Why do my lights dim when my neighbour's AC runs in the summer in Calgary?

Lights dimming in your home when your neighbour's air conditioner starts up indicates that you and your neighbour share a utility transformer, and the large inrush current from the AC compressor is causing a voltage drop that affects both homes. This is a utility-side issue, not a problem with your home's wiring, and while it is annoying, it is generally not dangerous — though there are situations where it warrants further investigation.

When a central air conditioning compressor starts, it draws a surge of inrush current that can be four to eight times its running amperage. A typical residential AC unit in Calgary runs at 15 to 25 amps but may draw 60 to 100+ amps for a fraction of a second during startup. If your home and your neighbour's home share the same utility transformer — which is common in Calgary neighbourhoods where one pad-mounted or pole-mounted transformer serves several houses — this large inrush current temporarily drops the voltage on the shared transformer. You see this as a brief dimming of your lights, particularly incandescent or halogen bulbs. LED bulbs may flicker rather than dim smoothly because their drivers are more sensitive to rapid voltage changes.

This issue is more noticeable during Calgary's summer heat events in July and August when multiple homes run their AC units simultaneously. While Calgary does not have the sustained summer heat of Toronto or the Lower Mainland, temperatures regularly reach 28 to 33 degrees in summer, and nearly every home in newer communities like Cranston, Mahogany, Seton, and Auburn Bay has central air conditioning. On the hottest days, multiple AC units cycling on and off on the same transformer can cause repeated voltage fluctuations.

If the dimming is brief (less than one second) and only occurs at startup, this is generally within normal utility operating parameters and is not harmful to your electrical system. However, if the dimming is prolonged, severe, or getting worse, it may indicate an overloaded or undersized transformer, a problem with the utility connections, or deteriorating neutral connections on the utility side. In these cases, contact ENMAX at 403-514-6100 (or FortisAlberta at 1-866-717-3113 outside Calgary city limits) to report the voltage fluctuation. ENMAX can monitor the transformer's load and voltage output and upgrade the transformer if it is undersized for the current neighbourhood load.

There are a few things worth checking on your side as well. A loose neutral connection at your meter base, service entrance, or panel can amplify voltage fluctuations from the utility. Under normal conditions, a solid neutral connection balances the two 120V legs of your service. When the neutral is loose or corroded, voltage imbalances become more pronounced, and external load changes — like your neighbour's AC starting — cause exaggerated dimming. Calgary's chinook cycling can loosen meter base connections over time, making this a real possibility in homes that have been through decades of chinook temperature swings. A licensed electrician can check your neutral connections and service entrance for tightness.

Whole-home surge protection is a worthwhile investment if you are experiencing regular voltage fluctuations from neighbourhood AC loads. A surge protector installed at your panel costs $250 to $600 installed and protects your electronics and appliances from voltage spikes that can occur when large motors start and stop. This is especially relevant in Calgary where the dry climate and chinook-related power fluctuations already put electronics at higher risk. If the dimming is significant enough to concern you, start by reporting it to ENMAX — transformer upgrades are their responsibility and cost you nothing. If you also want your home's service entrance and connections inspected, a licensed electrician can perform this check during a standard service call of $125 to $300.