My lights dim for a second when the AC or water pump kicks in — is that a problem?

Question

Every time the AC compressor or water pump starts, the lights briefly dim/flicker, then recover. It's not tripping anything (different from the breaker-trips-on-start issue) — just a momentary dim. - Is a momentary dim when a big motor starts normal? - Or is it a sign of a problem — loose connection, undersized circuit, voltage drop? - When does it need looking at? Trying to tell "harmless physics" from "get an electrician." What's actually happening when the lights dip?

Bilal H. · Accepted Answer

Short answer: a brief, slight dim when a large motor (AC compressor, water pump) starts is usually NORMAL — motors pull a big inrush current for a split second as they start, which causes a momentary voltage drop, and the lights dip until it settles. It's physics, not a fault, IF it's slight, brief and not getting worse. It becomes a concern when the dimming is DEEP, long, worsening over time, or accompanied by flicker/buzz/warmth at a fitting or the board — those point to a loose connection, an undersized/overloaded circuit, or a wiring problem worth checking.

Normal (don't worry) if: - The dim is slight and momentary (a fraction of a second) right as the motor starts, then steady. - It's a big load doing it (AC, pump) — the bigger the motor's inrush, the more noticeable. - Nothing else is wrong (no burning smell, no warm fittings, no worsening).

Get it checked if: - The dimming is deep or lasts more than a moment, or is getting worse over time. - Lights on many circuits dip hard, or a specific circuit flickers. - There's any buzzing, warmth or a burning smell at a socket, switch, fitting or the distribution board — that's a loose/high-resistance connection, which is a real fault (and a fire risk), not normal inrush. - Sensitive electronics reset/reboot when it happens.

What's behind a bad case: a loose or corroded connection (higher resistance = bigger voltage drop + heat), an undersized cable/circuit for the load, or the motor lacking a proper soft-start. On a big AC/pump, a soft-starter reduces the inrush and the dip.

So: a gentle, brief dip on a big-motor start = normal; a deep/worsening dip, or any warmth/buzz/smell = get a licensed electrician to check the connections and circuit sizing before it becomes a bigger problem. Anyone's "harmless flicker" turn out to be a loose neutral?

Jason S. · Answer

Reassuring — ours is a quick dip when the AC outdoor unit kicks in and nothing's warm or buzzing, so sounds like normal inrush. The bit I've filed away is the warning signs: if it starts dimming harder, or a fitting gets warm, or there's any buzz/smell, that's a loose-connection red flag and not to be ignored. Good to have the line between "physics" and "fault." I'll keep an eye on whether it ever gets worse rather than assuming it's always been fine.

Omar K. · Answer

Had a case where the dimming got noticeably worse over a few months and it turned out to be a loose connection at the board — an electrician found and tightened it, and the deep dips stopped. So the "is it getting worse" test is the useful one: a stable little flicker on a big motor start is one thing, a deepening one is your wiring telling you something. Worth acting on the trend, not just the symptom. Cheap to check a connection, expensive to ignore a hot one.

My lights dim for a second when the AC or water pump kicks in — is that a problem?

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