVAC system is controlled and powered by electricity. A technician who cannot diagnose electrical problems will struggle to efficiently service any modern heating or cooling equipment. The NATE Core exam dedicates approximately 20% of its questions to basic electricity - making it the single largest topic area.
This module covers the fundamental electrical concepts you must master: voltage, current, resistance, power, circuit types, and the instruments used to measure them.
Voltage is the electrical pressure that pushes current through a circuit. Think of it like water pressure in a pipe. Measured in volts (V). Common voltages in HVAC:
| Application | Voltage | Notes |
|---|---|---|
| Control circuits (thermostats) | 24V AC | From control transformer secondary |
| Single-phase residential power | 120V AC / 240V AC | Standard household circuits |
| Three-phase commercial power | 208V / 240V / 480V | Large commercial/industrial equipment |
| Low-voltage signals | 5-12V DC | Electronic controls, sensors |
Current is the flow of electrons through a conductor. Measured in amperes (amps, A). In HVAC, current draw tells you how hard a motor or component is working. A motor drawing more than its rated full-load amps (FLA) is being overworked and will overheat.
Resistance is opposition to current flow. Measured in ohms (?). Every component in a circuit has resistance. When resistance increases, current decreases (for the same voltage). Key applications:
Ohm's Law defines the relationship between voltage, current, and resistance:
V = I ? R (Voltage = Current ? Resistance)
Rearranged: I = V / R ?? R = V / I
Example: A 240V circuit has a load with 24? of resistance. Current = 240 ? 24 = 10 amps.
Power (measured in watts, W) is the rate at which electrical energy is consumed:
P = V ? I (Power = Voltage ? Current)
Also: P = I? ? R ?? P = V? / R
Example: A 240V air conditioner draws 15 amps. Power = 240 ? 15 = 3,600 watts = 3.6 kW.
In a series circuit, components are connected end-to-end in a single path. Current has only one path to flow.
HVAC application: Safety switches in a furnace control circuit are wired in series - if any safety opens (high limit, pressure switch, rollout), the entire circuit breaks and the furnace shuts down.
In a parallel circuit, components are connected across the same two points, providing multiple current paths.
HVAC application: The compressor, condenser fan, and other loads are wired in parallel to the 240V power supply - each operates at full voltage independently.
| Property | AC (Alternating Current) | DC (Direct Current) |
|---|---|---|
| Current direction | Reverses direction periodically | Flows in one direction only |
| Frequency (US) | 60 Hz (60 cycles/second) | No frequency - steady |
| HVAC use | Power distribution, motors, controls | Electronic controls, variable-speed drives |
| Voltage measurement | RMS (root mean square) | Actual measured value |
The most essential diagnostic tool for HVAC technicians. A multimeter measures:
Never measure resistance on an energized circuit - this will damage the meter and risk electrocution. Always verify the circuit is de-energized with a voltage measurement before taking resistance readings.
Measures current without breaking the circuit by clamping around a single conductor. The clamp senses the magnetic field produced by current flow. Key tips:
Capacitors are among the most commonly replaced HVAC components. They store electrical charge and serve two functions:
Start Capacitors: Provide an extra boost of current during motor startup, then disconnect from the circuit. Found on motors that need high starting torque.
Run Capacitors: Remain in the circuit during operation, improving motor efficiency and power factor. If a run capacitor fails, the motor may:
Rated in microfarads (?F or MFD). A capacitor reading more than ?6% outside its rated value should be replaced.
Capacitors store charge and can deliver a dangerous shock even after power is disconnected. Always discharge capacitors using an insulated discharge tool or a 20,000-ohm resistor before handling or testing.
Contactors are heavy-duty electromagnetic switches that control high-voltage loads (compressors, condenser fans). A low-voltage control signal (24V) energizes the contactor coil, which closes the high-voltage contacts supplying power to the load.
Relays are similar but handle smaller loads. Used extensively in control circuits to allow a low-voltage thermostat signal to switch higher-current loads like blower motors and electric heat strips.
Know these formulas cold: V=IR, P=VI. Know that in series circuits current is the same and voltage divides; in parallel circuits voltage is the same and current divides. Know that capacitors are rated in microfarads and that run capacitors stay in the circuit during operation while start capacitors disconnect after starting.