Voltage drop is the quiet performance-killer of solar and DC wiring. Every cable has some resistance, and pushing current through that resistance loses a bit of voltage along the way — energy that turns into heat instead of reaching your batteries, inverter, or loads. Our cable size calculator works the other direction (it picks a cable size for an allowable drop); this tool is for when you already have a cable in mind and want to know the actual drop it will produce.
The formula
For a DC or single-phase run, voltage drop is:
Voltage drop (V) = 2 × L × I × ρ ÷ A
Where L is the one-way cable length in meters, I is the current in amps, ρ (rho) is the conductor resistivity (0.0175 Ω·mm²/m for copper, 0.0282 for aluminium), and A is the cable's cross-sectional area in mm². The × 2 accounts for the current traveling out to the load and back. To express it as a percentage, divide by the system voltage and multiply by 100.
A worked example
Take a 6 mm² copper cable, 10 m one-way, carrying 20 A on a 48 V system:
- Voltage drop = 2 × 10 × 20 × 0.0175 ÷ 6 = 1.17 V
- As a percentage = 1.17 ÷ 48 × 100 = 2.4%
That's within the common 3% guideline, so this cable is fine for this run. If you dropped to a 4 mm² cable, the drop would rise to about 3.5% — over the limit, and a sign you'd want the thicker conductor.
What counts as acceptable?
The widely used rule of thumb is 3% or less for main power cables, and 5% maximum for the total across all runs. Lower is always better — every bit of dropped voltage is wasted energy. On low-voltage systems (12 V and 24 V) voltage drop is especially punishing, because even a small voltage loss is a big percentage of the system voltage. This is one of the strongest arguments for wiring larger systems at 48 V.
Three ways to reduce voltage drop
- Use a thicker cable. Doubling the cross-sectional area roughly halves the drop. This is the most common fix.
- Shorten the run. Drop is directly proportional to length — moving equipment closer helps directly.
- Raise the system voltage. Carrying the same power at a higher voltage means less current, and less current means dramatically less drop. Going from 24 V to 48 V is a big improvement.
Related tools
If the drop you get here is too high, use the cable size calculator to find the correct size for your target drop. To work out the current from your load, the daily load calculator and panel calculator can help you understand your system's power and current draw.
Frequently asked questions
How do I calculate voltage drop in a cable?
Voltage drop = 2 × one-way length × current × resistivity ÷ cross-sectional area. The ×2 covers the out-and-back path. For copper, resistivity is about 0.0175 Ω·mm²/m. Divide by system voltage for the percentage. This calculator does it all automatically.
What is an acceptable voltage drop for solar cables?
Keep it at or below 3% for main cables, and under 5% total. Lower is better since dropped voltage is wasted energy. Low-voltage 12V/24V systems need extra care because a small drop is a big percentage.
Why does voltage drop matter?
It wastes energy as heat, reduces voltage delivered to equipment, and can cause devices to underperform. Excessive drop on undersized cable also causes overheating — a safety hazard.
How can I reduce voltage drop?
Use a thicker cable, shorten the run, or raise the system voltage. Thicker cable is the usual fix; higher voltage is very effective because it lowers the current for the same power.