HOW-TO GUIDE
Why Does My Multimeter Show Voltage When Measuring a Disconnected Circuit?
By the OhmSavvy Research Team ·
Treat it as live until you have proven it dead
- A voltage reading on a circuit you switched off is a warning first and a curiosity second. Do not touch bare conductors until a loaded test shows no voltage.
- Use a CAT III-rated meter and leads for household wiring, and check the meter on a known live source before and after the test.
- Never open a panel or separate a shared neutral yourself. If the voltage turns out to be real, or you cannot identify every breaker feeding the box, call a licensed electrician.
Why a dead wire can read live
Two insulated wires lying side by side in a cable or conduit form a small capacitor. When one carries AC and the other is switched off, a little current crosses that capacitance into the dead wire. On its own the dead wire has nowhere to send that current, so it floats at some fraction of the live voltage set by how strongly it is coupled to the live conductor compared with how strongly it is coupled to ground. Long parallel runs couple more than short ones, and wires twisted together in the same jacket couple more than wires in separate cables.
A typical digital meter's voltage input is around 10 MΩ, chosen so that it barely disturbs the circuit under test. That is exactly why it shows the coupled voltage so clearly: the meter adds almost no path to ground, so the floating wire stays where the coupling put it, and readings of tens of volts on a 120 V system are common. Inductive coupling, where a current-carrying conductor induces a voltage along a parallel loop, can add to it on long runs, though in homes the capacitive effect is usually the bigger one.
Why is my multimeter reading voltage on a disconnected wire?
A wire you have unhooked from a fixture, switch or outlet is usually still lying in the same box and the same cable as conductors that are live. If it is disconnected at your end but still attached to a breaker or splice at the far end, it may simply be live, and that is the first thing to rule out. If it is truly disconnected at both ends, or its breaker is off, it has no source of its own, so it takes on whatever voltage the stray capacitance to its live neighbors gives it. Measured from that wire to ground or to a bare ground conductor, a standard meter can show anything from a few volts to well over half of line voltage.
The arithmetic shows why the number looks so convincing and why it means so little. Assume, purely as an illustration, 100 pF of coupling between a 120 V, 60 Hz hot and the dead wire beside it. At 60 Hz that capacitance behaves like a reactance of about 26.5 MΩ (1 ÷ (2π × 60 × 100 pF)). Put a 10 MΩ meter between the dead wire and ground and the two form a divider: the meter reads about 42 V (120 × 10 ÷ √(10² + 26.5²)). Yet the current behind that reading is only about 4.2 µA (120 V ÷ roughly 28.4 MΩ). Swap in a low-impedance input of about 3 kΩ and the same coupling produces only around 14 mV. A real feed has a source impedance far below 3 kΩ, so it barely notices the extra load and keeps nearly its full voltage.
Bars use a logarithmic scale: each step is ten times the last.
Real capacitance depends on how long the conductors run side by side and how tightly they are packed, so treat 100 pF as a teaching value, not a figure to expect from your cable. The shape of the result holds regardless: the higher the meter's input impedance, the more of the coupled voltage it shows, and a loaded reading drains it away. Three of the meters we list publish a way to do that loading for you. Fluke says the 117 has a selectable low-impedance mode to prevent false readings from ghost voltage, and specifies its Auto-V/LoZ function from 0.0 V. The Fluke 114 picks AC or DC automatically on a low-impedance input. The Fluke 113's VCHEK position is a LoZ voltage test of about 3 kΩ that falls back to a continuity check when no voltage is present, which is a quick way to sort an isolated dead wire from one still tied to something.
A dangling wire can also read DC volts or a drifting AC value if you measure it against something that is itself floating, such as a metal box with no ground connection, or with only one probe on the wire. Always measure between two defined points: the suspect wire and a known-good equipment ground, then the suspect wire and neutral. If both read tens of volts on the 10 MΩ input and near zero on LoZ, the wire is coupled, not fed. If the reading holds on LoZ, go to step 3 below before anyone touches it. Readings that wander rather than holding a value are covered in what causes unstable multimeter readings, and a circuit that shows full voltage but won't run anything is the subject of why a multimeter shows normal voltage when the circuit doesn't work.
Ghost voltage or a real feed?
| Symptom | Likely cause | How to check | Fix |
|---|---|---|---|
| Tens of volts AC, changes when you move the leads | Capacitive coupling from live wires in the same run | Re-test on LoZ or with a small load; voltage collapses | Nothing to repair; record it as coupled voltage |
| Full or near-full line voltage with the breaker off | Wrong breaker, a second feed or a backfeed | Reading stays the same on LoZ | Stop; find the real source or call an electrician |
| Voltage neutral-to-ground that rises when other loads run | Shared neutral on a multiwire branch circuit, or an open neutral | Watch the reading while other circuits switch on and off | Electrician identifies and isolates the circuit |
| A few millivolts to volts that wander with nothing connected | Floating meter input or open circuit on a bench | Short the probes; reading should drop to zero | Normal behavior; measure only with both probes on the circuit |
| DC voltage on unplugged equipment that slowly falls | Charged capacitor still holding energy | Watch the reading decay over seconds or minutes | Discharge safely before working on the device |
| Voltage on a switched lighting circuit with the switch off | Switch breaks only the hot; neutral and other legs still connected | Turn off the breaker, not just the switch, and re-test | Always isolate at the breaker for any wiring work |
1Prove the circuit dead
Electricians use a live-dead-live sequence for a reason. First, measure a known live source, such as a working outlet, to show the meter, leads and selected function are working. Second, measure the circuit you switched off, between every pair of conductors: hot to neutral, hot to ground, and neutral to ground. Third, go back to the known live source and confirm the meter still reads it. A meter that failed between steps one and three would otherwise give you a false dead reading.
01
Test known live
Proves meter and leads work
02
Test the circuit
Every conductor pair
03
Re-test known live
Meter still working?
04
Voltage present?
Re-test on LoZ or with a load
2Confirm with LoZ or a load
A low-impedance (LoZ) input replaces the 10 MΩ with a resistance in the region of a few kilohms. A coupled voltage cannot supply enough current to hold itself up across that, so the reading falls to near zero. A real source, even a weak backfeed, keeps most of its voltage. Fluke documents LoZ as one of the three features the 117 has and the 115 lacks, and Klein lists it on the MM720 for exactly this job; our Fluke 115 vs 117 comparison shows the difference side by side.
Without LoZ, you can make the same point by measuring with a known load in parallel, which is the approach described in the floating ground measurement guide. On mains wiring that load must be rated for line voltage and the wattage it will dissipate, so a meter with a built-in LoZ position is the simpler and safer route for most people. Avoid LoZ on electronic control or logic circuits, because the extra current it draws can upset them, and be aware that a LoZ test from hot to ground can trip a GFCI.
3When the voltage is real
If the reading stays put on LoZ, something is genuinely driving that conductor. The common household culprits are a mislabeled breaker, a box fed from two circuits, the second leg of a 240 V circuit where only one breaker was switched off, and a multiwire branch circuit whose shared neutral still carries current from the other half. That last one catches people out because neutral is assumed to be harmless: with load on the other circuit, an opened shared neutral can sit near full line voltage.
Less common sources include a generator, UPS or battery inverter wired without proper isolation, and equipment that stores charge, such as motor run capacitors and power supply filter capacitors. In every one of these cases the next step is to find the true source and isolate it, which in fixed wiring means a licensed electrician. Do not keep switching breakers at random while holding the conductors apart; record what you measured and where, and hand that information over.
4Floating inputs on bench electronics
On a workbench the same physics shows up at smaller scale. Leave the probes in the air or clip only the red lead to a disconnected board, and the display may wander through millivolts or even a volt or two as the leads pick up hum from nearby mains wiring and lighting. An unpowered board with long wires attached can show a similar drifting AC reading. None of this is a fault: the meter needs both probes on two points of a circuit for the number to mean anything, and shorting the probes together should bring it straight to zero.
A slowly falling DC voltage on a supply you have just unplugged is different. That is a capacitor discharging through whatever bleed path the design has, and on mains-powered equipment it can be hazardous for some time after the plug is pulled. Measure across the capacitor and wait for it to fall to a safe level, or discharge it properly, before working on the board. If the readings jump around rather than sitting at a steady wrong value, see what causes unstable multimeter readings.
What the meter is really telling you
Meters with LoZ
If you regularly test switched-off wiring, a meter with a LoZ position saves time and removes guesswork. Each of these picks lists low-impedance voltage in its maker's specifications.
Multimeters with a low-impedance (LoZ) voltage mode
LoZ, AutoVolt and built-in NCV in a True RMS, CAT III 600V body. Hands-on Tested.
LoZ for ghost voltages, CAT IV 600V / CAT III 1000V, IP42 and a 2 m drop rating. No NCV.
VCHEK: a LoZ voltage test of about 3 kΩ that drops to continuity when no voltage is present. No current ranges.
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Frequently asked questions
Why is my multimeter reading voltage on a disconnected wire?
A disconnected wire that still runs beside live conductors picks up voltage through stray capacitance, and a standard 10 MΩ meter input lets that coupled voltage show as tens of volts with only microamps behind it. First confirm the far end really is disconnected or its breaker is off, then re-measure on a LoZ setting or with a load. A coupled reading collapses; a real feed doesn't.
Is ghost voltage dangerous?
Coupled voltage by itself can only push a tiny current, so it usually cannot hurt you. The danger is assuming every unexpected reading is a ghost. The same display can come from a backfeed, a shared neutral carrying load current or a second breaker feeding the box. Prove the reading collapses under a load, and keep treating the conductor as live until you have.
Why does my multimeter show voltage between neutral and ground with the breaker off?
A small reading can be coupling. A larger, steady reading often means the neutral is shared with another circuit that is still on, so load current from that circuit flows through it and develops a voltage along the wire. Opening a shared neutral can expose you to full line voltage, so stop and have an electrician identify the multiwire circuit and switch off every breaker that feeds it.
Will a non-contact voltage tester tell me if the voltage is real?
No. A non-contact tester senses the electric field around a conductor, so it reacts to coupled voltage and can beep on a wire that is effectively dead. It can also stay silent on a live conductor that is shielded or deep in a box. Use it as a quick first look, then confirm with a contact meter, ideally on LoZ, using the live-dead-live sequence.
Why does the reading change when I touch the probe or move the leads?
Your body and the leads add their own stray capacitance to whatever the floating conductor is coupled to. Because the meter's input draws almost no current, those small changes shift the displayed voltage noticeably. A reading that changes with hand position or lead routing is a strong hint that nothing solid is driving the wire, but it is not proof.
Can I use LoZ mode on electronic circuits?
Generally not. A LoZ input draws far more current than a normal voltage range, which can disturb sensitive control or logic circuits and, when measuring from hot to ground, may trip a GFCI. Most manufacturers describe LoZ for electrical installation work. For electronics, use the standard high-impedance voltage function and, if needed, add a known resistor load deliberately.
Where to go next
Measuring Voltage on a Floating Ground Circuit →
Phantom readings, reference points and the loaded-measurement technique in detail.
Fluke 115 vs Fluke 117 →
The same meter body, but only the 117 has LoZ and NCV.
Why Does My Multimeter Show the Wrong Voltage? →
Setup mistakes, True RMS limits and circuit loading.
