Half the House Dead After a New Panel: Feeder vs Lug Decision Tree

Why this matters

When half the house goes dead right after a panel replacement, the cause is an open or de-energized leg, and the question is whether the break is upstream at the service feeder and main lug or downstream from a breaker that was never re-energized correctly. A 120/240V service delivers two hot legs; lose one and every 120V load on that leg dies while 240V loads run weak or not at all. Right after a panel swap, the usual suspects are a main lug torqued on a phase that lost contact, a feeder conductor not fully landed, a bus or main breaker pole not making, or simply breakers left off. Getting this right fast matters because a partially open service can also signal a far more dangerous open neutral, and because energized panel work to chase it demands proper procedure. Reading the two legs at the main and walking the open methodically is the whole game.

Symptom presentation

Immediately after the panel was changed, roughly half the receptacles and lights are dead while the other half work. 240V appliances may run abnormally or not at all. The dead loads cluster on one bus leg. The customer typically reports the outage started the moment the new panel went live. Resetting individual breakers does not restore the dead half if the leg feeding them is open upstream.

Quick checks

• At the main, measure L1-N and L2-N. A healthy service reads about 120V on each. If one leg reads near 0V and the other reads about 120V, one hot leg is open, this is the core finding.
• Measure L1-L2. About 240V means both legs are present at the bus; far below means a leg is missing at the main.
• Map the dead loads to bus position; if every dead breaker sits on the same vertical bus stab pattern, the open is at that leg's main lug or bus, not at individual branches.
• Inspect the main breaker lugs and feeder terminations for a conductor not fully seated, a lug torqued on insulation, or a stab not engaging the main.
• Confirm no branch breakers were simply left in the OFF or tripped-center position on the dead leg.

Isolation tree

Branch first on where the open appears. Read both legs at the main lugs (or line side of the main breaker) and at the bus. If a leg is present at the main lug but absent at the bus, the main breaker pole or its bus connection is the open, proceed to the main/bus branch. If the leg is absent at the main lug itself, the feeder/service conductor or its termination is open, proceed to the feeder branch.

Feeder branch: a service or feeder conductor not fully landed in the main lug, a lug tightened on conductor insulation, or a damaged conductor from the swap leaves that leg dead at the main. Confirm by reading line-side voltage on each feeder; the dead leg reads low. Re-land and torque the conductor to clean copper/aluminum and re-measure. For a service feeder, verify the utility side is delivering both legs before assuming the panel work.

Main/bus branch: if both legs are present at the main lugs but one is dead on the bus, the main breaker pole is not making or a bus stab is not seated. Confirm by reading across the main breaker (line to load) on the dead pole; a large drop means that pole is open. A main breaker not fully snapped onto the bus, or a bus stab bent during the swap, produces a dead half-bus.

Branch-position branch: if both bus legs read 120V at the bus but a group of circuits is dead, the affected branch breakers are off, tripped, or not seated on the bus stab. Re-seat and reset them; a breaker not fully engaged on the stab is dead and runs no load.

Neutral cross-check: if instead of one dead leg you find voltages swinging high on one side and low on the other, you have an open neutral, not an open hot. This is a separate, more hazardous fault, verify the neutral landing and bonding before anything else.

Confirming diagnosis

Confirm with the two-leg voltage map. The decisive reading is one leg at roughly 120V and the other near 0V referenced to neutral, with L1-L2 well below 240V, which proves one hot leg is open and the dead loads belong to it. Then localize by walking the voltage upstream: read the dead leg at the branch breaker (dead), at the bus stab (dead confirms the open is at or above the bus), at the main breaker load and line terminals, and at the feeder lug. The first point moving toward the service where the dead leg becomes live is just downstream of the open; the open is at the connection between the last live point and the first dead point. Re-torquing the suspect lug or re-seating the main and watching the dead leg come alive confirms the location. Always re-check L1-N and L2-N after the fix to confirm both legs are restored and balanced, and confirm the neutral is solid by verifying stable 120V on both legs under load.

Remediation

For an open feeder/service lug, de-energize, cut back to clean conductor, fully land it in the lug, and torque to the panel's specified value with a calibrated tool; confirm no insulation is clamped under the lug. For a main breaker or bus problem, re-seat the main fully onto the bus, replace a damaged main or bus stab, and verify both poles make. For branch breakers left off or unseated, fully engage each on its stab and reset. Re-torque all neutral and ground bar terminations and confirm exactly one main bonding jumper. After any repair, energize and re-read L1-N, L2-N, and L1-L2 to confirm both legs near 120V and 240V, then verify representative loads on the previously dead half operate normally under load.

References

• NFPA 70 (NEC) 408.41 and 408.3, Panelboard grounded-conductor terminations and conductor arrangement
• NFPA 70 (NEC) 110.14(A) and 110.14(D), Terminal connections and installation torque to listed values
• NFPA 70 (NEC) 250.24 and 250.28, Service neutral landing and main bonding jumper integrity
• NFPA 70 (NEC) 230.90 and 230.70, Service overcurrent and disconnect provisions
• NFPA 70E, Standard for Electrical Safety in the Workplace, energized panel work and PPE
• Manufacturer panel installation instructions and lug torque tables (e.g. Square D, Eaton, Siemens loadcenters)