Why do my lights dim when the kettle goes on?

Every metre of cable between the substation and your lamp has resistance. When a kettle draws 13A, that current flowing through the resistance produces a voltage drop, and the lamp sees slightly less voltage. That much is unavoidable and completely normal. What matters is how much, and whether it is getting worse.

Safety

Step by step

1. Establish what triggers it.Kettle, shower, oven, washing machine heating, a workshop tool starting. Big loads with high inrush — anything with a motor or a heating element — are the normal triggers. Note whether it is one appliance or all of them.
2. Judge the severity.A brief, barely noticeable dip as the kettle switches on, recovering instantly, is normal. A dim you would describe as "the lights go down", or one that persists for the whole time the appliance runs, is not.
3. Check whether it affects the whole house.If only the lights on the same circuit dip, the effect is local to that circuit. If lights on every circuit and both floors dip together, the drop is happening upstream of the consumer unit — in the tails, the meter, the service head or the network.
4. Watch for the serious pattern.Lights that flicker without you switching anything, lights that brighten as well as dim, appliances behaving oddly, or a persistent change after any electrical work: these suggest a high-resistance or failing neutral connection. That is a supply fault and it can cause serious damage and fire.
5. Consider the LED factor.LED lamps show voltage variation far more visibly than filament lamps did, because their drivers respond quickly. Some of the increase in reports of this symptom is simply that LEDs make an existing, normal dip visible.
6. Consider the supply capacity.In older properties, rural areas and long service cables, the supply impedance is genuinely higher and dips are more pronounced. It is not a fault, but your electrician can measure it and tell you where you sit.
7. Call 105 if the pattern is the serious one.In Great Britain, 105 connects you to your distribution network operator, free, at any hour. A suspected loose or failing service neutral is exactly what they exist for, and they will attend. Do not attempt to look inside the service head or the meter — both are sealed and neither belongs to you.
8. Get an EICR if it is not the supply.If the network confirms the supply is sound and the dip is pronounced, an electrician measuring earth fault loop impedance and inspecting the tails, main switch and circuit terminations will find a high-resistance connection inside the installation.

Tips

• Note the pattern in writing before calling anyone. "Whole house, every time the shower starts, worse than a year ago" is far more useful than "the lights flicker".
• If neighbours on the same supply report the same thing, it is almost certainly a network issue and worth mentioning when you call.
• If the dip started after any work — a new shower, an EV charger, a board change — say so. That is a strong pointer to a connection disturbed during the work.

Common mistakes

• Assuming it is just old wiring and living with it — A high-resistance connection generates heat at the point of resistance, and that point is often inside a sealed enclosure or a wall. It gets worse, not better.
• Opening the meter cupboard or the service head to look — Everything before the meter belongs to the network operator, is sealed, and is live at all times with no protection you can operate. It is one of the few genuinely lethal places in a domestic property.
• Blaming the lamps — LEDs make voltage dips more visible, but they do not cause them. Changing the lamps changes what you see, not what is happening.

If it doesn't work

Questions people ask