No Power Multiple Rooms Shared Neutral Vs Feeder Isolation Decision Tree

Why this matters

When several rooms lose power at once, the two leading causes are an open shared neutral on a multiwire branch and a lost feeder or subpanel feed. They present similarly (a block of rooms dark) but the fix and the hazard differ sharply. An open shared neutral leaves the hots live while loads see wild, series-divided voltages that destroy electronics; a lost feeder de-energizes everything downstream cleanly. Isolating which one you have, before randomly resetting breakers, protects equipment and points you straight at the failed connection rather than at the wrong panel.

Symptom presentation

• A cluster of rooms or outlets goes dead together, often on the same side of the house or the same subpanel.
• Some devices in the dead zone behave oddly (dim, flicker, or read strange voltage) rather than being fully dead, which points to an open neutral.
• Breakers appear on; nothing is obviously tripped.
• Voltage at dead outlets reads abnormal: very low on one, very high on another, or 120 V hot-to-ground but nothing hot-to-neutral.

Quick checks

• At a dead outlet, measure hot-to-ground, hot-to-neutral, and neutral-to-ground. Hot-to-ground present but hot-to-neutral absent, with neutral-to-ground elevated, signals an open neutral.
• Determine whether the dead rooms share one circuit, one multiwire branch, or one subpanel. A whole subpanel dead points to a feeder.
• At the panel, read both legs of the suspect feeder or breaker pair to ground.
• Look for a multiwire branch (two breakers, shared neutral) feeding the dead zone.

Isolation tree

Step 1. At a dead outlet, read hot-to-ground and hot-to-neutral.

• Hot-to-ground present, hot-to-neutral absent or abnormal, neutral-to-ground elevated: an open neutral. Go to Step 2.
• Both hot-to-ground and hot-to-neutral absent (truly dead, no voltage anywhere): a lost hot/feeder. Go to Step 5.

Step 2. Confirm it is a shared (multiwire) neutral and look at sibling circuits.

• The dead zone shares a neutral with circuits that show high voltage or stressed loads: classic open shared neutral, the loads are series-divided across the broken neutral. Go to Step 3.
• Single-circuit open neutral (no sibling sharing): trace that one neutral. Go to Step 4.

Step 3. Trace the shared neutral from the panel outward.

• Open at the panel neutral bar (loose lug, backed-out screw): re-terminate. With both hots de-energized, restore and torque the neutral.
• Open at a junction or device midway (a backstabbed or wire-nutted neutral that released): find the last live point and the first dead point; the open is between them. Repair the connection.

Step 4. Single open neutral.

• Open at a device termination or splice: locate by the live/dead boundary and re-terminate.

Step 5. Lost hot/feeder path. Read the feeder or breaker legs at the panel.

• One leg of a subpanel feeder reads 0 V to ground: an open feeder conductor or a loose feeder lug. De-energize upstream, inspect and re-terminate the feeder lug; check the upstream breaker.
• Both legs dead at the subpanel: the feeder breaker is off/failed or the feeder is open at the source. Inspect the upstream breaker and the feeder terminations.
• A single branch breaker dead with feeder present: the branch breaker tripped or failed, or the branch hot is open. Test/replace the breaker and trace the branch.

Confirming diagnosis

A confirmed open shared neutral shows hot-to-ground present, abnormal hot-to-neutral, elevated neutral-to-ground, and sibling circuits at strange voltages; it resolves when the broken neutral is re-terminated and all affected outlets return to a steady 120 V. A confirmed feeder loss shows one or both feeder legs at 0 V to ground with no neutral anomaly; it resolves when the feeder lug or upstream breaker is restored. The hot-to-neutral versus hot-to-ground pattern at the dead outlet is the deciding test. Document the three voltage readings before and after.

Reading the voltage signature

The three-reading pattern at a dead outlet sorts the two faults cleanly. An open neutral shows hot-to-ground at full 120 V (the hot is still connected) but hot-to-neutral collapsed or wandering, with neutral-to-ground elevated well above its normal near-zero, because the broken neutral now floats at whatever the series loads divide it to. A lost hot or feeder shows hot-to-ground at 0 V; there is simply no source. A useful tell on an open shared neutral is that the abnormal voltages shift as loads in the affected zone turn on and off, since the series divider rebalances; a feeder loss is steady at zero regardless of what is switched. Reading neutral-to-ground specifically, rather than only hot-to-neutral, is what exposes a floating neutral that a quick hot-to-ground check would miss.

Remediation

• Open shared neutral at the panel bar or a midpoint splice: de-energize both hots, re-terminate the neutral under proper torque, eliminate backstabbed neutrals on the run.
• Single open neutral at a device: locate by the live/dead boundary and re-terminate.
• Open feeder conductor or loose feeder lug: de-energize upstream, inspect and re-terminate; verify the upstream breaker.
• Tripped or failed branch breaker: test and replace; trace the branch for the underlying fault that tripped it.

References

• NFPA 70 (NEC) Article 210.4, Multiwire Branch Circuits (shared-neutral requirements).
• NFPA 70 (NEC) Article 300.13(B), Continuity of Grounded Conductors (open-neutral hazard, do not interrupt under load).
• NFPA 70 (NEC) Article 215, Feeders (feeder conductors and terminations).
• NFPA 70 (NEC) Article 408, Switchboards, Switchgear, and Panelboards (bus and lug connections).
• NFPA 70 (NEC) Article 110.14, Electrical Connections (termination integrity).