A momentary dip in your lights when the AC compressor kicks on is usually normal and comes from inrush current, the brief surge a motor draws the instant it starts. It becomes a problem when the flicker lasts more than a second, keeps getting worse, or happens alongside buzzing, warm outlets, or a breaker that trips.
TL;DR
- A quick dip lasting under a second when the AC starts is typically harmless inrush current, most noticeable in older homes.
- Flicker that lingers, worsens over weeks, or spreads to the whole house points to a loose connection, an aging panel, or a failing breaker.
- San Diego’s summer AC load spikes push marginal 100-amp panels toward their limit, especially in older housing stock.
- Flicker paired with buzzing, a burning smell, or a warm panel is a stop-and-call situation, not a wait-and-see one.
- The general causes of flickering lights, beyond just the AC trigger, are covered in our full flickering lights guide.
Why AC startup causes a normal dip in the first place
An AC compressor is a motor, and motors draw far more current for a fraction of a second at startup than they do once running. This inrush current can be three to six times the compressor’s normal running amperage, pulling briefly on the same electrical system that’s feeding your lights. That momentary extra draw causes a small voltage dip across the circuit, which shows up as a quick dim or flicker.
This is more noticeable in older San Diego homes, particularly houses built in the 1960s through the 1980s in areas like Clairemont, Rancho Bernardo, and older parts of El Cajon and La Mesa, where panels were sized for a smaller electrical load than modern homes carry. If the dip lasts under a second and the lights return to full brightness right away, that’s the system doing what it’s designed to do under a normal, brief surge.
When it’s more than a normal dip
Pay attention to how the flicker behaves over time, not just whether it happens. A dip that’s been the same for years and hasn’t changed is a different situation than one that’s getting more noticeable or lasting longer each time the AC cycles.
Watch for these signs that point past normal inrush: the flicker lasts more than a second or two rather than an instant, it’s gradually gotten worse over recent weeks or months, it affects lights throughout the house rather than just the ones nearest the AC circuit, it happens with other appliances too and not only the AC, or the AC’s breaker has started tripping along with the flicker. Any of these means the AC startup is exposing a real electrical issue rather than just demonstrating normal motor behavior.
What’s actually behind persistent or worsening flicker
A loose connection. Loose connections at the breaker, the panel’s bus bar, or a junction somewhere in the circuit create resistance, and resistance under a sudden load like AC startup shows up as a more pronounced dip than a tight connection would produce. This tends to get worse over time as the connection degrades further, not better.
A panel near its capacity limit. San Diego summers push AC units to run longer and harder, and an older 100-amp panel that was already close to its rated load from other circuits can show voltage dips more severely under that seasonal strain. This is common in coastal-adjacent inland neighborhoods and older west-side Chula Vista housing stock, where the original panel predates central air conditioning. Our panel upgrade guide covers what it takes to move a maxed-out panel to a size that handles modern loads without the strain showing up as flicker.
A failing breaker. Breakers wear out after decades of cycling, and a worn breaker develops more internal resistance than a healthy one, which produces a bigger voltage drop under the same load. If the flicker is isolated to lights on the same circuit as the AC and has worsened as the breaker has aged, the breaker itself may be the failing component rather than the wiring around it. A breaker that’s degrading this way often starts tripping outright before long. Our breaker keeps tripping guide covers how to tell a worn breaker from a simple overload.
If the AC-triggered flicker is one symptom among several, including lights that flicker for reasons unrelated to the AC, utility-side issues, or bulb compatibility problems, our full flickering lights causes guide breaks down the complete list of causes beyond this specific trigger.
Safe checks vs. when to stop and call
You can safely note when the flicker happens and whether it’s getting worse. Time it: does it clear in under a second, or does it linger? Check whether it happens with other large appliances starting, like the dishwasher or a well pump, not just the AC. None of that requires opening a panel or touching wiring.
What you shouldn’t do yourself: open the electrical panel to inspect connections, tighten breaker or bus bar connections, or keep running the AC on a circuit that’s showing worsening flicker alongside buzzing or a burning smell. Homes further inland, including an electrician in El Cajon, see this pattern intensify during peak summer heat when AC units run for hours at a stretch, and a marginal connection that’s fine in spring can become a real problem by August.
Frequently asked questions
Is it normal for lights to dim slightly every time the AC turns on?
Yes, a brief dip under a second is typical inrush current from the compressor motor starting, and it’s more noticeable in older homes and panels sized for smaller loads. If it’s been consistent for a long time and doesn’t linger, it’s not usually a sign of a problem.
How do I know if AC-triggered flickering has gotten worse?
Compare how long the dip lasts and how deep it looks compared to what you remember from months ago. A dip that used to clear instantly but now takes a second or two, or that’s visibly darker than before, has changed for a reason, and that reason is worth having checked.
Could my panel be too small if my lights flicker every time the AC starts?
It’s possible, especially in older San Diego homes on original 100-amp service that also run central air, an EV charger, or other large loads. A panel running close to its rated capacity shows voltage dips more noticeably under a sudden draw like AC startup. A load calculation can confirm whether the panel is the limiting factor.
Does the AC breaker tripping along with the flicker mean something different?
Yes. A breaker that trips, rather than just a light dimming, points to an overload or a fault on that circuit specifically, not just a normal voltage dip shared across the panel. That combination is worth diagnosing sooner rather than later.
Should I stop running my AC if the lights flicker when it starts?
Not for a brief, consistent dip. If the flicker has become longer, deeper, or is joined by buzzing, a burning smell, or a warm panel, stop running that circuit and have it checked before continuing to use the AC on it.
When to call us
A dip that’s stayed the same for years usually doesn’t need a service call. Flicker that’s worsening, lasting longer, or showing up with buzzing, warmth, or breaker trips is worth having diagnosed before San Diego’s summer heat pushes it further. Our electrical troubleshooting service traces whether the cause is a loose connection, a failing breaker, or a panel that’s outgrown your home’s load. Call us at (858) 988-5580 for a same-day estimate.