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Voltage drop and cable size: how to choose and why it matters

· 6 min read ·

Learn to calculate a cable’s voltage drop and the minimum cross-section for it, why current capacity is another criterion and how to check the standard.

Why voltage drop matters

Every cable has a small resistance. When current flows through it, part of the voltage is left along the way: that is the voltage drop. It becomes heat in the cable and represents energy that the panels generated and the home did not receive. The longer the cable, and the thinner, the larger the drop.

0.0175 × 60 mcopper resistivity × length (out and back)÷4 mm²cable cross-section=0.26 Ωcable resistance
10 Astring current×0.26 Ωcable resistance=2.6 Vvoltage drop, 0.66% of 400 V

How the calculation works

The calculation uses the resistivity of copper, about 0.0175 Ω·mm²/m at 20 °C, and the total length of the circuit: out and back, that is, twice the distance. If the cable heats up, the resistance rises: in very hot cables, the real value can be about 20% higher. The examples assume a DC circuit, or single-phase AC with power factor 1, and ignore reactance.

10 A4 kW ÷ 400 V, in the string
60 mout and back over 30 m
26 Wlost in the cable (10 A × 2.6 V)

Work out the cross-section from the drop

Enter the power, the voltage, the length (one way) and the drop you accept. The calculator shows the minimum cross-section for the drop and the next standard size up. If no size in the table (up to 120 mm²) meets the target, it warns instead of showing a smaller size.

Interactive

Work out the cable size from the voltage drop

Enter the power, the voltage, the length (one way) and the drop you accept. The calculator shows the current, the minimum cross-section, the standard size and the real drop.

10.0 Acurrent
2.6 mm²minimum cross-section for the drop
4 mm²standard cross-section
0.66%real drop, with cable loss below26.3 W

Copper, out and back, at about 20 °C. Hot cables have more resistance. This only checks the voltage drop: the cable’s maximum current (ampacity), the installation method and the protection follow your country’s standard. Single-phase AC or DC circuit, power factor 1, no reactance.

Three cases

The same steps give quite different results depending on the voltage and the current. With the drop chosen as the target in each case:

Standard cross-section that meets the voltage drop alone (mm²)

DC string: 4 kW, 400 V, 30 m, 1% target4 mm² · 0.66%
48 V bank: 3 kW, 5 m, 1% target25 mm² · 0.91%
AC circuit: 5 kW, 230 V, 20 m, 3% target2.5 mm² · 2.65%

The criteria that add up

Voltage drop

Depends on length

  • Measures the loss of voltage and energy
  • Grows with current and distance
  • Solved by increasing the cross-section

Current capacity

Depends on heat

  • Avoids excessive cable heating
  • Changes with the installation method and temperature
  • Bundled cables run hotter

Protection

Fuse or breaker

  • Must protect the chosen cable
  • Must act before the cable overheats
  • Follows the circuit’s standard
The final cross-section meets all three at once.

On the values and the standards

Do

  • Calculate the drop using the out-and-back length
  • Check the current capacity and the protection separately
  • Allow for hotter cables, on roofs and in exposed conduits
  • Leave the final design to a qualified professional

Avoid

  • Use the same cross-section in every section without calculating
  • Accept a high drop just because the cable “can carry the current”
  • Copy tables from another country or another voltage
  • Splice cables of different cross-sections without a criterion

Draw it in Editor Solar

This is Editor Solar itself, already loaded with the diagram from this article. Move the equipment, edit labels, connect new parts and export to PNG, SVG or PDF. It is free and runs right in your browser.

The DC cable between the panels and the inverter, with length, cross-section and voltage drop noted.

Try it: Double the cable length on the diagram to 60 m and redo the voltage drop. Does the 4 mm² cross-section still meet the drop you accept?

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Open this diagram in the full editor ↗

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