Electrical Fundamentals for HVAC
Ohm's law, wiring, capacitors, contactors, transformers, motor types, and circuit sizing for HVAC equipment.
- Apply Ohm's law to calculate voltage, current, and resistance in HVAC circuits
- Explain the function of capacitors, contactors, relays, and transformers
- Compare PSC and ECM motor characteristics and troubleshooting
- Read equipment nameplates to determine MCA and MOP values
- Identify the purpose and operation of sequencers and hard start kits
Lesson 1
Ohm's Law, Voltage, Current & Resistance
The Foundation of Electrical Troubleshooting
Electrical problems are the most common service calls in HVAC. Every technician must understand the basic electrical relationships to diagnose motors, compressors, contactors, and control circuits safely and accurately.
Ohm's Law
Ohm's law states: V = I x R (Voltage equals Current times Resistance).
This is the fundamental electrical formula. From it, you can derive:
- I = V / R (Current equals Voltage divided by Resistance)
- R = V / I (Resistance equals Voltage divided by Current)
Voltage, Current, and Resistance
Voltage (V or E) is electrical pressure - the force that pushes electrons through a circuit. Residential HVAC systems typically operate at 120 VAC or 240 VAC for power circuits.
Current (I or A) is the flow of electrons through a conductor, measured in amperes (amps). Higher current means more electron flow and more work being done.
Resistance (R) is opposition to current flow, measured in ohms. Every component in a circuit has some resistance. Higher resistance means less current for a given voltage.
Control Voltage
HVAC systems use 24 VAC as the standard low-voltage control voltage. A step-down transformer converts line voltage (120 or 240 VAC) to 24 VAC to power the thermostat circuit, gas valve, contactor coil, and other control components.
Safety First
Always verify voltage is absent before working on electrical components. Use a multimeter to test - never assume power is off. Line voltage (120/240 VAC) can be lethal. Even 24 VAC can cause burns under certain conditions.
Series vs Parallel Circuits
| Property | Series Circuit | Parallel Circuit |
|---|---|---|
| Current | Same through all components | Divides among branches |
| Voltage | Divides across components | Same across all branches |
| Resistance | Adds up (R1 + R2 + R3) | Decreases (1/Rt = 1/R1 + 1/R2) |
| Example | Safety switches in a furnace | Multiple receptacles on a circuit |
Ohm's law (V = I x R) is the foundation of all electrical troubleshooting. Typical HVAC control voltage is 24 VAC, supplied by a step-down transformer. Always verify voltage is absent before working on electrical components.