In a Miami house the air conditioner starts dozens of times a day, and every time it does, the compressor motor pulls a burst of current for a moment. If your lights dip at that moment, you will notice it constantly. Sometimes that dip is normal. Sometimes it is the first sign of a failing part in the air conditioner, a weak connection in the panel, or a problem with the service coming into the house. The details tell you which.
Why lights dip when the AC starts
A large motor needs several times its running current for a fraction of a second as it gets going. That surge flows through the same service wires, meter and panel as your lighting. The wires have a little resistance, so the surge pulls the voltage down slightly for an instant, and the lights dim. LED bulbs can show it as a flicker rather than a dim. On a healthy system with an air conditioner in good condition, the dip is small and over before you are sure you saw it.
Usually normal
- A brief, slight dim when the outdoor unit starts, then steady light.
- The same small dip every time, not getting worse over the months.
- Only the lights; nothing else in the house resets, buzzes or stops.
Worth a closer look
- A deep dip. Lights drop noticeably, or electronics restart.
- A long dip. The lights stay dim for a second or more, or flicker repeatedly as the unit struggles to start.
- Getting worse. It used to be barely visible and now everyone notices.
- Lights getting brighter as well as dimmer. Some rooms brighten while others dim. That is not a motor-start effect; it can be a lost neutral. See the lost-neutral warning.
- Other signs. Buzzing from the panel or the outdoor unit, a breaker that trips, or a burning smell. See burning smells and buzzing.
The usual causes when it is not normal
- The air conditioner is working too hard to start. A worn capacitor or a struggling compressor draws more starting current for longer. That is a job for your HVAC contractor, and it is worth booking: the same strain shortens the life of the compressor.
- A loose or corroded connection. At the breaker for the air conditioner, at the main lugs in the panel, or at the meter. A poor connection adds resistance, so every surge drops the voltage further and heats the connection. In coastal homes, corrosion is a common contributor; see salt air corrosion.
- A problem on the utility side. The service drop, the meter or FPL's equipment. If neighbors see the same thing, or the flicker happens with no AC starting, it may be FPL's to check. FPL's page on flickers explains the causes on its side and how to report them.
- An undersized service in a home that has added air conditioning, appliances and a car charger over the years. Our list of signs a panel needs replacing includes dimming when large appliances start.
What you can check
- Watch when it happens. Only when the AC starts, or also with the dryer, microwave or well pump? Every room, or one circuit?
- Ask a neighbor. If they see flicker at the same times, call FPL first.
- Note the AC's behavior. Does the outdoor unit hum or click several times before running? Does it trip its breaker? That points to the unit.
- Look, do not touch. With the panel door open (not the cover), look for rust, stains or scorch marks, and listen for buzzing.
What we check
We measure the voltage at the panel while the air conditioner starts, which shows how deep the dip really is. We check the condition and tightness of the connections at the meter base, main breaker, main lugs and the AC breaker, and look for heat and corrosion. If the dip comes from the unit, we will tell you so you can call your HVAC contractor. If it comes from the service, we will tell you whether it is FPL's side or yours; our guide to FPL's side or your house explains the split. Our lighting repair page covers flicker that is limited to one fixture or circuit.
For flicker that happens all over the house and not just with the AC, read lights flickering throughout the house.
Questions people ask
Can flicker damage my electronics?
A brief dim is unlikely to. Repeated deep dips and surges can be hard on electronics and on motors, which is one reason to find the cause. Whole-house surge protection is a separate safeguard worth considering; see surge protection.
Will a new air conditioner fix it?
If the old one was the cause, often. Modern units with soft-start or variable-speed compressors usually start more gently. If the cause is a connection or the service, a new unit will flicker the lights too.
Need it looked at?
KRW Electric (EC13012832) troubleshoots and repairs lights that dim or flicker when the air conditioning starts in Pinecrest, Coconut Grove, South Miami and Miami Beach, with same-day service when scheduling permits. Call (305) 359-7312 and tell us what you are seeing. We will ask a few questions, tell you whether it sounds urgent, and give you the earliest time we have. Our before you call checklist covers what to have ready, and what happens on a repair visit explains the rest.
Reviewed by Kyle White, KRW Electric Company LLC — licensed electrical contractor EC13012832.
