DC voltage drop is not some "looks good on paper" calculation but a very practical issue. When solar panels on the roof generate energy, it must reach the inverter. However, the cable is not perfect: it has resistance, and because of this resistance, a small amount of voltage is lost. This lost voltage is converted into heat, meaning it's a loss.
If this loss is too great, not only does efficiency decrease, but the cable and connectors also heat up more. In the long run, this can lead to faster aging, loose connections, and even contact problems. This is why DC side cables must be dimensioned thoughtfully, not just by "feel."
In practice, the goal is to keep the DC side voltage drop below 1-2%. This is not a written law but a proven professional guideline. A higher value than this indicates unjustified loss.
What does voltage drop mean in practice?
The calculation is not complicated, and an simplified formula is more than sufficient at the installer level. For copper wire, you can use:
ΔU = 2 × L × I × 0.0175 / A
where
L = one-way cable length in meters
I = string operating current (A)
A = cross-section (mm²)
0.0175 = specific resistance of copper
It is very important: the DC circuit consists of a round-trip wire, which is why 2 × L is included in the formula. If the inverter is 25 meters from the array, we are effectively calculating with 50 meters of wire.
The percentage voltage drop is calculated as follows:
ΔU% = (ΔU / U_string) × 100
Let's look at a real-world example
Suppose a string's operating current is 10.5 A. The distance between the roof and the inverter is 25 meters, the cable has a cross-section of 4 mm², and the string's operating voltage is 350 V.
First, we calculate the voltage drop:
ΔU = 2 × 25 × 10.5 × 0.0175 / 4
ΔU ≈ 4.6 V
This means that approximately 4.6 V of the 350 V is "lost" in the cable.
Now let's look at it as a percentage:
ΔU% = (4.6 / 350) × 100
ΔU% ≈ 1.3%
This is a perfectly acceptable value. With these parameters, 4 mm² is an appropriate choice.
What happens if we use a thinner cable?
If we were to use a 2.5 mm² cable in the same situation, the voltage drop would be roughly over 2%. The system would work, but:
- energy loss would be greater,
- the wire would heat up more,
- in the long run, it would be less reliable.
This is where common sense comes in: the cost of the cable is dwarfed by the cost of the inverter and panels, but the loss occurs during every sunny hour. If the distance is greater, it is worth considering a thicker cross-section.
What an installer should keep in mind
It is not necessary to perform detailed calculations for every job, but the logic must be understood. Voltage drop depends on three factors: current, wire length, and cross-section. If the distance increases or the current is high, the cross-section must also be increased.
The most common mistake is to choose a cable based only on current, without considering the length. Another typical mistake is forgetting the round-trip length. The system may still work, but not optimally.
