What this usually means
A brief, slight dip when a large motor starts can be normal. A pronounced dim, a dim that persists, or one that affects the whole house points at the circuit, the service, a connection, or the utility supply — and a loose service or neutral connection can be a genuine hazard rather than a nuisance.
The common causes
An electrician works through 5 conditions that all produce do my lights dim when an appliance starts. They are listed so you can describe what you are seeing accurately — not so you can decide which one applies, because several of them look identical from outside the equipment.
- Motor inrush on a shared circuit — Well pumps, compressors and power tools draw a large starting current for a moment.
- Undersized or shared circuit — A heavy appliance sharing a circuit with lighting.
- Loose connection — A loose service, neutral or panel connection can cause voltage to swing as load changes.
- Service capacity — A service near its limit shows the strain when a large load starts.
- Utility-side condition — A supply problem outside the property, which is the utility's to resolve.
What to note before you call
For this symptom specifically, 5 observations do most of the work. All of them can be noted without opening any electrical equipment. Having them ready turns a vague call into a productive one and shortens the diagnostic visit.
- Whether one room dims or the entire house
- Which appliance triggers it
- Whether lights ever brighten unusually, which is more serious
- Whether neighbours see the same thing
- Any flicker, buzzing or burning smell
What happens next
An electrician tests the circuit, isolates the cause and explains what the repair involves before doing it. Where this points at panel and service upgrades, that page covers what the work usually includes and what changes the scope.