Symptom
A meter reads more than a couple of volts between neutral and ground at a receptacle or panel; power supplies and controllers reset; a UPS alarms on wiring fault; equipment fed from the circuit is unusually noisy; lights flicker or brighten as loads change; a neutral connection is warm.
The short answer
High Neutral-to-Ground Voltage
Neutral-to-ground voltage at a point in the system is the voltage drop along the neutral conductor between that point and the main bonding jumper at the service, where neutral and ground are connected. A fraction of a volt to about two volts under load is normal on a branch circuit. Higher values come from a long or undersized neutral carrying heavy current, harmonic current from electronic loads adding on a shared neutral, a loose neutral connection, an open neutral, or a missing main bonding jumper. A reading near zero at a loaded subpanel is its own warning: it usually means an extra neutral-to-ground bond downstream, which puts neutral current on the ground system. Measure the voltage at the point, at the panel, and at the service, measure the neutral current, and compare with the drop the conductor should produce.
Key points
- Neutral-to-ground voltage is the voltage drop on the neutral from the point of measurement back to the service bond.
- Under two volts on a loaded branch circuit is normal. Several volts means high current, a long run, harmonics, or a bad connection.
- A reading that swings as loads change points at a loose neutral; an open neutral is a hazard to every 120 V load.
- Near zero at a loaded subpanel means an extra bond downstream, and that puts neutral current on the ground.
- Measure the neutral current and compute the expected drop; the difference from the reading is the bad connection.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Long or heavily loaded branch circuit | Measure neutral current with a clamp meter and compute the drop from the conductor resistance and length. If the reading matches, the circuit is doing what physics says; reduce load or the run length. |
| Shared neutral with unbalanced loads | A multiwire branch circuit carries the difference of the phase currents on the neutral. Clamp it under normal load. |
| Harmonic current from electronic loads | Drives, switch-mode power supplies, and lighting ballasts put triplen harmonics on the neutral that add rather than cancel. A true RMS clamp shows neutral current above the phase imbalance. |
| Loose or corroded neutral connection | The reading changes sharply as loads switch and the connection is warm. Thermal imaging under load finds it. This is the dangerous one. |
| Open neutral | 120 V loads see voltages that swing toward 0 and 240 V as the load balance changes; lights flicker bright and dim. Treat as an emergency. |
| Missing main bonding jumper at the service | Neutral-to-ground voltage floats everywhere and may be large. Verify the bond at the service disconnect. |
| Extra neutral-to-ground bond in a subpanel or a device | Near zero neutral-to-ground at a loaded subpanel; measurable current on the ground conductor. Separate the neutrals and grounds in the subpanel. |
| Generator or transfer switch neutral arrangement | A generator with its own neutral bond and a transfer switch that does not switch the neutral creates two bonds when on generator. Check the neutral switching and the generator bonding against the design. |
What the reading is
At the service, the neutral and the equipment ground are connected by the main bonding jumper, so neutral-to-ground voltage there is zero. Everywhere downstream, the neutral carries load current back to the service and drops voltage along the way, while the ground carries no current and stays at the service potential. The meter reading between them at any point is therefore the voltage drop on the neutral from that point back to the bond. It rises with load current, conductor length, and conductor resistance, and it jumps when a connection in the path goes bad.
V_NG ≈ I_N × R_N
- V_NG = the neutral-to-ground voltage at the point of measurement
- I_N = the current in the neutral conductor
- R_N = the resistance of the neutral conductor from the point back to the service bond, including every connection along the way
A 100 foot run of 12 AWG copper has about 0.16 ohm of resistance one way; at 10 A of neutral current that is 1.6 V, and that is normal. The same circuit reading 6 V at 10 A has 0.6 ohm in its neutral, and about half an ohm of it is a connection that is failing.
What the value suggests
| Reading at a loaded point | Interpretation | Next step |
|---|---|---|
| 0 V at a loaded subpanel or receptacle | An extra neutral-to-ground bond nearby is shorting the reading | Clamp the ground conductor for current; inspect the subpanel neutral bar for bonding |
| 0.2 to 2 V | Normal voltage drop under load | None, unless equipment is sensitive; document the baseline |
| 2 to 5 V | High load, long run, or harmonics | Measure neutral current and compute the expected drop; check for shared neutrals and electronic loads |
| More than 5 V, steady | Poor connection, undersized neutral, or missing service bond | Thermal scan connections under load; verify the main bonding jumper |
| Any value that swings as loads switch | Loose neutral | Find the warm connection; treat as urgent |
| Values that swing with 120 V equipment misbehaving | Open neutral | De-energize; this destroys equipment and starts fires |
Diagnostic procedure
- 1
Measure at three points
Neutral-to-ground at the equipment, at the panel that feeds it, and at the service. The drop should accumulate along the path. A large step between two points puts the problem in that segment.
- 2
Measure the neutral current
True RMS clamp meter on the neutral at the panel, under the load that produces the symptom. Compare with the phase currents; neutral current above the imbalance is harmonic current.
- 3
Compute the expected drop
Conductor size and length from the drawing give the resistance; times the current gives the expected voltage. A reading close to it is a design question. A reading well above it is a connection.
- 4
Scan the connections
Under load, a thermal camera or a careful hand near, not on, the terminations finds the hot one: the neutral bar, the breaker panel neutral lugs, splices in junction boxes, receptacle back-stab connections.
- 5
Check the bonds
At the service, confirm the main bonding jumper. At every subpanel, confirm the neutral bar is isolated from the enclosure and the grounds are on a separate bar. Clamp the ground conductor leaving each panel; it should carry no current.
- 6
Check the generator path
If the site has a generator, read the neutral-to-ground voltage and the ground current on utility and on generator. A change between the two points at the neutral bonding and switching arrangement.
- 7
Fix and verify
Tighten or replace the connection, separate the bonds, enlarge the neutral, or rebalance the loads. Re-measure under the same load.
Why it matters to control equipment
Most control power supplies tolerate a few volts of neutral-to-ground without complaint. Problems begin with the causes behind the reading rather than the reading itself: a loose neutral produces voltage dips and transients that reset controllers, harmonic current heats the neutral and the transformer, and an extra bond puts current on the grounding system that then appears as a potential difference between panels and as ground loops on instrument signals. A UPS that alarms on a wiring fault is usually seeing neutral-to-ground voltage above its threshold, and it is right to complain.
The extra bond
A neutral-to-ground connection anywhere other than the service creates a second path for neutral current: through the ground conductors, conduits, water pipes, and building steel back to the service. The neutral-to-ground reading near that bond drops to zero, which can look like an improvement, while the ground conductors carry current they were never meant to carry and every enclosure on the way sits at a slightly different potential. Bonding screws left in subpanels, bonded neutrals in equipment such as generators and some transfer switches, and neutrals landed on ground bars by mistake are the usual sources. Clamping the ground conductor at each panel finds them: it should read zero.
Frequently asked questions
- What neutral-to-ground voltage is acceptable for a PLC panel?
- Under two volts under load is the usual guidance, and most control power supplies are unaffected below that. Above it, investigate the cause rather than the number; the same causes produce transients and ground current that hurt the equipment more than a steady two volts does.
- My reading is exactly zero at a panel far from the service. Is that good?
- Only if that panel is unloaded. A loaded panel far from the service has to show some drop. Zero almost always means the neutral is bonded to ground in that panel or nearby, and the ground system is carrying part of the neutral current. Clamp the ground conductor to confirm.
- Can a drive cause high neutral-to-ground voltage?
- A single-phase drive or a bank of switch-mode power supplies draws current in pulses rich in third harmonic, and on a three-phase four-wire system those add on the neutral instead of cancelling. The neutral current can exceed the phase current, and the drop follows. A true RMS clamp on the neutral shows it; the fix is a larger neutral, separate neutrals per phase, or harmonic mitigation.
- Does an isolation transformer fix it?
- A transformer creates a separately derived system with its own neutral-to-ground bond at the transformer, so the panel it feeds starts from a fresh zero and is unaffected by the upstream neutral drop. It is the standard cure for a control panel fed by a long, noisy, heavily loaded circuit, and it also blocks a good deal of common-mode noise. It does not fix the upstream problem, which still needs attention.
Related topics
- Missing Equipment GroundAn enclosure, motor, or device with no effective equipment ground: how it shows up as a tingle, a fault that does not trip, surge protection that does nothing, or instrument noise; how to test the path safely; and why a ground rod is no substitute.
- Ground Loop SymptomsHow a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
- Floating Reference Between PanelsSignals between two panels that misbehave because the panels do not share a DC reference: inputs that never turn on or half turn on, strange voltages to ground, and noise that comes and goes. How DC commons should be bonded and how to cross a panel boundary.
- UPS for Control PanelsKeeping the controller, the radio, and the SCADA server alive through an outage: AC and DC UPS types, what to put on the UPS and what not to, sizing for runtime and inrush, the battery as a maintenance item, the alarms a UPS must provide, and what happens when the UPS is the thing that fails.
- Panel Power SuppliesThe 24 V DC supply behind everything in a modern panel: sizing with real inrush and duty, single versus redundant with diode modules, how a switch-mode supply behaves under a short, distribution and fusing per branch, monitoring, and the failure modes that take out a whole panel at once.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
Direct contact
Have a controls question?
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