WiremanWireman · Open or loose service neutral: troubleshooting for electricians · 2023 NEC ·
Open or loose service neutral
Lights brighten and dim as loads switch, electronics die, or current shows up on water pipes. Find where the neutral is failing.
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How it works
On a 120/240 V service, the neutral carries only the difference between the two legs’ 120 V loads. If it opens, the loads on each leg end up in series across 240 V, and the voltage splits by how much load is on each side: the lightly loaded leg climbs well above 120 V and the heavy one sags. Whatever neutral current is left looks for another way back, through the grounding electrode conductor, metal water pipe, cable TV shields and even the neighbor’s service. Neutrals usually fail at a connection: the drop connectors, the meter socket neutral, the main panel neutral lug, or a corroded underground neutral.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What are you seeing?
- Lights brighten and dim as loads switchTest: Load each leg and watch the voltageExpect: Both legs stay close to each other (within a few volts) and move together as load changes. L1 to L2 near 240 V.
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Volts across the lug, or it’s hotA failing neutral lug in the panel
- About 0 V, lug is coolThe neutral is open upstream of the panel
- Legs move together, staying balancedThe service neutral is holding
- Balanced now, but it comes and goesAn intermittent neutral
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Some circuits high, others lowTest: Load each leg and watch the voltageExpect: Both legs stay close to each other (within a few volts) and move together as load changes. L1 to L2 near 240 V.
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Volts across the lug, or it’s hotA failing neutral lug in the panel
- About 0 V, lug is coolThe neutral is open upstream of the panel
- Legs move together, staying balancedThe service neutral is holding
- Balanced now, but it comes and goesAn intermittent neutral
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Shocks or current on water pipes or cableTest: Clamp the grounding and bonding conductorsExpect: Well under 1 A on grounding and bonding conductors. The service neutral carries the load imbalance.
- Several amps on the grounding conductorsTest: Load each leg and watch the voltageExpect: Both legs stay close to each other (within a few volts) and move together as load changes. L1 to L2 near 240 V.
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Volts across the lug, or it’s hotA failing neutral lug in the panel
- About 0 V, lug is coolThe neutral is open upstream of the panel
- Legs move together, staying balancedThe service neutral is holding
- Balanced now, but it comes and goesAn intermittent neutral
- Loaded leg drops, the other risesTest: Measure across the neutral connections you can reachExpect: About 0 V across the lug, and no hot spot.
- Little or no currentThe service neutral is holding
- Several amps on the grounding conductorsTest: Load each leg and watch the voltageExpect: Both legs stay close to each other (within a few volts) and move together as load changes. L1 to L2 near 240 V.
- A burned or melted neutral connectionA failed neutral connection
- Lights brighten and dim as loads switchTest: Load each leg and watch the voltageExpect: Both legs stay close to each other (within a few volts) and move together as load changes. L1 to L2 near 240 V.