HOW-TO GUIDE
Why Do My Lights Flicker When the Refrigerator or AC Starts?
By the OhmSavvy Research Team ·
Warning signs that skip the troubleshooting
- Burning smell, a warm or discolored receptacle or switch, buzzing or crackling at the panel: switch off the appliance, kill power at the main if you can do so safely, and call a licensed electrician. Call 911 for smoke or fire.
- Lights going noticeably brighter, or one part of the house brightening while another dims: unplug sensitive electronics and call the utility or an electrician the same day.
- Never open the panel, the meter base, or the air conditioner's outdoor disconnect or service panels to investigate.
What startup current does
A motor at rest has very little opposing it electrically, so when power is first applied it draws a large current until it spins up. Compressors are harder still, because they start against the pressure in the refrigerant system. This surge is called inrush or locked-rotor current, and for a compressor it commonly runs to several times the steady running figure, though only for a small fraction of a second to roughly a second.
Every wire, terminal and transformer between the utility and the compressor has some resistance. Current through resistance produces a voltage drop, so during the surge, the voltage left for everything else on the same path falls. The lights are simply the most visible load on that path. When the compressor reaches speed, the current settles and the voltage recovers, which is why the flicker is a single blink rather than a steady dimming.
How deep a dip is normal
The size of the dip depends on how much wire the surge shares with the lights. A refrigerator plugged into the same branch circuit as the kitchen lights shares the whole run back to the panel, out and back. A central AC on its own 240 V circuit shares only the service conductors and the utility transformer with the lights, so the whole house dips, but by less.
As a rough illustration, 12 AWG copper has a resistance of about 1.6 ohms per 1,000 feet. If a compressor briefly draws an assumed 30 A, the extra drop on a shared circuit grows with the length of the run:
A couple of volts for an instant is hard to notice. Seven or eight volts on a 120 V circuit is clearly visible. A loose terminal adds its own resistance on top, so a circuit that used to be fine and now flickers hard may have a connection that is starting to fail.
Symptoms and likely causes
| Symptom | Likely cause | What to check | Who should fix it |
|---|---|---|---|
| Faint whole-house blink as AC starts | Normal compressor inrush on the service | Whether it is getting worse | No action if stable |
| Deep dip, AC hums or clicks before starting | Weak start capacitor or hard-starting compressor | Time from click to running | HVAC technician |
| Kitchen lights dip when the fridge cycles | Fridge and lights on one branch circuit | What else is on that circuit | Electrician to split or check the circuit |
| Dip worse than it used to be, warm outlet | Loose terminal or splice on the circuit | Stop using the outlet | Electrician |
| Some lights brighten while others dim | Loose or failing neutral | Readings on several circuits | Utility or electrician, same day |
| Neighbors see dips too, worse at peak times | Undersized or overloaded utility transformer | Ask neighbors; keep a log | Electric utility |
1Make the checks you can do without tools
- Match the blink to the machine. Stand where you can hear the fridge or the outdoor unit and watch a light. If the flicker lines up with the compressor starting every time, you have found the trigger.
- Note the reach. One room only suggests a shared branch circuit; every room suggests the service or the AC's own startup.
- Listen to the start. A compressor that hums, clicks off and retries, or takes several seconds to settle is drawing its surge for longer than it should.
- Read the panel directory. If the fridge and lights are labeled on the same breaker, that explains a local dip. Resetting or switching breakers is fine; opening the cover is not.
- Remove extension cords. A refrigerator on a light-duty cord or power strip adds resistance; plug it directly into a wall receptacle as its manual usually asks.
2Measure the dip and the surge
Two measurements turn the flicker into numbers. Use a meter rated CAT III 600V or higher, inspect the leads, and wear safety glasses.
The voltage dip. Put the meter's probes in a receptacle on the same circuit as the flickering lights, set AC volts, and start Min/Max. Wait for the fridge or AC to start, then read the minimum. Bear in mind that a general-purpose meter may not register the full depth of a very short dip. Compare with the idle reading and with the same test on a receptacle in another room.
The startup current. For a plug-in refrigerator, put a line splitter between its plug and the wall and clamp the meter on the splitter's single-conductor loop, never around the whole cord. With an inrush mode selected, the clamp captures the surge when the compressor starts. Comparing that figure with the running current, and with the circuit's breaker rating, shows how much the appliance is asking of the circuit. Our guide to measuring AC current with a clamp meter covers the technique, and the clamp meter buying guide explains what inrush modes do.
A central AC is hardwired at 240 V, and clamping its supply means opening the outdoor disconnect or an equipment panel. Leave that to an HVAC technician or electrician, who can measure its starting current against the nameplate locked-rotor figure.
When it is a loose neutral
A US home gets two 120 V legs and a shared neutral. While the neutral is solid, a surge on one leg only lowers voltage a little. If the neutral connection loosens, at a splice, at the panel or out at the service, the two legs stop being held at their proper values. A large 120 V load on one side then drags that side down while the other side rises, sometimes well above normal. The giveaway is lights on some circuits brightening as others dim, often when the fridge, microwave or a hair dryer runs.
This is not a flicker to live with. It can overheat connections and damage electronics on the high side. Unplug sensitive equipment, and call the utility, who will check their side of the meter, and an electrician for the house side. Our page on half the house losing power covers the related case where one leg drops out completely.
Who to call
Clamp meters with an inrush mode
600 A AC with inrush mode and simultaneous V + A display, CAT IV 600V. No DC amps.
Budget AC/DC clamp to 1000 A with AC inrush, LoZ and NCV. Resolution not published.
Inrush plus watts, VA and power factor, CAT IV 600V / CAT III 1000V. One-year warranty.
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Frequently asked questions
Is it normal for lights to dim when the AC kicks on?
A brief, slight dim across the house as a central air conditioner starts is common, because its compressor draws a large surge through the service wires and the utility transformer. It becomes a concern when the dip is deep, lasts longer than a blink, has recently got worse, or comes with breakers tripping or the compressor struggling to start.
Why do only the kitchen lights flicker when the fridge starts?
The refrigerator and those lights probably share a branch circuit, so the fridge's startup surge flows through the same wire and connections that feed the lights. A long run, a heavily loaded circuit or a loose terminal on that circuit makes the dip bigger. An electrician can confirm what is on the circuit.
Can a hard-start kit stop the flicker from my air conditioner?
On some systems an HVAC technician may fit a hard-start or soft-start device to shorten or soften the compressor's startup. Whether it suits your unit depends on the equipment and the manufacturer's guidance, so ask the technician rather than adding one yourself.
Could the flicker damage my appliances?
Short, shallow startup dips are something appliances are designed to ride through. Repeated deep sags, or a neutral fault that pushes some circuits well above normal voltage, can stress motors and electronics. That second pattern is the one to act on quickly.
Do LED bulbs flicker more than incandescent bulbs when a motor starts?
They can behave differently. An incandescent filament dims smoothly during a sag because it holds heat. An LED bulb's driver may ride through a small dip with no visible change, or, in cheaper bulbs and some dimmer combinations, show a sharp blink or flutter. Swapping bulbs can change how noticeable the flicker is, but it does not change the voltage dip itself.
Where to go next
How to Check for Voltage Fluctuations in Your Home →
Logging, Min/Max and plug-in monitors for dips that come and go.
Washing Machine Keeps Tripping the Breaker →
Another appliance whose fault shows up at a particular moment.
Why Does My Portable Generator Shut Off? →
Starting watts versus running watts when the supply is small.
