Voltage Reads Normal But Device Won't Run: A Misleading-Reading Decision Tree

Why this matters

A digital multimeter (DMM) shows a clean 120 V at the receptacle, the load shows 240 V at the disconnect, yet the equipment will not run. The reading looks like proof the circuit is healthy, and an inexperienced tech walks away calling it a "bad appliance." That conclusion is wrong often enough to cost a return trip, a warranty dispute, and the customer's confidence. The fault is real, but it hides behind a no-load voltage measurement that a high-impedance meter happily passes. Understanding why the good reading lies is the difference between a 20-minute diagnosis and a parts-replacement guessing game.

Why the good reading misleads

A standard DMM has an input impedance around 10 megohms. It draws microamps to display a number. A loose terminal, a corroded splice, a partially open conductor strand, or a backstabbed connection with high resistance will still pass enough microamps to read full system voltage at no load. The instant a real load draws current, that high-resistance point develops a voltage drop across itself, and the voltage reaching the device collapses.

This is the single-point versus path problem. The meter measured potential at one point against neutral or ground. It did not measure the circuit's ability to deliver current through the entire path under working conditions. A circuit that reads 120 V open and sags to 90 V under a 12 A load has roughly 2.5 ohms of unwanted series resistance somewhere in the path, dissipating about 360 watts of heat at the fault point. That heat is the fire risk, and the open-circuit reading never showed it.

Symptom presentation

• Device hums, clicks a contactor, or shows a controller fault but will not start or run.
• Lights on the same circuit dim noticeably the moment the device tries to start.
• Voltage at the device terminals is correct with everything off, low when the load engages.
• Heat, discoloration, or a faint odor at a junction box, breaker, receptacle, or splice.
• Intermittent operation that worsens when the circuit warms up.

Quick checks

Confirm the complaint, then prove it under load instead of at rest.

• Verify nameplate voltage and phase. A 208 V motor on a 240 V single-phase supply, or vice versa, will misbehave even with a perfect circuit.
• Read voltage at the device with the load off, then again while a helper attempts to start it. A drop greater than 3 percent on a branch circuit (about 3.6 V on 120 V) under normal current points to a high-resistance path, not a bad device.
• Clamp the conductor and read current draw. Zero current with the device calling means an open control or open line conductor. High locked-rotor current that will not transition to running current points to a mechanical or supply problem.

Isolation tree

1. No-load voltage good, loaded voltage sags more than 3 percent. A series high-resistance fault. Move the meter probe upstream point by point with the load running: device terminals, last receptacle, junction box, breaker lug, breaker line side. The voltage will be correct upstream of the bad connection and low downstream of it. The connection between the last good and first bad reading is the fault.

2. No-load voltage good, loaded voltage holds, device still dead. The supply path is fine. Look at the device's own control circuit: a control transformer, an interlock, a thermal cutout, or a controller lockout. Measure across the load contactor coil while it is called.

3. No-load voltage good, but only one leg of a 240 V load reads correct under load. One ungrounded conductor is feeding through a high-resistance termination or a partially open breaker pole. Read L1-to-ground and L2-to-ground under load. The bad leg collapses.

4. Voltage reads normal at the receptacle but the device shows nothing. Confirm the device is actually connected to that receptacle and the cord or plug is intact. A backstab connection inside the receptacle can pass open-circuit voltage to the face while failing at the load.

Confirming diagnosis

After de-energizing and following lockout, open the suspect connection. The confirming evidence at a high-resistance termination is visual and tactile: oxidized copper, a darkened terminal screw, a loosened backstab spring, melted insulation, or a heat-tinted breaker lug. Re-terminate, then repeat the loaded-voltage test. The drop should disappear, and the device should run at rated voltage. A torque check against the manufacturer's marking on the lug or device confirms the mechanical fix.

If the connection looks clean and the drop persists, suspect an undersized or damaged conductor. Compare measured drop against the expected drop for the conductor length and gauge; an excess indicates a damaged or undersized run.

Remediation

• Re-terminate or replace the high-resistance connection. Replace backstabbed devices with screw or pigtail terminations on 15 A and 20 A branch circuits where heat is involved.
• Torque all reworked lugs and terminals to the manufacturer's value per NEC 110.14(D).
• Replace a damaged conductor run rather than splicing in a heat-affected zone.
• Re-verify under full load and confirm the voltage drop is within the recommended limits of NEC 210.19 informational notes (3 percent branch, 5 percent total).

References

• NFPA 70, National Electrical Code, Article 110.14, Electrical Connections (termination and torque).
• NFPA 70, National Electrical Code, Article 210.19 informational note (branch-circuit voltage drop guidance).
• NFPA 70E, Standard for Electrical Safety in the Workplace (energized work justification, PPE, boundaries).
• UL 486A-486B, Wire Connectors (termination performance and heat-cycling behavior).