WiremanWireman · Lights dim when the A/C starts: troubleshooting for electricians · 2023 NEC ·
Lights dim when the A/C starts
A blink or a long dip every time the compressor starts. Normal inrush, a hard-starting compressor, or too much resistance in the supply.
Before you start
How much electrical experience do you have?
This sets how much detail each step shows. You can change it any time.
Showing detail
How it works
A compressor motor draws its locked-rotor current (LRA on the nameplate, several times the running current) for a fraction of a second as it starts. That current through the impedance of the transformer, the service conductors and the A/C circuit drops the voltage, and lights show it as a blink. A short, small dip is normal. A deep dip means the supply has too much impedance or a loose connection. A long dip means the compressor is slow to start. A dip where some lights go brighter is a neutral problem.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What do the lights do?
- A quick blink, under a secondTest: Measure the dip at the panelExpect: A dip of a few volts for well under a second, about the same on both legs.
- Small, brief, equal on both legsNormal compressor inrush
- Large dip on both legsTest: Compare the dip at the main and at the A/CExpect: Most of the dip shows at both places. A small additional drop at the disconnect is normal.
- Main dips about as muchHigh impedance in the service or utility supply
- Main holds, disconnect dips a lotToo much impedance in the A/C circuit
- Much larger dip on one legA high-resistance connection on one leg
- Dim for several seconds, or the A/C strugglesTest: Measure the starting currentExpect: A peak at or below the nameplate LRA that falls to running current in well under a second.
- Falls to running current quicklyTest: Compare the dip at the main and at the A/CExpect: Most of the dip shows at both places. A small additional drop at the disconnect is normal.
- Main dips about as muchHigh impedance in the service or utility supply
- Main holds, disconnect dips a lotToo much impedance in the A/C circuit
- Stays high for seconds, or tripsA hard-starting compressor
- Falls to running current quicklyTest: Compare the dip at the main and at the A/CExpect: Most of the dip shows at both places. A small additional drop at the disconnect is normal.
- Some lights dim and others get brighterA loose or open service neutral
- A quick blink, under a secondTest: Measure the dip at the panelExpect: A dip of a few volts for well under a second, about the same on both legs.