Voltage Drop: The Hidden Fault Decision Tree

Why this matters

Voltage drop is the fault that passes every "is it connected" test and still kills equipment. The wire reads continuous, the breaker is on, the device gets some voltage, but a hidden resistance somewhere in the path is eating a chunk of it under load. Motors run hot and short-cycle, heaters underperform, controls drop out, contactors chatter. Because the loss only shows up when current flows, a static check finds nothing. Learn to test under load and you will solve "ghost" problems that have stumped other techs.

What voltage drop actually is

Every connection and conductor has a little resistance. Push current through resistance and you lose voltage across it. A clean circuit loses almost nothing. A corroded lug, a loose terminal, an undersized wire, or a tired contact can lose a large fraction, and that lost voltage turns into heat right at the bad spot. The rule of thumb: a circuit should deliver nearly its full supply voltage to the load under normal running current. Significant loss means a problem in the path.

Start here: measure at rest, then under load

Safety first: this is live testing. Use a meter rated for the circuit, wear proper PPE, and keep one hand clear. You cannot find voltage drop on a dead circuit, so you must measure energized.

1. Read voltage at the source (panel, transformer, supply terminals) with the load off, then with the load running.
2. Read voltage at the load with it running.
3. Compare. If the source holds steady but the load sees noticeably less while running, voltage is dropping somewhere between them. If the source itself sags hard under load, the problem is upstream of your circuit (utility, transformer, main feeders).

Localize the drop: walk the path

The trick is to measure the voltage difference across each segment while current flows. Put your meter leads on the two ends of a single section (across a connection, a length of wire, a switch, a contactor) with the load running.

• A good section drops nearly nothing. You will read close to zero across it.
• A bad section drops a meaningful voltage. That reading is pointing right at your fault. The connection or component you are measuring across is the resistance.

Walk it methodically from source toward load (or load toward source). The first section that shows a real drop is your suspect.

Branch on what you find

• If a single connection or terminal drops voltage: that lug, splice, or screw is loose or corroded. De-energize, clean or remake the connection, retorque to spec, re-test under load. This is the most common find.
• If a switch, contactor, or relay drops voltage across its contacts: the contacts are pitted or burned. Replace the device.
• If a long wire run drops voltage with no single bad connection: the conductor is undersized for the length and load, or it is damaged/corroded internally. Upsizing the wire or shortening the run is the fix.
• If the neutral or return side drops voltage: a bad neutral is just as damaging as a bad hot, and it is the one techs forget to check. Test the return path too.
• If the source itself sags under load: the problem is upstream. Escalate to the feeder, transformer, or utility.

Confirm and document

After the repair, re-run the under-load test. The load should now see nearly full voltage and the section that was dropping should read near zero. Note the corrected reading. A voltage-drop fix you cannot show in numbers is a fix the next tech (and the warranty) will question.

Why static checks miss it

Continuity and resting-voltage tests pass a high-resistance connection because almost no current flows during the test. The resistance only "matters" when full load current pushes through it. That is why the device works at idle and fails under load, and why you must test energized and loaded.

References

• NFPA 70 (National Electrical Code) - conductor sizing and voltage-drop guidance.
• OSHA 29 CFR 1910 Subpart S - safe energized-work practices and PPE.
• Manufacturer documentation - minimum operating voltage at the load.
• See related: Why It's Getting Hot, Grounding: The Why and the Quick Check, Measuring Current Draw Against the Rating.