Why Do My Lights Dim When the AC Turns On?

Why do my lights dim when the AC turns on?

Your AC compressor pulls several times its normal running current for a split second as it starts. That surge drags your home’s voltage down for a moment, and lights on the same service dip. A slight blink that’s over in well under a second, the same as always, is normal. Deeper or longer dimming is worth a closer look.

In July your AC cycles on and off all day, so you see that blink a lot. Let’s sort out when it’s harmless and when it isn’t.

Normal blink or warning sign?

Normal looks like this: a small dip, gone almost before you notice it, and it’s done that for years.

These are not normal:

• Dimming that’s deep, or lasts more than a second or two
• Dimming that’s getting worse over the summer, or happens when other appliances start too
• Lights that flicker at random, with nothing switching on
• A burning smell, or a panel that feels warm

If you smell burning or see smoke, get everyone out and call the fire department first.

The one symptom that means call today

Picture this: the AC kicks on, the kitchen lights sag, and the lamp in the living room gets brighter. Some lights dimming while others brighten points to a loose or failing neutral connection. That can push damaging voltage into your electronics, and it’s a fire risk. Call an electrician or your utility right away. Don’t wait for the weekend.

Did it start after you switched to LED bulbs?

If you swapped to LEDs this spring and the flicker seems new, the bulbs may be the only thing that changed. LEDs show small voltage dips more visibly than the old incandescent bulbs did. Same dip, easier to see. If it’s still brief and shallow, it’s the same normal blink you’ve always had.

What causes the bigger dips

When the dimming crosses the line, the cause is usually one of these:

• The AC itself. A weak capacitor or an aging compressor that starts hard pulls a bigger surge.
• Loose connections. A loose or corroded connection at a breaker, in the panel, at the meter or at the utility connection makes every surge hit harder.
• Your service or circuits. A service that’s small for your home’s loads, or lights sharing a circuit with air-handling equipment, will sag every time the AC starts.
• The utility side. A problem with the transformer or the service line. If your neighbors see the same thing, that’s a strong hint.

A safe check you can do yourself

Next time the AC kicks on, watch and take notes. Which lights dim: one room or the whole house? How long does it last? Does anything get brighter? Whole-house dimming points to your service or the utility. One room points to a single circuit.

How it gets fixed

1. An electrician tightens or repairs loose connections and checks your voltage under load.
2. An HVAC technician can add a soft starter to your AC, which cuts the startup surge.
3. Lights that share a circuit with your air handler can move to their own circuit.
4. If capacity is the real problem, your panel or service gets upgraded. Our post on upgrading from a 100-amp to a 200-amp panel explains when that makes sense.
5. If the trouble is outside your house, your utility checks its side.

If your AC is tripping its breaker instead of just dimming the lights, read why your AC keeps tripping the breaker. Infiniti Electric is licensed, insured and available 24/7 in Riverside, and our electrical troubleshooting starts with measuring voltage while your AC starts. Bring your notes and contact us.

AC and dimming lights questions