Is a 3×25 A main breaker enough for a wallbox?

The short answer

In most houses, yes. An 11 kW wallbox draws 16 A on each phase, so a 3×25 A main breaker leaves 9 A per phase for the rest of the house. That's fine for a normal evening. The trouble starts when charging coincides with big appliances — and there's a simple fix for that before you think about upgrading the breaker.

How to find out what breaker you have

• On your electricity bill — the supply point lists the breaker rating, for example “3×25 A”.
• On the breaker itself in the meter cabinet — it's marked something like B25 or C25. The number is the current in amps.

If the bill shows a single phase (for example “1×25 A”), you have a single-phase supply. You can still charge, just more slowly — see below.

What draws how much

• 3.7 kW wallbox = 1×16 A (single phase)
• 7.4 kW wallbox = 1×32 A (single phase)
• 11 kW wallbox = 3×16 A
• 22 kW wallbox = 3×32 A — more than a 3×25 A breaker allows

That's why houses almost always get 11 kW. More on that in 11 or 22 kW.

The thing to watch: night-time

Cars charge mostly at night. But night is also when the low-tariff loads switch on: water heater, storage heaters, heat pump. Add evening cooking on an induction hob, the oven or the tumble dryer.

When all of that meets the charging, the main breaker trips. Not straight away, but typically at the worst moment — a winter evening with the heat pump and water heater both running.

Fix no. 1: dynamic load management

The wallbox gets a meter on the house's main supply and watches what the rest of the household draws. Switch on the oven and charging throttles itself; switch it off and charging picks back up. The main breaker stays on and you don't have to think about it.

Most current wallboxes support this; some need an add-on meter. It costs far less than upgrading the breaker — and there's no paperwork with the grid operator.

Fix no. 2: set a lower charging current

A wallbox can be capped at a lower current — say 10 A instead of 16 A per phase. The car then charges at about 6.9 kW. Over an eight-hour night that's still more than 50 kWh, hundreds of kilometres of range. For a normal daily commute it's more than you need.

It's the cheapest route, but less clever: the wallbox is always throttled, even when the house isn't drawing anything else.

What if you have 3×20 A or a single phase

3×20 A: with the wallbox at a full 16 A, only 4 A per phase is left, which isn't much. Load management is practically a must here, or a charging current of around 10 A.

1×25 A: a single-phase wallbox capped at 16 A, i.e. 3.7 kW. That adds roughly 29 kWh overnight — enough for ordinary commuting. Switching to a three-phase supply is possible, but it's a bigger job and involves the grid operator.

When to upgrade the breaker

An upgrade makes sense when big loads run in parallel regularly (heat pump, direct electric heating, several cars) and even with load management the car wouldn't get charged in time. You apply to the grid operator — PREdistribuce in Prague, usually ČEZ Distribuce outside it. Expect a one-off fee for the capacity increase, and a higher fixed monthly charge on your bill, since that depends on the breaker size. How it works in Prague is covered in connecting a wallbox in Prague.

How we do it

At the survey we look at the main breaker, the consumer unit and what runs in the house — especially at night. Then we recommend whether it's enough to configure the wallbox, add load management, or whether upgrading the breaker makes sense. In most cases you don't need to upgrade.