Why do my lights flicker when I turn on a large appliance like the dryer or AC?

A brief, momentary flicker when a large appliance starts up is usually normal — it is caused by the sudden inrush of current that motors draw at startup, which momentarily drops voltage on shared circuits. However, if the flickering is severe, prolonged, or happens on circuits that should be independent from the appliance, it could indicate a more serious issue that warrants professional investigation.

When your central air conditioner compressor kicks on, it can draw 3 to 5 times its normal running current for the first fraction of a second. A typical residential AC unit might run at 20 amps but briefly pull 60 to 100 amps at startup. This momentary surge causes a small voltage drop throughout your electrical system, and your lights — particularly LED lights, which are very sensitive to voltage fluctuations — respond with a visible flicker. The same thing happens with dryers, well pumps, and any motor-driven appliance. If the flicker is barely noticeable and lasts less than a second, your system is likely functioning normally.

The concern arises when flickering is pronounced, lasts more than a second or two, or affects lights throughout the house. In older GTA homes with 60A or 100A panels — common in post-war bungalows and split-levels across Scarborough, Etobicoke, and North York — the electrical service may simply be undersized for modern demands. A home that was designed to power lights, a stove, and a radio is now running central air, a dryer, a dishwasher, a home office, and possibly an EV charger. The main service conductors and panel cannot deliver enough current without voltage drop, especially during the startup surges of large appliances. A panel upgrade from 100A to 200A service, which costs $2,000 to $5,000 in the GTA, resolves this by providing substantially more electrical capacity.

Loose connections anywhere in your electrical system — at the panel, at the meter base, at outlets, or at wire splices inside junction boxes — can also cause flickering, and this is the scenario that should concern you most. A loose connection creates resistance, and resistance creates heat. Over time, that heat damages wire insulation, oxidizes connection points, and creates conditions for arcing — the leading cause of electrical fires in residential buildings. If your lights flicker randomly without any large appliance cycling, or if you notice flickering accompanied by buzzing sounds, warm cover plates, or an occasional burning smell, you have a potentially dangerous wiring fault that needs immediate professional attention.

There is also the possibility that the issue originates outside your home. If your neighbours experience similar flickering at the same times, the problem may be on Toronto Hydro's side — a failing transformer, a loose connection at the pole, or voltage regulation issues on the distribution line. Contact Toronto Hydro at 416-542-8000 to report the issue, and they will investigate their equipment at no charge.

A licensed electrician can perform a voltage drop test and load analysis to determine whether your flickering is benign startup behaviour or a symptom of undersized service, loose connections, or failing equipment. Toronto Electrical Repair can connect you with local licensed electricians through the Toronto Construction Network for a professional assessment.