Why a starting motor can dim the lights
An air conditioner's compressor and a furnace's blower both run on electric motors. The U.S. Department of Energy's staff report on electricity markets and reliability explains that a shortage of reactive power makes voltage drop, and gives the everyday example: it's what you see "when the lights dim as an electric motor starts." The report is about the power grid, not home wiring, but the effect it describes is the same one many people notice when their heating or cooling kicks on.
Timing is a clue, not a diagnosis. Dimming that lines up with the equipment starting tells you when it happens. It doesn't tell you which part, connection or section of the electrical supply is responsible.
A quick dip isn't automatically harmless
The Consumer Product Safety Commission's Guide to Home Wiring Hazards lists dim or flickering lights among its warnings of potential hazards, describing lights that "dim" often. Its 1992 National Electrical Safety Month release names dim or flickering lights alongside overheated plugs and receptacle covers, lingering odors, power outages, sparks and damaged insulation, and says that if you experience any of these warning signs, a professional electrician or electrical inspector should be called to inspect your home.
So "it only happens when the AC starts" isn't enough to call it normal. There's no official number of seconds that makes a dip safe. Instead, pay attention to the pattern: is the dip barely noticeable or dramatic? Has it always happened, or is it new? Does the light come right back to normal brightness? Does it happen once at startup, several times during one start, or keep fluctuating while the equipment runs?
When to call an electrician
Arrange an electrician's assessment if the dimming is severe, getting worse, lasts beyond startup or keeps fluctuating. Do the same, sooner, if you also notice any of the other warnings in the CPSC guide: buzzing or sizzling from the electrical system, sparks or bright flashes, an odor of hot insulation, or switch plates, receptacle covers, cords or plugs that are hot to the touch or discolored from heat. Don't wait for a breaker to trip before taking these seriously. If breakers are already tripping, our guide to why a breaker trips when you use a kitchen appliance explains what that can mean.
Note the pattern without investigating the wiring
Useful notes for the electrician: which rooms or lights are affected, whether the lights recover right away, whether the equipment starts normally, and whether the dip happens with heating, cooling or just the fan. Mention any unusual sounds or smells you've already noticed, but don't approach suspect equipment to check.
These are observations, not a do-it-yourself test. Don't remove electrical covers, tighten connections or probe wiring, and don't keep switching the equipment on and off just to make the dip happen again. The CPSC guide warns that any shock, even a mild tingle, may be a warning of danger.
Ask for an assessment, not a predetermined fix
Start with a qualified electrician for the lighting and supply concern, and ask whether an HVAC technician should also look at how the equipment starts. The symptom on its own isn't a basis for choosing a new panel or an add-on part for the heating or cooling system, so ask what findings support any repair that's proposed. CPSC's 1992 release also says a thorough electrical inspection is strongly advised if a home is more than 40 years old or if additional outlets or high-wattage appliances have been added. Our checklist for hiring a home repair contractor in Philadelphia and Bucks County covers what to get in writing, and our guide to GFCI and AFCI protection explains two kinds of safety devices an electrician may bring up.
If you ever see smoke or fire, don't stay to find the cause. The U.S. Fire Administration's fire extinguisher guidance says that when you're unsure whether it's safe to use an extinguisher, alert others, leave the building and call 911 from a mobile or neighbor's phone.