WiremanWireman · Lights flicker or dim: troubleshooting for electricians · 2023 NEC ·
Lights flicker or dim
One fixture, one circuit or the whole house. Separates harmless from a failing neutral.
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How it works
Lights show voltage changes before anything else does. A short dip when a big motor starts is normal. Flicker on one fixture is usually the bulb, the dimmer or a loose connection in that box. Whole-house flicker, or lights that get brighter when something else turns on, points to the service, and a bad neutral is the one to worry about.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Which lights flicker?
- One fixture or lampIs it an LED on a dimmer?
- YesLED and dimmer mismatch
- No, or it flickers with the dimmer at fullA bad lamp, socket or loose connection in that fixture
- Everything on one circuitTest: Check voltage drop under loadExpect: A drop of less than about 5% (6 V on 120 V).
- Drops less than 6 VThe circuit itself is fine
- Drops more than 6 VA loose connection on that circuit
- Lights all over the houseWhen does it happen?
- A quick dip when the A/C, dryer or a big motor startsTest: Measure the dip at the panelExpect: A dip of a few volts for under a second, both legs about equal.
- Small, brief dip (under about 5 V)Normal motor-start dip
- Large dip on both legsToo much resistance in the service
- One leg drops while the other risesLoose or open service neutral
- Some lights get brighter while others dimLoose or open service neutral
- Randomly, or in wind or stormsTest: Compare the two legsExpect: Both legs within a few volts of each other, and they stay close as loads change.
- Legs stay balanced, but both jump aroundA supply-side problem
- Legs move apart as loads changeLoose or open service neutral
- A quick dip when the A/C, dryer or a big motor startsTest: Measure the dip at the panelExpect: A dip of a few volts for under a second, both legs about equal.
- One fixture or lampIs it an LED on a dimmer?