Short answer: A short dip in the lights when the air conditioner starts is normal: the compressor draws several times its running current for a fraction of a second and the voltage sags. Flicker that has been getting worse, that lasts more than a blink, that happens when other things run, or that comes with a buzzing panel or warm outlets is not normal. That is a loose or corroded connection somewhere between the utility and your lights, or a service that is too small for the house, and a loose neutral in particular can start a fire.
What is actually going on
An air conditioner compressor is the largest motor in the house. On start-up it pulls "locked rotor" current, several times its running draw, for a fraction of a second. Every conductor between the utility transformer and your lights has some resistance, and for that instant the voltage everywhere in the house drops a few volts. Incandescent and cheap LED bulbs show it as a blink. That is the harmless version, and if it has always done it and it has not changed, it is a design characteristic, not a fault.
Three things turn it into a problem:
- A loose or corroded connection. Resistance at a bad joint (at the meter, the main lugs, a breaker, a neutral bar, or a splice in a box) makes the voltage dip much bigger, and the joint heats up every time. Flicker that is getting worse over months is this.
- A loose or open neutral. The neutral carries the imbalance between the two 120-volt legs. If it is loose, one leg goes high and the other goes low every time a big load starts: some lights get brighter while others dim. That pattern is the emergency one. Appliances on the high leg get over-voltage; the joint arcs.
- Undersized service. A 1970s house with a 100-amp service, or a 150-amp panel now carrying a modern kitchen, two AC systems and an EV charger, will sag on every start because the whole service is near its limit. The flicker is a symptom of a panel that needs upgrading.
How to tell which one you have
- Does it happen with anything else? AC only, brief, unchanged for years: normal. Also with the dryer, the oven, the microwave: the service or a connection.
- Do some lights get brighter while others dim? That is a neutral problem. Stop here and call an electrician today.
- Is it getting worse? A widening dip over weeks means a connection is degrading.
- Look and listen at the panel. Buzzing, a hot smell, a breaker that is warm to the touch, scorching around a breaker or the main: connection problem. Do not open the panel cover yourself.
- Check the panel brand. Federal Pacific (FPE Stab-Lok) and Zinsco panels are widely known for breakers that fail to trip and bus connections that overheat; home inspectors in both markets flag them routinely. If you have one, the flicker is a reason to replace it, not to tighten it.
- Look at the meter base and the service drop outside. Corrosion, a loose weatherhead, or damaged cable is a utility-side connection and often the whole explanation on a barrier island.
The honest DIY version
Keep homeowner checks to observation. Note which lights flicker, what equipment is running, and whether the problem is new or worsening. Do not remove the electrical-panel cover or replace a suspect outlet or switch to diagnose flickering. Have a qualified electrician identify the cause and make any needed repair, especially if there is heat, discoloration, buzzing or a burning smell.
When it is serious
- Some lights brighten while others dim. Open or loose neutral. Electrician today; if you smell burning, main breaker off and call.
- A warm outlet, a warm breaker, a buzzing panel, a burning smell anywhere.
- Flicker plus tripped breakers with normal use.
- Suspected aluminum branch wiring, often found in homes wired from the mid-1960s to the mid-1970s. Do not open the panel to identify it. If you cannot identify the wiring safely, have a qualified electrician check it. Flickering, hot faceplates or a burning-plastic smell warrant professional evaluation.
- An FPE or Zinsco panel with any of the above.
What is left after the electrician leaves — and who does it
The electrician tightens or replaces the connection, or replaces the panel, or tells you the service needs upgrading. What is left depends on which. A panel replacement usually needs the wall around it patched and painted, and sometimes the panel relocated out of a closet or bathroom where code no longer allows it, which is drywall and trim work. A service upgrade in an older Charleston house often becomes the moment to deal with the rest of the wiring, and if that touches the kitchen it turns into a panel-and-kitchen conversation. We do the electrical work through licensed electricians and the finish work around it as one job.
Charleston and York County notes
Charleston: a lot of the 1960s–70s stock in West Ashley, James Island and North Charleston has 100-amp services and aluminum branch wiring, and inspectors flag FPE panels here every week. On the barrier islands and along the marsh, salt air corrodes the meter base, the weatherhead and exterior boxes, so a flicker that started after a few years on Sullivan's or the Isle of Palms is often a corroded exterior connection rather than anything inside. York County: the flicker we see most in Fort Mill, Tega Cay and Rock Hill is a house from the late 1990s or 2000s that has since added a second HVAC system, a pool pump and a car charger to a 150- or 200-amp panel that was full the day it was installed. That is a load calculation, then a service upgrade, not a tightened lug.
Need this fixed?
Diagnosis and repair are licensed electrical work. See our electrical division and the guide to choosing an electrician; published ranges are in the Charleston electrical cost guide and the York County electrical cost guide.
Southeastern Renovation is a licensed South Carolina general contractor (CLG.127339) working in Charleston and York County. Tell us what you found or call (843) 718-8177.
Sources
- US Consumer Product Safety Commission — aluminum wiring repair guidance: cpsc.gov/s3fs-public/516.pdf
- Electrical Safety Foundation International — home electrical safety: esfi.org/home-electrical-safety/
General guidance for homeowners in Charleston and York County, South Carolina, written from what we see on jobs. Conditions vary house to house; anything structural, gas or electrical is licensed work in South Carolina and should be looked at in person.
Frequently Asked Questions
Is it normal for lights to dim when the AC turns on?
A brief, slight dim as the compressor starts is normal and has to do with the start-up current of the motor. It should be momentary, the same every time, and not getting worse. Anything beyond that is a connection or service problem.
What does it mean when some lights get brighter and others dimmer?
A loose or open neutral. One side of the service goes high and the other low every time a big load starts. It is a fire and appliance-damage risk. Call an electrician the same day.
Can flickering lights cause a fire?
Flickering itself no, but the loose or corroded connection that causes worsening flicker heats up every time a load starts and can arc. That is how many electrical fires begin. Worsening flicker is a reason to have the panel and service checked.
Do I need a bigger electrical panel if my lights flicker?
Sometimes. If the flicker happens with several large loads and the house has added AC, appliances or an EV charger to an older 100- or 150-amp service, a load calculation will often show the service is undersized. If it is one loose connection, tightening it is enough.
Are Federal Pacific and Zinsco panels really a problem?
Yes. Both have well-documented histories of breakers failing to trip and bus connections overheating, and home inspectors in South Carolina flag them routinely. Flicker in a house with one of these panels is a reason to replace the panel rather than repair it.
Why do my lights dim when the dryer starts?
The same mechanism as the AC: a large load starting draws a short surge and the voltage dips for a moment. A brief dim is common; dimming that lasts, is getting worse, or comes with warm outlets, a burning smell or a buzzing panel needs a licensed electrician.
