New Panel Shows Voltage Imbalance at Commissioning: Decision Tree

Why this matters

A freshly installed or swapped panel that reads unequal leg-to-neutral voltages at commissioning is signaling a connection problem you can fix before energizing loads - or a service condition you must verify. The two most common causes are a loose or incomplete neutral connection (so the legs float against an imbalanced load) and an open or high-resistance main connection on one leg. A small imbalance from naturally uneven load is normal; a large or load-tracking split is a fault. Catching it at commissioning, before the customer connects sensitive electronics, prevents overvoltage damage and a callback.

Symptom presentation

At commissioning you measure the two legs to neutral and find them unequal - for example one leg near 125 V and the other near 115 V, or worse, one climbing toward 140 V while the other sags toward 100 V as loads switch. Line-to-line may read normal while the leg-to-neutral split is wrong. The imbalance may be steady or may swing as you add load to one side.

Quick checks

• Read L1-L2, L1-N, and L2-N together with minimal load. Note all three.
• Compare the split. A few volts of difference under uneven load is normal; a large split, or one that grows as you load one leg, is a neutral or main-connection fault.
• Verify the neutral landed and torqued at the main neutral lug and that the main bonding jumper is correct for a service panel (neutral bonded only at the service).
• Confirm both main lugs and both bus legs are tight and seated.
• Sanity-check line-to-line. A healthy 120/240 V service reads roughly 240 V across the legs and about 120 V each to neutral. If line-to-line is correct but the two leg-to-neutral readings are lopsided, the legs are fine and the neutral is the suspect.
• Apply a small known load to one leg and watch the readings move. On a good neutral both legs stay near nominal; on an open neutral the loaded leg sags and the unloaded leg rises in step.
• Confirm the panel is wired as a service or a subpanel before judging the bonding: a subpanel with a bonded neutral, or a service with an unbonded neutral, both produce wrong readings and a hazard.

Isolation tree

Test the neutral first, then each leg's source connection.

1. Both legs near nominal, small difference, stable under load: normal.
   • Uneven 120 V loading naturally splits leg-to-neutral by a few volts. No fault. Document and proceed to load commissioning.
2. Legs swing opposite each other and track load (one rises, one falls): open or high-resistance neutral.
   • The neutral connection is loose, not landed, or high-resistance, so the legs float and divide voltage by the imbalance of the loads. Check the panel neutral lug, the service neutral splice, and the meter neutral.
   • This is the most dangerous outcome: the lightly loaded leg can overvolt to well above 120 V. Correct the neutral before connecting any loads.
3. One leg low and steady, the other near nominal, line-to-line low: high-resistance main connection on one leg.
   • A loose main lug, a poor meter jaw, or a bad bus connection on one leg drops that leg under load. Read across the main lug and the bus stab under load; a drop across one connection localizes it.
   • Distinguish from a utility issue by reading the meter line side; if the imbalance is present at the meter line terminals with the panel correct, refer to the utility.
4. Imbalance present at meter line side, panel connections correct: service or utility.
   • The split originates upstream of your work. Document and refer to the utility for the service drop, transformer, or primary.
5. Bonding and grounding errors. A neutral not bonded at the service, or bonded in more than one place, distorts the reference and can read as imbalance. Verify exactly one neutral-to-ground bond at the service and none downstream; confirm the grounding electrode conductor is landed. A floating or multiply-bonded neutral both produce unstable leg-to-neutral readings and are code violations as well as hazards.

When you correct a connection, re-read all three voltages before moving on. The signature of a fixed neutral is both legs returning to within a couple of volts of each other and staying there as you shift a test load from one leg to the other.

Confirming diagnosis

• Normal: small, stable leg-to-neutral difference that does not grow with one-sided load.
• Open/high-resistance neutral: legs swing opposite and track load; correcting the neutral connection brings both legs back to nominal.
• One-leg high-resistance main: one leg sags under load with a measurable drop across a single main or bus connection; line-to-line reads low.
• Service/utility: imbalance present at the meter line side with all panel connections verified.

Remediation

• Neutral fault: land and torque the neutral at the panel and service splice to the marked value; remake the meter neutral if loose; antioxidant on aluminum. Re-read both legs to neutral to confirm recovery.
• Main/bus connection: clean and torque the affected main lug and reseat the bus connection; replace heat-damaged hardware.
• Bonding error: verify the main bonding jumper bonds neutral to ground only at the service, and that subpanels keep neutral and ground separated.
• Service/utility: refer with documented readings.
• Re-commission: confirm balanced, stable leg-to-neutral voltages before connecting branch loads.

References

• NFPA 70 (NEC) Article 408 - Panelboards, busbars, and panel commissioning.
• NFPA 70 (NEC) Article 250 - Grounding, bonding, main bonding jumper, neutral continuity.
• NFPA 70 (NEC) Article 230 - Service conductors and the service neutral.
• NFPA 70 (NEC) Article 110 - Terminations and torque.
• NFPA 70E - Energized service-work practices and PPE.