The lights dim when our shower is on, and it’s making our guests anxious

Property Clinic: Voltage drop is normal but should not be at a level that causes nuisance

If there is a significant diversion of a large amount of power to one outlet in a house, then other areas must compensate.

Our house is relatively new, built 15 years ago, but one issue has remained unresolved all this time: every time someone has a shower all the lights dim down. When the shower stops, they brighten up again.

We never really minded; it was even a bit of an in-house joke. But recently it happened during a visit by ‘the high-profile in-laws’ – before coronavirus, I must add. Anyway I can still see yer wan looking around anxiously, “Oh, oh what’s happening?” Please tell me what needs to be done to avoid future mortification.

I note your guests visited before this time of social distancing. Times, habits and regulations change. Your house is about 15 years old so the regulations that applied to your house will be different from today’s stricter requirements.

Older installations will not incorporate an RCBO or residual current breaker with over-current protection. This means the installation may be more prone to “voltage drop” or dimming lights as you describe.


The problem arises as electrical showers place a very significant draw on power. The delivery of electricity into a home is at a constant rate or voltage. If there is a significant diversion of a large amount of energy or power to one outlet, then other areas must compensate. The most obvious indication of this is dimming lights when showers or an immersion heater is switched on.

Voltage drop is normal but generally will be barely noticeable and should not be at a level that causes nuisance. There are situations when the problem can be exaggerated. This can happen if the house in question is at the end of the power line where a number of houses are served by a single transformer. Loose electrical connections can also exaggerate the dimming lights effect.

As with all issues when dealing with electrical installations you should engage a qualified electrician. They will give the installation the once over and check connections and the like. Given that this is not an essential repair – you have survived fifteen years without remedying it – I’m sure you will be happy to forgo resolution for a few more months until Government restrictions on non-essential commercial activity and home visits are lifted.

Hopefully when your guests are allowed to return, the dimming of the lights will be a happy reminder of times past. Then you can move on and get the problem sorted.

Noel Larkin is a chartered building surveyor and a member of the Society of Chartered Surveyors Ireland,