Wireman · Neutral overheating from harmonics: troubleshooting for electricians · 2023 NEC ·

Neutral overheating from harmonics

A hot neutral or transformer with balanced phases. Triplen harmonics from electronic loads adding on a shared neutral.

Before you start

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How it works

Electronic loads like computer power supplies, LED drivers and drives draw current in pulses instead of a smooth sine wave. Those pulses contain harmonics, currents at multiples of 60 Hz. In a 3-phase 4-wire wye system, the 60 Hz currents from balanced phases cancel on the neutral, but the triplen harmonics (3rd, 9th, 15th) line up and add. A shared neutral can then carry as much as or more than a phase conductor even with perfectly balanced phases. When most of the load is nonlinear, the neutral counts as a current-carrying conductor for ampacity adjustment, and an average-responding meter reads these currents wrong, so use true RMS.

Step by stepFull flowchart

Every question and answer in this guide. Tap “Step by step” to walk through it.

What did you find?
- A neutral or neutral lug is hot or discoloredTest: Clamp each phase and the neutralExpect: Neutral current roughly equal to the imbalance between the phases, and well below phase current when the phases are balanced.
  - Phases balanced, neutral still highTest: Confirm the harmonic contentExpect: On a mostly linear load, the neutral current is mostly 60 Hz.
    - Mostly 180 Hz (3rd harmonic)Triplen harmonics overloading the neutral
    - Mostly 60 HzUnbalanced phase loading
  - Neutral tracks the phase imbalanceUnbalanced phase loading
  - Neutral current is normal, lug is hotA loose or damaged neutral termination
- Neutral current close to or above phase currentTest: Clamp each phase and the neutralExpect: Neutral current roughly equal to the imbalance between the phases, and well below phase current when the phases are balanced.
  - Phases balanced, neutral still highTest: Confirm the harmonic contentExpect: On a mostly linear load, the neutral current is mostly 60 Hz.
    - Mostly 180 Hz (3rd harmonic)Triplen harmonics overloading the neutral
    - Mostly 60 HzUnbalanced phase loading
  - Neutral tracks the phase imbalanceUnbalanced phase loading
  - Neutral current is normal, lug is hotA loose or damaged neutral termination
- The transformer feeding the panel runs hotTest: Clamp each phase and the neutralExpect: Neutral current roughly equal to the imbalance between the phases, and well below phase current when the phases are balanced.
  - Phases balanced, neutral still highTest: Confirm the harmonic contentExpect: On a mostly linear load, the neutral current is mostly 60 Hz.
    - Mostly 180 Hz (3rd harmonic)Triplen harmonics overloading the neutral
    - Mostly 60 HzUnbalanced phase loading
  - Neutral tracks the phase imbalanceUnbalanced phase loading
  - Neutral current is normal, lug is hotA loose or damaged neutral termination
- High neutral-to-ground voltage at receptaclesTest: Measure neutral to ground at the load and the panelExpect: A small N-G voltage at the receptacle, rising with load. Near 0 V at the service.
  - High at the receptacle, low at the panelA shared branch neutral carrying harmonic or excess current
  - High at the panel tooTest: Clamp each phase and the neutralExpect: Neutral current roughly equal to the imbalance between the phases, and well below phase current when the phases are balanced.
    - Phases balanced, neutral still highTest: Confirm the harmonic contentExpect: On a mostly linear load, the neutral current is mostly 60 Hz.
      - Mostly 180 Hz (3rd harmonic)Triplen harmonics overloading the neutral
      - Mostly 60 HzUnbalanced phase loading
    - Neutral tracks the phase imbalanceUnbalanced phase loading
    - Neutral current is normal, lug is hotA loose or damaged neutral termination