Switchboard Roulette: The Risk Hiding Behind Big Batteries

My first proper job was at a nuclear power station. If you wanted to change anything you started with a drawing.

You marked the change up in red. An engineer reviewed it. The drawing office turned the red ink into real drawings. The work was done to that drawing. Someone checked the install against the markup, and signed off the drawing ‘as-built’.

I always thought that would be overkill for a domestic switchboard. Until I read a thread on one of the big Australian solar Facebook groups this week.

A savvy installer posted that inspectors are now auditing battery and solar jobs for busbar overload. Underneath, a dozen qualified electricians and a couple of engineers argue about whether a residential busbar can actually be overloaded by adding an inverter to it. One quotes Kirchhoff. Another posts a photo of a report from work. A third says move the inverter supply to the opposite end of the bar and you’re fine. A fourth says no, the bar is one conductor and the currents add. They are all confident. They cannot all be right.

Here is the problem.

Too Many Passengers On The Busbar

The main switch on many Australian houses is 63A. The busbar feeding the rest of the board is often rated 80A. When the only source on the board was the grid, life was simple. Grid in at 63A, busbar easily carried 63A, everyone went home.

Now we strap a battery and a solar inverter onto the same busbar. The grid still supplies up to 63A. The inverter pushes another 32A or more. The kitchen, the EV charger, the heat pump, and the spa pull more current than the conductors in the board were ever built to carry. Depending on how the board is arranged, wires in that switchboard can now carry the combined current from the grid and inverter sources long enough to exceed their rating – and melt.

AS/NZS 4777.1 clause 3.4.2 has explicitly banned this since 2021. No conductor in the installation shall be overloaded by summation of grid and supplementary supply. The standard exists. How many recent big battery installs have even considered this is another question.

And the loads are getting worse. Karl, one of the FB contributors, notes that 145A through a residential switchboard happens at his place every other day. Three EVs, cold winter, induction cooktop, two splits, electric hot water. The maximum demand assumption that sized your board in 1998 is dead.

The underlying problem is that much of the residential sector now runs serious currents through boards assembled over decades of piecemeal upgrades, often with no accurate documentation.

For the record, I hate bureaucracy. But paperwork for high-current electrical work earns its keep, the same way a structural engineer signing off your house plans earns her keep. Nobody calls that red tape. It’s the reason your roof is still over your head.

Keep A Paper Trail

Here is what we should do.

Before any battery, solar, or EV charger goes on, someone qualified draws the existing board. Every cable, every breaker, every busbar rating, every conductor size.

The proposed change is added to the drawing in red. The worst-case current at every point on the busbar is calculated. If anything exceeds rating, the design changes before a cover comes off. After the install, the person signing the compliance cert confirms the work matches the drawing. The drawing lives with the property and gets updated the next time anyone touches it.

Lots of people reading the previous paragraph will think I’ve lost the plot: “Typical bloody engineer!”.

But this is not radical. It is what every industry that runs serious power already does. And it is arguably what AS/NZS 3000 section 8.1.2 already demands when it says any alteration must be verified not to impair the safety of the existing installation.

Drawing Power

In my experience, you cannot confidently verify the safety of a modern energy system without a drawing. Domestic boards are now carrying commercial currents. The paperwork needs to catch up.

Will it? Unlikely. Because the market now rewards speed and low headline prices above careful engineering.

But right now, too much residential electrical work still relies on memory, assumptions, and optimism. That worked when houses drew 40A peaks and had one energy source. It is a far riskier bet in the age of solar, batteries, EVs, and all-electric homes pulling 145A.

Phase Shift is a weekly opinion column by SolarQuotes founder Finn Peacock. Subscribe to SolarQuotes’ free newsletter to get it emailed to your inbox each week along with our other home electrification coverage.