Voltage 120 No-Load, 180 On One Leg: Open Neutral vs POCO Decision Tree

Why this matters

When one leg of a 120/240 V single-phase system reads near 120 V with no load but swings up toward 180 V (and the other leg sags toward 60 V) the moment a load comes on, you are watching an open or high-resistance neutral. The neutral is the reference that splits the 240 V across the two legs evenly; lose it and the two legs become a series voltage divider across the loads, so the lightly-loaded leg floats high and the heavily-loaded leg collapses. The danger is real: high voltage on a leg cooks 120 V appliances and electronics, and a flickering, brightening-then-dimming house is a fire and equipment-loss risk. The diagnostic split that matters most is where the open neutral lives, on the utility (POCO) side or inside the customer's service, because that decides who fixes it and how you make it safe right now.

Symptom presentation

The customer reports lights brightening in one part of the house while dimming in another, sometimes in time with the refrigerator or HVAC cycling. Electronics fail or behave erratically; bulbs burn out fast. At the panel, with loads off, both legs read near 120 V to neutral and 240 V leg to leg. Apply load to one leg and that leg's voltage drops while the other climbs well above 120 V; leg-to-leg may still read near 240 V. The voltages "see-saw" around the 240 V total as loads shift. This is the signature of a neutral that cannot carry the imbalance current.

Quick checks

• Confirm the see-saw. Read each leg to neutral with loads off, then switch a known 120 V load onto one leg and watch both legs move oppositely. Movement confirms an open/high-resistance neutral.
• Read leg-to-leg. If leg-to-leg holds near 240 V while leg-to-neutral see-saws, the two hots are intact and the neutral is the suspect.
• Check the main panel neutral. Inspect the neutral/ground bar and the main neutral lug for looseness, corrosion, or heat discoloration.
• Look outside. Inspect the service drop/lateral neutral, the weatherhead, the meter base, and any visible splices for damage or burning.
• Note scope. Whole-house symptoms point upstream toward the service neutral; symptoms limited to one circuit or subpanel point downstream.

Isolation tree

1. Confirm the neutral is open, not a hot problem. See-saw legs with steady ~240 V leg-to-leg confirms neutral. Steady legs with one missing entirely is a lost hot, a different fault.
2. Bound the fault: main lugs. With the main off and the service de-energized where you safely can, check the main neutral connection at the panel and the neutral bar. A loose, corroded, or burned main neutral lug is a common in-panel cause. Torque to spec or repair.
3. Meter base and service entrance. Inspect the neutral through the meter base, the meter jaws, and the service-entrance conductors. Burned meter-base neutrals and corroded lugs are frequent. If the open is at or before the meter on the line side, it is utility territory.
4. Service drop / lateral (POCO side). If the in-panel and customer-side connections are sound and the open is in the drop, the weatherhead splice, or the underground lateral, the fault is on the utility neutral. This is a POCO repair, not a contractor repair.
5. Subpanel and feeder neutrals. If symptoms are limited to a subpanel, check the feeder neutral and its terminations between main and subpanel; a downstream open neutral see-saws only the loads it serves.

Confirming diagnosis

Confirm before assigning the fix. The definitive test is location bounding: move your neutral reference toward the source. If you read the see-saw at the main panel neutral but a solid, stable reference at the line side of the meter (or at the point where the utility neutral lands), the open is between those two points and on the customer's equipment. If the see-saw is present even at the service-entrance/utility connection point, the open is in the utility's drop or lateral and is theirs. Re-torquing a suspect lug and watching the see-saw vanish under the same load confirms that connection was the open. Never declare it fixed without re-applying load and seeing both legs hold near 120 V.

Remediation

• In-panel neutral (loose/corroded/burned lug or bar): de-energize, clean or replace the affected lug/bar, torque to the listed value, and re-test under load.
• Meter base neutral: repair or replace the meter base neutral connection (coordinate with the utility for meter pull/seal).
• Customer service-entrance neutral conductor damaged: replace the affected conductor and terminations.
• POCO-side open (drop, weatherhead splice, lateral): stop, document the readings, and turn it over to the utility. Advise the customer to limit loads until repaired to protect equipment.
• After any repair, confirm balanced legs (near 120 V each) under realistic load before closing out.

References

• NFPA 70 (National Electrical Code) Article 200 (use and identification of grounded conductors) and Article 250 (grounding and bonding) for neutral/grounded-conductor requirements.
• NEC Article 230, Services, for service-entrance conductor and equipment requirements.
• NFPA 70E, Standard for Electrical Safety in the Workplace, for energized-work PPE and approach boundaries.
• Local utility (POCO) service rules defining the demarcation between utility and customer responsibility.