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How to Identify L1 / L2 / N Issues
USA: Split-Phase 120/240V & 3-Phase Wye 120/208V AC
First – Know Normal Voltage Readings
1) Residential Split-Phase 120/240V (2-wire hot + neutral)
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L1 ↔ N: 120V
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L2 ↔ N: 120V
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L1 ↔ L2: 240V
2) Commercial 3-Phase Wye 120/208V (L1/L2/L3 + N)
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L1 ↔ N: 120V
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L2 ↔ N: 120V
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L3 ↔ N: 120V
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L1 ↔ L2 / L2 ↔ L3 / L1 ↔ L3: 208V
Step 1: Measure with Multimeter (AC Voltage)
Use AC voltmeter, measure L1-N, L2-N, L1-L2 (add L3 if 208V).
Normal Healthy Readings
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Line to Neutral: 115–125V
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Line to Line:
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120/240V system: 230–250V
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120/208V system: 200–215V
Step 2: Interpret Abnormal Readings (Fault Diagnosis)
Case A: One Line to Neutral Low (e.g. L1-N = 95–110V, L2-N normal)
Cause
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Loose/burnt L1 wire, bad breaker lug, corroded connection
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Overloaded circuit on that leg
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Undersized wiring long run
Case B: L1-N high, L2-N low (e.g. 132V / 108V) L1-L2 still ~240V / 208V
#1 Most Dangerous Fault: Open / Loose Neutral
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Unbalanced voltage will damage electronics, inverters, batteries
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Common at main panel neutral bar, loose wire nut, broken neutral splice
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Quick Field Rule Summary
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Uneven L1-N / L2-N voltage → suspect Neutral first
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One leg low only → bad that hot line (L1/L2) connection
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Line-Line voltage normal but L-N unbalanced → always neutral fault
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Sub-panel: Neutral & Ground must not be bonded.
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