DC string cable is the cheapest part of an array and one of the easiest to get wrong. Undersize it and you lose energy every hour of every day for twenty-five years, quietly, with nothing on the monitoring portal to tell you why.
The formula
For a DC run, current flows out and back, so the cable length in the calculation is doubled:
Vdrop = (2 × L × I × ρ) ÷ A
Copper has a resistivity of about 0.0172 Ω·mm²/m at 20 °C, but cable on a roof does not sit at 20 °C. At an operating temperature near 70 °C it rises to roughly 0.021 Ω·mm²/m. Use the hot figure — designing at 20 °C understates the loss by about a fifth.
A worked example
Twelve modules in series, Vmp 41.5 V and Impp 11 A each, with a 30 m run from the roof to the inverter. String voltage is 12 × 41.5 = 498 V; string current stays at 11 A.
| Conductor | Voltage drop | As % of 498 V | Verdict |
|---|---|---|---|
| 2.5 mm² | 5.54 V | 1.11% | Marginal |
| 4 mm² | 3.47 V | 0.70% | Good |
| 6 mm² | 2.31 V | 0.46% | Better than needed |
Working the 4 mm² row through: (2 × 30 × 11 × 0.021) ÷ 4 = 13.86 ÷ 4 = 3.47 V. As a fraction of the 498 V string that is 0.70%, comfortably inside the usual 1% design target for the DC side.
Voltage drop is not the only constraint
A cable that passes the voltage-drop check can still be the wrong cable:
- Current rating after derating. Published ampacity assumes a reference condition. Bunched cables, conduit and 60 °C+ roof temperatures all derate it.
- Voltage class. String voltages of 1000–1500 V DC need cable rated for it. Ordinary 1100 V AC building cable is not the same rating.
- UV and temperature endurance. Solar DC cable is cross-linked and rated for 25 years of outdoor exposure at 90 °C continuous. PVC building cable on a roof goes brittle and cracks long before the modules it feeds are finished.
- Fault current. Conductors must survive the prospective fault current for the time the protection takes to clear it.
Before you order
- Calculate with the hot resistivity figure, not the 20 °C one.
- Use the real one-way route length including vertical drops and slack, not the straight line.
- Check the derated ampacity for the actual installation method.
- Confirm the cable voltage class covers the maximum string Voc at minimum temperature.
- Specify DC-rated cable for DC and AC-rated cable for AC — they are not interchangeable.
Frequently asked
Does a longer string need thicker cable?
Usually the opposite. Adding modules in series raises voltage but leaves current the same, so the absolute drop is unchanged while the percentage drop falls. Higher string voltage is the cheapest way to reduce cable loss, within the inverter's input limits.
Can I use aluminium?
On long AC runs it is common and economic. On DC strings it is rarely worth it — the higher resistivity needs a larger area for the same drop, and the termination discipline aluminium requires is easy to get wrong on a roof.
What size do most residential strings use?
4 mm² covers the large majority of domestic runs. 6 mm² appears on long runs or high-current strings. 2.5 mm² is usually only appropriate for very short runs.
