Electrical & Controls
Ohm's law calculations, electrical components (capacitors, contactors, relays, transformers), thermostat wiring, and control circuit troubleshooting for HVAC systems.
- Apply Ohm's law and the power formula to calculate voltage, current, resistance, and wattage in HVAC circuits
- Identify the function of capacitors, contactors, relays, and transformers in HVAC equipment
- Interpret standard thermostat wiring designations (R, Y, G, W, C, O/B)
- Troubleshoot common electrical faults using multimeter readings
Lección 1
Ohm's Law and Electrical Fundamentals
The Three Electrical Properties
Every HVAC electrical circuit involves three fundamental properties: voltage (measured in volts, V), current (measured in amps, A), and resistance (measured in ohms). Understanding how these three relate to each other is the basis for every electrical diagnosis you will make in the field.
Voltage is electrical pressure - the force that pushes electrons through a conductor. In residential HVAC, you encounter 24V control circuits, 120V for blower motors and ignition, and 240V for compressors and electric heat strips.
Current (amperage) is the flow rate of electrons through a conductor. The amount of current a component draws tells you how hard it is working. A compressor drawing more amps than its rated load amperage (RLA) is a sign of mechanical or electrical problems.
Resistance is opposition to current flow. Every wire, motor winding, and electrical component has resistance. Resistance creates heat - which is useful in electric heat strips but damaging in loose wire connections.
Ohm's Law
Ohm's law defines the mathematical relationship between voltage, current, and resistance:
V = I x R (Voltage equals Current times Resistance)
From this, you can derive: I = V / R and R = V / I
Practical example: A 10 kW electric heat strip on a 240V circuit draws how many amps?
P = V x I, so I = P / V = 10,000W / 240V = 41.7 amps
The power formula (P = V x I, where P is watts) connects electrical measurements to energy consumption. For electric heat, 1 kilowatt produces 3,413 BTU/h. So a 10 kW heat strip produces 34,130 BTU/h.
Series vs. Parallel Circuits
In HVAC, safety controls are wired in series - the high-pressure switch, low-pressure switch, and other limits are all in the same circuit path. If any one opens, the entire circuit breaks and the compressor stops. This is intentional - every safety must be satisfied for the system to run.
Loads like compressor contactors and gas valves are often wired in parallel across the power source. Each load receives full voltage regardless of whether other loads are operating.
Series Safeties, Parallel Loads
On the exam, if a question states safety switches "all must be closed for the system to run" - that describes a series circuit. If the question asks about multiple loads that each receive full voltage independently - that describes a parallel circuit.
Ohm's law (V = I x R) and the power formula (P = V x I) let you calculate any unknown electrical value from two known values. Safety switches are wired in series so any single open switch stops the system - 1 kW of electric heat produces 3,413 BTU/h.