Lights Dimming When an Appliance Turns On: Causes, Fixes, and When to Worry

You switch on the washing machine and the kitchen lights dip for a moment. The kettle clicks on and the living room lamp dims briefly. It is one of the most common electrical concerns in UK homes, and it is natural to wonder whether something is wrong.

The short answer is: a brief, barely noticeable dim when a large appliance starts is usually harmless. But persistent, severe, or worsening dimming is a warning sign that should not be ignored. The difference between the two comes down to what is causing the voltage to drop — and whether the cause is a normal electrical characteristic or a developing fault.

This guide explains the electrical science behind dimming lights in plain language, covers the most common causes, helps you tell the difference between normal and dangerous, and tells you exactly when to call an electrician. If you are an electrician, the later sections cover the diagnostic approach and relevant BS 7671 regulations.

Every cable in your home has resistance. It is tiny — fractions of an ohm per metre — but it is always there. When current flows through a cable, some of the voltage is "used up" overcoming that resistance. This is voltage drop, and it is an unavoidable consequence of Ohm's law: voltage drop equals current multiplied by resistance (V = I x R).

Under normal conditions, the voltage drop across your home's cables is small enough that you never notice it. Your lights receive close to the full 230V supply and shine at their normal brightness. But when a large appliance switches on and suddenly draws a high current, the voltage drop across the shared supply cables increases proportionally. The voltage available to your lights temporarily decreases, and they dim.

Think of it like water pressure in a house. If someone turns on the garden hose full blast, the shower pressure drops momentarily because the pipes are shared. The same principle applies to electricity — the supply cables are shared, and a sudden large current draw reduces the voltage available to everything else on that supply.

BS 7671 Regulation 133.2 addresses this directly. It requires that conductor cross-sectional area is selected to limit voltage drop to permissible levels, ensuring that connected equipment receives adequate voltage under normal load conditions. For lighting circuits, the maximum permissible voltage drop is typically 3% of the nominal supply voltage — approximately 6.9V on a 230V supply.

Not all dimming has the same cause, and not all causes carry the same risk. Here are the most common reasons your lights might dim when an appliance switches on:

This is the critical question. Here is how to distinguish between harmless dimming and a sign of a developing fault:

If you recognise any of the items in the "potentially dangerous" column, do not wait. The combination of loose connections and high current is exactly how electrical fires start. The connection heats up under load, the heat loosens it further, the resistance increases, it gets hotter — it is a vicious cycle that can end in a fire.

Certain appliances are well-known for causing momentary dimming because of their high startup current. Understanding which appliances are the usual culprits helps you determine whether the dimming is expected behaviour or something to worry about.

Modern appliances increasingly use soft-start circuits, inverter-driven motors, and electronic controls that reduce the inrush current significantly. If you have recently replaced an old appliance with a new one and the dimming has stopped, the soft-start technology is the reason.

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One of the most common underlying causes of dimming lights is the supply infrastructure within the property. The meter tails — the cables connecting the electricity meter to the consumer unit — carry the total load for the entire house. If these cables are undersized, the voltage drop across them increases with every additional amp of load.

In older UK properties (pre-1990s), it was common to install 6mm or even 4mm meter tails. These were adequate for the electrical loads of the time — a few lights, a cooker, and perhaps an immersion heater. Today, with electric showers, multiple high-power kitchen appliances, EV chargers, and heat pumps, the same meter tails may be carrying far more current than they were designed for.

BS 7671 Regulation 528.3 requires that voltage drop is calculated and limited to ensure equipment operates correctly. The voltage drop calculator can help determine whether the existing cables are adequate for the current load. If they are not, the solution is straightforward: upgrade the meter tails and, if necessary, the supply cable.

Within the installation itself, shared circuits can also contribute to dimming. If the lighting circuit and a high-power socket circuit share a distribution board with inadequate busbar connections or undersized main switch, the voltage drop under load will be higher than necessary. A consumer unit upgrade can resolve this by providing better connections and adequate ratings for modern loads.

Sometimes the problem is not inside your home at all. The electricity supply from the Distribution Network Operator (DNO) can be the root cause. Supply-side issues include:

If your electrician determines that the supply voltage is consistently low or drops significantly under load, the next step is to contact your DNO. They are obligated to maintain the supply voltage within the statutory limits. You can find your DNO by entering your postcode on the Energy Networks Association website.

Not every instance of dimming lights requires a professional visit. But certain signs demand immediate action. Call a qualified electrician if:

A qualified electrician will carry out a systematic investigation, typically starting with visual inspection and tightness checks at the consumer unit, then measuring supply voltage under load, earth fault loop impedance, and insulation resistance on affected circuits. An EICR is the most thorough approach and will identify all defects in the installation, not just the one causing the dimming.

When a customer reports dimming lights, the diagnostic approach should be systematic. Start with the supply and work inward:

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