HVAC Technician Academy
Learn how to find hidden resistance in HVAC circuits by measuring voltage drop across contacts, switches, safeties, wiring, and connections while current is actually flowing.
Voltage drop testing measures the difference in electrical potential across a component or section of wiring while the circuit is operating. A healthy closed connection should usually drop very little voltage. Excessive voltage across a closed contact, switch, connector, or wire section can reveal unwanted resistance that an ordinary continuity test may miss.
Many electrical faults only appear when current flows.
A corroded connector may show continuity with power off.
A burned contactor may still pass voltage with no meaningful load.
A loose connection may look normal until a compressor, relay, or contactor coil is energized.
| Test | Circuit Condition | What It Tells You |
|---|---|---|
| Resistance / continuity | Power removed | Whether an electrical path exists |
| Voltage drop | Circuit operating under load | How much electrical potential is being lost across the path |
Ideally, a good closed contact acts almost like a piece of wire.
That means very little voltage should be lost across it.
Exact acceptable values depend on the circuit, current, component, and manufacturer.
The important principle is:
An open switch or contact in an energized circuit can show most or all of the available circuit voltage across it.
This often confuses technicians.
They measure 24 volts across a float switch and assume: "The float switch has 24 volts, so it must be good."
In reality, full voltage across the device may indicate the device is open.
Consider a simplified cooling circuit:
The transformer measures: 27 VAC from R to C.
But the contactor coil only receives: 19 VAC while energized.
Somewhere between the transformer and the coil, about 8 volts are being lost.
Instead of replacing the contactor or transformer, measure across each closed component under load.
| Test Point | Example Reading | Interpretation |
|---|---|---|
| Across thermostat contact | 0.1 VAC | Very little loss |
| Across float switch | 0.2 VAC | Low loss |
| Across corroded connector | 7.2 VAC | Major unwanted resistance |
| Across remaining wiring | 0.3 VAC | Low loss |
In this example, the corroded connector is clearly consuming most of the missing voltage.
Voltage drop testing is not limited to 24-volt circuits.
It can also help identify high resistance across line-voltage contacts.
A closed contactor feeding a compressor should not consume a significant portion of the supply voltage.
Excessive drop across a closed power contact may indicate:
Study: AC Contactor Diagnostic Guide
A high-resistance connection may look normal when almost no current flows.
When the load energizes, current creates a voltage drop across that resistance.
The relationship is explained by Ohm's Law:
If resistance exists and current increases, voltage lost across that resistance becomes easier to see.
A resistive connection does more than reduce voltage.
It can create heat.
That may result in:
Technician Case File
Transformer voltage with no load: 27.0 VAC.
Technician initially suspects a weak transformer.
During contactor operation: coil voltage drops to 18.9 VAC.
Voltage drop tests are performed through the Y circuit.
Across thermostat: minimal drop.
Across float switch: minimal drop.
Across one outdoor wire splice: 7.3 VAC.
Splice is opened and found heavily corroded.
After repair: coil voltage stabilizes and chatter disappears.
Lesson: The voltage-drop test identified a fault that continuity and unloaded voltage testing could easily miss.
Disconnect voltage: normal.
Compressor terminal voltage while running: low.
Technician measures across each closed contactor pole.
One pole shows an abnormal voltage drop under compressor load.
Contact is badly burned.
The supply itself was not the primary fault. Voltage was being lost across a resistive contact.
1. Understand the circuit. Know the source, path, load, and return.
2. Confirm the problem occurs under load.
3. Measure source voltage during operation.
4. Measure voltage at the load.
5. Determine how much voltage is missing.
6. Divide the circuit into sections.
7. Measure across each closed switch, connector, contact, or wiring section.
8. Identify the section consuming abnormal voltage.
9. Repair the connection or component.
10. Retest the complete circuit under full operating load.
| Observation | Possible Direction |
|---|---|
| Normal source voltage, low load voltage | Voltage being lost somewhere in the circuit path |
| Large drop across closed contact | High-resistance contact |
| Large drop across connector | Corrosion, looseness, or damaged connector |
| Full supply voltage across open switch | Open point in energized circuit |
| Minimal drop across all path components but source collapses | Possible overloaded/weak source or upstream supply issue |
When voltage is missing at the load, do not guess which component is bad. Find where the voltage is being dropped.
1. Why can continuity testing miss a high-resistance connection?
2. What should voltage drop across a healthy closed switch generally look like?
3. Why must voltage drop testing usually be performed under load?
4. What can full supply voltage across an open switch indicate?
5. If source voltage is normal but load voltage is low, what should you do next?
1. A resistive connection may still have electrical continuity when very little test current is flowing.
2. It should generally be very low because a healthy closed contact should have little resistance.
3. Current flowing through resistance creates the measurable voltage loss you are trying to identify.
4. The open switch may be the point where the circuit's electrical potential is being dropped.
5. Measure voltage drop across sections of the operating circuit until the unwanted loss is located.
Return to the complete electrical training hub.
24-Volt Controls →Trace R, C, Y, G, W, and control circuits under load.
Contactor Diagnostics →Apply voltage-drop testing to contactors and power circuits.
Voltage-drop testing is normally performed on energized equipment and may expose the technician to hazardous line voltage. Only qualified personnel should perform live electrical testing. Use properly rated instruments and PPE, follow manufacturer and applicable electrical-safety procedures, and never perform resistance or continuity measurements on an energized circuit.
Burned contacts, loose connections, damaged wiring, and weak safeties can pass simple continuity tests and still fail under load. Voltage-drop testing helps locate the actual resistance.
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