Lights Brighten On One Leg Dim On The Other Open Neutral Decision Tree

Why this matters

When lights and appliances on one half of the panel go bright (and electronics start dying) while the other half goes dim, you are looking at an open or high-resistance neutral upstream of the split, most often the main service neutral or a feeder neutral. With the neutral reference lost, the two 120V legs are no longer independently referenced to ground; they sit in series across 240V, and the midpoint floats according to the relative load on each leg. The lightly loaded leg climbs toward 240V and burns out everything plugged into it; the heavily loaded leg sags. This is an active emergency: it destroys electronics in real time and can energize chassis. Recognizing the floating-midpoint pattern fast is the difference between a quick utility callout and a customer's whole house of equipment dying.

Symptom presentation

Classic report: half the house lights flare brighter and bulbs pop, the other half dims, and the imbalance flips as loads turn on and off. Customers describe TVs and microwaves failing, dimmers flickering, a low hum, and 240V appliances behaving oddly. Ask whether it started after a storm, a utility line touch, recent panel or meter work, or seemingly nothing. A symmetrical brighten-one-side, dim-the-other pattern is nearly pathognomonic for a lost neutral; it is not a branch-circuit fault.

Quick checks before isolation

Confirm the pattern with a meter. Read line-1-to-neutral and line-2-to-neutral at the panel. Healthy is roughly 120V each and 240V line-to-line. An open neutral shows the two line-to-neutral readings summing to ~240V but split unevenly (for example 150V and 90V), shifting as loads change, while line-to-line stays near 240V. That signature alone confirms a neutral problem. Do not start pulling branch circuits; the fault is on the neutral path common to both legs, which is upstream of the branches.

Isolation tree

Step 1 - Confirm open neutral by the floating midpoint. With the meter on the two line-to-neutral readings, switch a large 120V load on the dim side. If the dim side's voltage rises and the bright side's falls (the midpoint follows the load), the neutral is open between the panel and its reference. A solid neutral would hold both legs near 120V regardless of load shifting. This test isolates the problem to the neutral conductor, not the hots.

Step 2 - Locate the open point: utility versus customer side. Open the meter base or main disconnect (utility coordination required to pull the meter). Measure from the panel neutral bar to a known good earth ground (a driven rod or a metal water main bond) under load. If the panel neutral bar floats above ground potential under load, the neutral is open between the panel and the utility transformer - this includes a loose neutral lug at the meter, a corroded meter jaw, a failed service-entrance neutral, or a utility-side break. A neutral bar that holds at ground potential while branches still misbehave points to a sub-feed neutral problem instead.

Step 3 - Connection-by-connection on the customer side. Inspect, in order of probability, the main neutral lug torque at the panel, the neutral-to-ground main bonding jumper (a missing or loose main bond can mimic an open neutral), the meter base neutral, and the service-entrance cable neutral for corrosion or a burned strand. A panel that has had aluminum SE cable for decades commonly fails at the meter or main lug from oxidation creep. Torque-check and thermal-scan each.

Step 4 - Subpanel feeders. If the main service neutral is sound but one subpanel shows the floating pattern, the open neutral is in that feeder: a loose feeder neutral at either end, or on older installs an improper neutral-ground bond at the subpanel masking a feeder neutral break. Confirm the subpanel neutral and ground are correctly separated (post-2008 four-wire feeder) and the feeder neutral is intact and torqued at both ends.

Confirming the diagnosis

A confirmed open service neutral shows the two line-to-neutral voltages summing to 240V but splitting unevenly and shifting with load, while line-to-line stays near 240V, and the panel neutral bar floats above earth under load. Tightening or replacing the specific failed neutral connection collapses the split back to near 120V/120V and the bar returns to ground potential. A confirmed utility-side break shows the same pattern persisting after every customer-side neutral is verified tight, which is the handoff point to the power company.

Remediation

Repair the specific neutral defect: torque the main neutral lug to spec, re-terminate or replace a corroded meter-base or service-entrance neutral, restore a missing main bonding jumper, or upsize/replace a degraded aluminum SE neutral. Where the open is on the utility side of the point of demarcation, the power company owns the repair - de-energize, secure the service, and hand off with your measurements. After repair, re-balance verify: both line-to-neutral legs near 120V under shifting load, neutral bar at earth potential, and the main bond present and correct per NEC 250.24 and 250.28.

References

• NEC 2023 Article 250.24 - Grounding service-supplied AC systems, main bonding
• NEC 2023 Article 250.28 - Main bonding jumper and system bonding jumper
• NEC 2023 Article 250.32 - Buildings supplied by a feeder, separate neutral and ground at subpanel
• NEC 2023 Article 300.13(B) - Continuity of grounded conductors
• ANSI C84.1 - Voltage Ratings for Electric Power Systems and Equipment