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All electrical work builds on three fundamental quantities: voltage, current, and resistance. Understanding how they relate to each other - through Ohm's Law - is the foundation of every electrical diagnosis you will ever make.
Voltage is electrical pressure - the force that pushes electrons through a conductor. Think of it like water pressure in a pipe. Higher voltage = more pushing force. Measured in volts (V).
| HVAC Application | Voltage | Notes |
|---|---|---|
| Thermostat/control circuits | 24V AC | From control transformer secondary |
| Standard residential power | 120V AC | Indoor fan motors, controls, convenience outlets |
| Residential compressor/condenser | 240V AC (single-phase) | Most residential AC equipment |
| Commercial equipment | 208V, 240V, or 480V (three-phase) | Large compressors and air handlers |
Current is the flow of electrons through a conductor. Think of it like the volume of water flowing through a pipe. Measured in amperes (amps, A). In HVAC: a motor drawing more current than its nameplate rating is being overworked - it will overheat and fail prematurely.
Resistance is opposition to current flow. Every component has resistance. Measured in ohms (?). Key insight: when resistance increases in a circuit (due to a loose connection, corroded contact, or failing component), current decreases and heat is generated at the high-resistance point.
V = I � R � (Voltage = Current � Resistance)
Rearranged: I = V ? R �� R = V ? I
Examples:
A 240V compressor circuit has 24? resistance. Current = 240 ? 24 = 10 amps
A circuit reads 24V and draws 2A. Resistance = 24 ? 2 = 12 ohms
P = V � I � (Power in watts = Voltage � Current)
Also: P = I� � R �� P = V� ? R
Example: A 240V unit drawing 15A uses 240 � 15 = 3,600 watts = 3.6 kW
Components connected end-to-end with one current path.
HVAC example: Furnace safety switches (high limit, rollout, pressure switch, flame sensor) are all wired in series in the control circuit. Any single switch opening stops the furnace.
Components connected across the same two points with multiple current paths.
HVAC example: The compressor, condenser fan motor, and crankcase heater are all connected in parallel to 240V - each operates at full voltage, independently.
| Property | AC (Alternating Current) | DC (Direct Current) |
|---|---|---|
| Direction | Reverses direction 60 times/second (60 Hz in US) | Flows in one direction only |
| Voltage measurement | RMS (root mean square) - 120V RMS = 170V peak | Direct measured value |
| HVAC applications | Power supply, motors, control transformers | Electronic controls, variable-speed drives, ECM motors |
| How transformers work | AC voltage can be stepped up or down with transformers | DC cannot be transformed without electronics |
Memorize V=IR and P=VI. In series: current same everywhere, voltage divides, total R adds up. In parallel: voltage same everywhere, current divides, total R decreases. Safety switches in furnaces are series - any one opening kills the circuit. Loads (compressor, fan) are parallel - each gets full voltage.