Current, voltage, and resistance

In HVAC, electricity does three daily jobs:

1. Power — compressor, condenser, blower, pump motors.
2. Control — energize 24 V coils that close contactors and relays.
3. Signal — sensors that report temperature or pressure to boards and thermostats.

You do not need to be a construction electrician; you need to measure with purpose and not invent paths for current.

Voltage (V) is the potential difference between two points. It is what pushes electrons through a conductor.

Typical field values:

0 V between R and C does not mean "no power in the building." It means no difference at those two points — often because the control circuit is open, not because line power is gone.

Current (I) is measured in amperes (A) — charge passing a point per second.

Mental rules:

• The same current flows through elements in series.
• In parallel, total current splits among branches.

In HVAC diagnosis, motor amperage is a load picture:

• Start: brief peak (LRA on compressors).
• Run: compare to nameplate FLA/RLA.
• High amperage with normal mechanical load → capacitor, lost phase, bearings, restriction.
• Low amperage → reduced load or low voltage (investigate context).

Resistance (R) in ohms (Ω) limits current for a given voltage.

Conductors have low R; motor windings moderate R; a burned contact or loose connection adds high local R → heat and voltage drop.

A healthy contactor coil might read tens of ohms. Open = infinite; shorted = near zero — and the fuse or transformer suffers.

V = I × R

Rearrange: I = V / R, R = V / I.

Electrical power (work and heat in watts):

P = V × I

A useful ½ hp motor is roughly 370 W. The nameplate shows voltage, amperage, and sometimes service factor. The breaker protects the conductors and installation per code — it does not "limit horsepower."

DC flows one direction — batteries, some board circuits after rectification.

AC reverses direction many times per second. North American HVAC equipment typically runs at 60 Hz.

Set the meter to V~ (AC) or V⎓ (DC) for what you are testing. Measuring 24 V AC on a DC scale gives wrong or misleading readings.

On AC, the meter usually displays RMS (effective) voltage — not the brief peak of each cycle.

You will see:

• V — volts
• A, FLA, RLA, LRA — amperes in different motor states
• W or kW — power
• Ω — ohms (coils, heaters)
• µF — microfarads (capacitors; covered later)

Memorize the difference between FLA (running) and LRA (locked rotor at start). Comparing live amps to the wrong label wastes time.

Before you trust a number:

1. De-energize when checking resistance — power off, lockout if required.
2. Live voltage checks: hands clear, proper PPE, know where your other hand is.
3. Record where you measured (line side of contactor vs motor terminals matters).

Electricity is not mysterious — it follows rules. Your job is to apply them one step at a time.

You should be able to say out loud:

1. Voltage pushes; current flows; resistance limits.
2. V = I × R is how I interpret meter readings.
3. Zero volts between two points is about that path, not the whole building.
4. Motor amperage tells a story only when compared to nameplate and mechanical load.

When you can teach that to a helper, this lesson did its job.

Situation. Rooftop unit: blower motor "not getting power." You measure 120 V at the disconnect and 0 V at the motor terminals with the contactor de-energized.

How to think it.

• The motor needs a complete path through the contactor and any limits — not just voltage at the disconnect.
• With the thermostat calling, you measure 24 V at the contactor coil and 120 V on the load side when the contactor pulls in. The problem was an open high-limit in series with the motor, not "dead power."

Takeaway: voltage readings must follow the actual circuit path from source to load.