Why Your Lights Dim When the Air Conditioner Starts

The compressor kicks in, the lights dip for half a second, and everything carries on. It has probably been happening for years.

It is not the air conditioner failing, and it is not something you can fix by replacing a light fitting. It is a measurement, and what it is measuring is the cable feeding your house.

What the dip is telling you

When a compressor starts, it draws several times its running current for a moment. That surge has to travel the whole way from the street to your switchboard.

Every cable has resistance. Push a large current through it and the voltage at the far end drops. If the cable is generously sized for the load, the drop is small enough that nobody notices. If it is undersized, the drop is big enough to visibly dim every incandescent and some LED fittings in the house.

So the dimming is not the air conditioner's fault. The air conditioner is just the biggest thing you own, and it is revealing something that was already true.

Why it is worse in summer, and worse out west

Two things stack up.

Cable ratings fall as it gets hotter. A cable's current rating assumes an ambient temperature. On a 42 degree afternoon in a roof space that is hotter still, the same cable carries less than it does in July. Supply that is marginal all year becomes inadequate exactly when you are asking the most of it.

Western Sydney runs hotter than the coast by a wide margin on the same day. Penrith, Blacktown and Liverpool regularly sit several degrees above the eastern suburbs. That is why supply failures out there cluster in January and February instead of spreading evenly through the year, and why consumer mains upgrades are the single most common job we do in those regions.

If your main switch trips on hot afternoons and holds perfectly the rest of the year, that is the same story with the volume turned up.

The usual culprit: original mains on a modern house

Here is the pattern we see constantly.

A house built in the 1960s or 70s was connected for lighting, a stove, a fridge and not much else. The consumer mains installed then were sized for that, and on plenty of properties they are still 6mm cable.

Since then the same house has picked up ducted air conditioning, an induction cooktop, a heat pump hot water system, a dishwasher, a pool pump and increasingly a car charging overnight. The cable never changed.

The mains are not dangerous simply for being old. They are just the narrowest part of the path, and everything else is now asking for more than they were built to deliver.

What the symptoms mean together

Worth a call if you have noticed any of these:

• Lights dimming when the compressor starts, or when the oven and kettle run together
• The main switch tripping on hot days and holding the rest of the year
• Breakers that will not hold once several appliances are running
• A meter box that is warm to the touch
• Any smell of hot plastic near the board

The last two are different from the others. Heat and smell at the switchboard are not a capacity annoyance; they are a reason to stop using heavy loads and call the same day.

What actually fixes it

Three possibilities, in ascending order of work.

New consumer mains. Replacing the cable from the meter to the switchboard with something properly sized. This alone resolves a lot of dimming complaints, and it is Level 2 work because the supply comes off at the network side to do it.

A supply upgrade. Where the service itself is the constraint rather than just the mains, the whole supply path gets uprated: service line, mains, metering and board.

Three phase. Where the load is genuinely large, converting to three phase spreads it across three conductors instead of stacking it on one. It is the right answer for big houses with ducted systems, pools, workshops and car charging, and the wrong answer for a modest house that just needs bigger mains. We will tell you which you are.

In almost every case the board gets replaced at the same time, because there is little sense installing mains that can carry more into a board that cannot distribute it. Pre-1990s boards also frequently sit on backing panels containing asbestos, which is identified and planned for before work starts rather than discovered halfway through.

How it gets worked out

Not by guessing. Maximum demand is calculated against the standard, based on what the property actually runs and what you are planning to add. That calculation decides the cable size, and it is the difference between an upgrade that solves the problem and one that moves it.

If you are thinking about a car charger, mention it up front. Doing the mains once for both is considerably cheaper than doing it twice.

Frequently asked questions

Is dimming dangerous, or just annoying? On its own it is a symptom rather than a hazard, and plenty of installations have lived with it for years. What makes it worth acting on is that the same underlying condition, a supply working close to its limit, runs every termination in the path warmer than it should be. It is also occasionally a sign of a developing neutral fault, which is a different and more urgent problem.

Does this mean I need three phase power? Usually not. Most properties that think they need three phase need bigger single phase. The answer comes from a maximum demand calculation rather than from any single appliance, and three phase earns its place where there are large or sustained loads such as ducted air conditioning, pool plant or workshop machinery.

Could it be the air conditioner rather than the supply? The unit is the trigger, not usually the fault. A compressor draws hard for a moment at startup, and that inrush is what exposes a marginal supply. A soft starter can reduce the symptom, which is worth knowing, but it treats the trigger rather than the cause.

Why do my lights dim only on hot days? Cable ratings fall as ambient temperature rises, so a supply that copes for most of the year can be inadequate in February. If it happens in summer and not in winter, that points at capacity rather than at a fault.

Will upgrading the switchboard fix it? Generally no, and this is the most common wasted spend on this problem. Voltage drop happens across the cable feeding the board, not in the board itself. A new switchboard on the same undersized mains dims exactly the same way.

How is it actually diagnosed? By measuring, not guessing: voltage at the board under load, the size and length of the consumer mains, the condition of the terminations, and the calculated maximum demand against what the connection can carry. That tells you whether the answer is the mains, the connection, or neither.

Before you call an air conditioning company

If the dimming started the day the unit went in, the unit is the trigger, not the fault. An air conditioning technician cannot fix voltage drop in your consumer mains, and a regular electrician cannot legally replace them.

Call 0485 824 714 and describe what happens and when. Free quotes, and we cover all of Sydney and the Blue Mountains on both the Ausgrid and Endeavour Energy networks.