Identifying an Open or Backfed Neutral

General field practice for qualified electricians. The equipment manufacturer's instructions, your adopted code edition, and the AHJ govern where they differ.

⚠ An open neutral can put a full 240 V across 120 V loads, and a neutral can carry current even with its own breaker OFF. Treat every neutral as live until you have measured it. Service conductors on the utility side (drop, lateral, meter) are the utility's. Call them.

1 · Symptoms

Lights that brighten and dim when large loads start. Electronics or bulbs failing on one side of the house.
Some 120 V outlets reading high (130–200 V) while others read low.
Appliances acting oddly, a shock felt from plumbing, or a burning smell at the panel or meter.

2 · Service open neutral test

At the main lugs, measure L1–N, L2–N, and L1–L2 with the house loads on.
Turn on a big 120 V load on one leg, like a space heater or hair dryer, and measure again.
Open or poor neutral: L1–L2 stays ≈240 V but the two L–N readings split and shift (for example 150 / 90 V) as loads change. They still add up to about 240.
Check the neutral lug at the panel and at the meter base. If the problem is upstream of the meter, call the utility. Lift loads off the affected leg until it's fixed.

3 · Branch-circuit open neutral

At the outlet: hot–ground ≈120 V, but hot–neutral ≈0 V (or a ghost reading). The neutral is open between there and the panel. Check backstabbed receptacles and wire nuts upstream. The last working outlet is a good place to look.
Multiwire branch circuit (shared neutral): an open neutral puts the loads on the two legs in series across 240 V. The smaller load sees most of the voltage and burns out. Fix it immediately, and make sure there's a handle tie or 2-pole breaker (210.4(B)).

4 · "Backfed" neutral: current where it shouldn't be

Neutral from another circuit: turn the circuit's breaker OFF and clamp its neutral. Any current means another circuit's load is returning on it (crossed or "borrowed" neutrals). This is a common cause of AFCI/GFCI trips. Trace it and give each circuit its own neutral.
Current on the EGC or raceway: clamp the grounding conductor. Current there means a neutral-to-ground connection downstream of the service. Common causes are a bonded subpanel, which 250.24(A)(5) prohibits on the load side (keep neutrals and grounds separate in subpanels), a neutral touching a box, or a generator with a bonded neutral on an interlock.
Voltage on a "dead" neutral: with the breaker OFF, a neutral can still carry a dangerous voltage if it is shared or crossed. Measure before you open a neutral splice.
Clamp check: clamp the hot and neutral of one circuit together. The reading should be ≈0 A. Any net current is leaving on another path, such as ground or another circuit's neutral.

Reference only — not engineering, legal, or code-compliance advice. Results are estimates for qualified persons and must be verified against the NEC edition adopted where the work is done (your selected edition), manufacturer instructions, and the local AHJ. Use is subject to the Terms of Use & Disclaimer. — Morningstar Electric LLC