Quick answer
Lights that dip for a fraction of a second when the AC starts are normal: a compressor draws several times its running current for an instant at start-up. Dimming that lasts a second or more, worsens over the summer, or comes with a hum or a tripped breaker points to a weak capacitor or a tired compressor. Lights that brighten or flicker house-wide are an electrical supply fault.
Your lights dim when the AC turns on because the compressor motor needs a burst of current to get moving. For a fraction of a second it draws several times what it uses while running, the voltage on the house wiring sags, and the lights show it. A quick blink is normal. How long the dip lasts, and whether it is getting worse, tell you if something needs repair.
A blink shorter than a second is normal compressor inrush
Every compressor has two current ratings on the outdoor unit's nameplate. RLA, rated load amps, is what it draws while running. LRA, locked-rotor amps, is what it pulls in the instant before the rotor turns, and it is several times the RLA figure. That surge is over in a fraction of a second, and during it the lights on nearby circuits dip and recover.
How visible the dip is depends on the house. On a 100 A service, common in 1950s and 60s bungalows and in pre-war houses upgraded from 60 A, the same surge is a larger share of what the service can deliver. A long branch run from the panel to the condenser adds voltage drop of its own. Some LED bulbs and dimmers also show a flicker that an old incandescent bulb would have hidden.
Dimming that lingers or worsens: a weak capacitor or a failing compressor
If the lights sag for a full second or more, or the dip is deeper than it was last summer, the compressor is taking too long to reach speed. The usual cause is the run capacitor, a metal can in the outdoor unit rated in microfarads that gives the motor the phase shift it needs to start and run. Capacitors lose capacity with heat and age. One that has drifted below the tolerance printed on its label leaves the compressor straining at every start.
A capacitor is a $150–$350 repair and the most common AC fix there is; the symptoms are listed in how to tell if an AC capacitor is bad. If the capacitor tests good and the compressor still pulls high current for too long, the compressor itself is wearing, with tight bearings or tired windings. That is the stage where it starts tripping the breaker.
Hard-start kits and soft starters: what each one does
Two add-on parts reduce a start-up dip, and they do different jobs. A hard-start kit is a start capacitor and relay that give the compressor extra torque for the first instant, so it reaches speed sooner; it helps an aging compressor and shortens the dip without lowering the peak much. A soft starter is an electronic module that ramps the compressor up and cuts the inrush itself, which suits a 100 A service or a house that sometimes runs on a generator.
Neither part fixes a bad capacitor, a loose connection or a supply fault, so testing comes first. The price depends on the compressor size and the part chosen, and it is quoted in writing after a diagnostic visit of $90–$150 that is folded into the repair. When the unit is due for replacement anyway, an inverter-driven condenser ramps up gradually and produces almost no dip.
When the cause is the electrical supply, not the air conditioner
Some dimming is not an HVAC problem at all. A loose neutral, a worn connection at the meter base or panel, or a failing utility connection will also make lights react when a large load starts, and these are fire and shock hazards. The signs are different from a compressor start:
- Some lights get brighter while others dim. That pattern points to a loose or open neutral.
- Lights also dip when the dryer, microwave, kettle or a well pump starts.
- Flicker comes and goes with wind or rain, or neighbours on the same transformer see it too.
- You hear buzzing or smell hot plastic at the panel or the meter.
For those, call a licensed electrical contractor, whose ECRA/ESA licence number should be on the quote, or your hydro utility, and not an HVAC company. The utility looks after its own service conductors to the house: Burlington Hydro, Oakville Hydro, Milton Hydro, Alectra Utilities in Hamilton, Mississauga and Brampton, Halton Hills Hydro in Georgetown and Acton, Toronto Hydro, or Hydro One on rural lines in Caledon and Flamborough. Lights that brighten, or a burning smell at the panel, should be reported the same day.
What to check yourself before calling anyone
- Time the dip. Under a second and unchanged from past summers needs nothing.
- Note which lights dim: one room that shares a circuit, or the whole house.
- Listen at the outdoor unit as it starts. A hum or buzz for a second or two before the fan and compressor get going suggests a weak capacitor.
- Check the condenser's breaker, and do not keep resetting one that trips.
- Read the nameplate and write down the LRA and RLA for the technician.
On the visit, a technician measures the capacitor against its rating, the start-up current with a clamp meter and the voltage at the contactor as the compressor starts, which separates an equipment fault from a wiring one. ZK Mechanical's AC repair covers Burlington, Oakville, Milton, Hamilton, Mississauga and Brampton, and typical repair bands are on the pricing page. Call (647) 801-1252 or book online. Start-up and electrical symptoms are grouped in the cooling answers index.