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    electrical troubleshooting

    Dim lights when large appliance starts

    Common symptoms: lights dim; lights flicker on appliance; voltage drop; lights dim when ac starts; lights dim when fridge cycles; lights brighten

    Stop and call a pro if:

    • lockout-tagout-required
    • service-neutral-suspicion

    Step-by-step diagnostic flow

    1. Step 1

      If diagnosis involves opening the panel, establish an electrically safe work condition first. For voltage observation only, the panel cover stays on.

    2. Step 2

      When the appliance starts, which lights dim?

      Same-circuit dimming is normal in moderation. Whole-house dimming is the diagnostic flag.

    3. Step 3

      How significant is the dimming on circuits OTHER than the appliance's?

    Possible outcomes

    Stop — establish electrically safe condition before opening the panel

    high confidence

    Panel-interior work requires verified zero energy per NFPA 70E §120.5.

    Safe next steps
    • Keep the cover on for voltage observation, or de-energize the main and verify before opening

    Expected motor inrush — minor dimming on the same circuit is normal

    high confidence

    Motor and compressor inrush is 5–8× run current for a fraction of a second. A brief, small dim on the same branch is expected and not a fault.

    Safe next steps
    • No action required if the dim is barely perceptible and only on the appliance's circuit
    What to document for a pro
    • If it worsens over time, document and re-check

    Lights dim AND brighten at the same moment — open neutral signature

    high confidence

    Simultaneous dim/brighten across paired circuits is the canonical open-neutral signature. The neutral path has high resistance somewhere upstream — possibly at the panel neutral bus, the service-entrance neutral, or a multi-wire branch circuit splice.

    Safe next steps
    • See the 'Open neutral diagnosis (MWBC)' workflow
    • Until diagnosed, unplug sensitive electronics on the affected circuits
    • Treat this as a do-not-defer item
    What to document for a pro
    • Whether dimming/brightening is concentrated on certain breakers
    • Whether the home has overhead or underground service
    Open neutral risks appliance damage and fire

    Possibly a loose service connection or panel neutral

    medium confidence

    A clear whole-house dim transient that recovers fast is characteristic of a loose conductor — at the main lugs, at the meter, or at the utility tap.

    Safe next steps
    • Visually inspect the main panel cover for scorch marks
    • Have a licensed electrician torque-check the main lugs and neutral bus
    • If service is overhead, the utility can check the drop and meter
    What to document for a pro
    • Time of day the dim happens most
    • Whether it's worse in cold weather (thermal contraction loosens joints)

    Lights stay dim — likely undersized service, voltage drop, or utility-side issue

    medium confidence

    Persistent dim while a large appliance runs indicates sustained voltage drop. Causes: undersized service, undersized branch conductor for the load, or a utility-side regulation problem.

    Safe next steps
    • Note the appliance amp draw and the panel main amp rating
    • If it's a large new appliance (EV charger, well pump, central AC), a load calc and possibly service upgrade may be needed
    • Have the utility check supply voltage at the meter while the appliance is running
    What to document for a pro
    • Appliance nameplate amps
    • Panel main amp rating
    • Wire gauge on the appliance branch
    Diagnostic guidance only. If unsure, stop and call a licensed professional — gas, electrical, and refrigerant work is hazardous to untrained users.

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