Electricity Fundamentals

Voltage, current, resistance and power, and how they show up in HVAC line and control circuits.

Concept

Beginner

#fundamentals#electrical#safety#controls

Safety

De-energize and lock out before servicing whenever possible. Discharge capacitors before handling. Never bypass or defeat a safety control.

What it is

Voltage

Electrical pressure, the difference in potential that pushes current through a circuit. Measured in volts.

Current

The flow of electrons through the circuit, measured in amperes.

Resistance

Opposition to current flow, measured in ohms.

Power

The rate of work being done, measured in watts. In HVAC it appears as heat, motor torque and compressor capacity.

Why it matters

A large share of HVAC service calls are electrical. Being able to trace a circuit, take a reading and interpret it is the difference between replacing the failed part and replacing parts until something works.

Circuit	Typical voltage	What it feeds
Line voltage single phase	120 or 240 volts	Compressors, blowers, condenser fans, heaters
Line voltage three phase	208, 240 or 480 volts	Commercial compressors and motors
Control voltage	24 volts alternating current	Thermostats, contactor coils, gas valves, safety switches
Low voltage direct current	Typically 5 to 24 volts	Control boards, sensors, communicating equipment

How it works

Alternating current reverses direction many times per second, which is what allows transformers to step voltage down and induction motors to turn. Most HVAC controls run on 24 volt alternating current produced by a transformer inside the equipment.

A complete circuit needs a source, a path out, a load and a path back. Switches and safety devices are placed in the path so that opening any of them stops the load. Reading voltage across an open switch shows source voltage, reading voltage across a closed switch shows nearly zero.

Safety

Treat every circuit as energized until you prove otherwise

De-energize and lock out before servicing whenever the task can be done with the power off. When a live reading is genuinely required, use rated personal protective equipment, a meter rated for the circuit, and keep one hand clear. Capacitors store a dangerous charge after power is removed and must be discharged safely before handling.

Never bypass a safety control

Pressure switches, rollout switches, limits and float switches exist because a failure mode can injure someone. Jumping one to make equipment run is not a repair and is never an acceptable way to leave a system.

Tools involved

A digital multimeter rated for the circuit category you are working on
Clamp ammeter
Non contact voltage tester as a first check only, never as proof of de-energization
Lockout and tagout devices