When the inverter operates on the AC side, we are in the same world as with any other electrical consumer or producer: current flows in the wire, heat is generated, and protection must function.
The only difference is that here we are not consuming, but feeding back into the grid.
Many faults occur not on the DC side, but here, in the AC circuit. Typical symptoms include:
- unjustified inverter shutdown in sunshine
- overheating miniature circuit breaker
- intermittent RCD tripping
- thermal fault starting from a loose connection
And these often stem from a simple sizing error.
1. How much current flows on the AC side?
The nominal power (kW) of the inverter alone doesn't say much. For the wire and the miniature circuit breaker, the current is what matters. For this, we use very simple relationships.
For a single-phase inverter
The formula:
I (A) = P (W) / U (V)
In Hungary, U = 230 V.
If the inverter is 5 kW, then:
P (W) = 5 * 1000 = 5000 W
I = 5000 / 230
I = 21.7 A
This means that in full sunshine, the inverter loads the AC circuit with approximately 22 amps.
This is already a significant current. For comparison:
a 16 A socket circuit would be insufficient for this.
For a three-phase inverter
Here, the power is distributed across three phases.
The formula:
I (A) = P (W) / (1.732 * U (V))
U = 400 V.
The same 5 kW on three phases:
I = 5000 / (1.732 * 400)
I = 5000 / 692.8
I = 7.2 A
So, approximately 7 A per phase.
This clearly shows why it is "easier" to electrically handle larger systems on three phases. The current is smaller, and the load is more even.
2. Miniature circuit breaker selection
This is a very important conceptual point. The primary task of the miniature circuit breaker is: to protect the wire against overcurrent. The goal is not to protect the inverter.
If the inverter outputs 22 A (5 kW single-phase), then:
- a 20 A miniature circuit breaker is very likely to trip at peak production.
- a 25 A is already a more realistic choice.
- a 32 A, however, is only justified if the wire is also sized for it.
An oversized breaker is just as much a fault as an undersized one.
If the wire is 4 mm² and designed for 25 A, then a 32 A breaker is no longer targeted protection.
3. Wire cross-section
Many installation errors occur here. Based on the current, a certain cross-section might be "enough on paper," but in reality, several factors matter:
- the cable length
- the installation method (in walls, conduits, thermal insulation)
- the ambient temperature
- the quality of the connection points
If the inverter is 6 meters from the distribution board, with good ventilation, the situation is different than if it is 35 meters away, running in an attic space, in an environment of 45–50 °C in summer.
With long wires, two things happen:
- heating increases
- voltage drop increases
And here comes the phenomenon that many do not understand. The inverter trips not because it is "faulty," but because the local voltage rises due to the long AC cable.
4. Voltage drop
No need for complex engineering formulas here. The installer's level of thinking goes like this:
- short cable → smaller problem
- long cable → larger cross-section justified
If the AC cable run is longer than 30–40 meters for systems around 5–8 kW, then you shouldn't rely on "feeling." Often, the system works, but it shuts down at peak production. In such cases, the installer looks for a grid problem, while the real cause is their own AC side sizing.
Let's look at a specific example
8 kW three-phase inverter
cable length: 28 meters
400 V grid
Let's calculate the current.
P (W) = 8 * 1000 = 8000 W
I = 8000 / (1.732 * 400)
I = 8000 / 692.8
I = 11.5 A
Approximately 11–12 A per phase.
This doesn't seem like much. However, over 28 meters, voltage drop can already occur. In such cases, a larger cross-section is often justified, even if a smaller one would be sufficient based on the current. This is the point where there is a difference between a "minimally sufficient" and a "stably operating" system.
5. RCD issue
Here, we don't calculate, we think. Due to the inverter's electronics, not all RCD types are suitable. If the wrong type is chosen, it can lead to unjustified tripping or inadequate protection.
This is the point where the inverter manufacturer's specifications are always the guiding principle.
Here, one should not work based on "how we usually do it."
Summary – the practical minimum
On the AC side, you don't need to think in complex formulas, but in clear logic. An installer should at least clearly see that:
- a 5 kW single-phase inverter means roughly 22 A,
- with such a current, you do not install a 20 A circuit breaker, as it would operate at its limit,
- if the cable length increases, not only the current matters but also the voltage drop – in such cases, it is worth considering a larger cross-section,
- and the RCD is not a routine matter, but a decision to be adapted to the inverter type.
It's not about the depth of engineering calculations, but about the installer understanding why things are happening in the system. If this mindset is present, the AC side will not cause surprises with tripping, overheating, or inverter failures.
