Size cables for current and voltage drop
Cables must carry the current without overheating (ampacity) and without wasting too much voltage. Low voltage DC needs very heavy cable, so voltage drop usually decides the size. Use the DC cable voltage drop calculator on this page.
How
- Find the maximum current in each circuit: array to controller, controller to battery, battery to inverter, and each load circuit.
- Measure the one way cable length. Voltage drop counts both the positive and negative conductors, so the formula uses twice the length.
- Voltage drop (V) = 2 times length (m) times current (A) times resistivity, divided by cable area (mm2). Copper is about 0.0172 ohm mm2 per m at 20 °C, rising when hot.
- Pick a target drop. Victron suggests under 2.5% for DC wiring. The US NEC informational notes suggest 3% for branch circuits and 5% overall.
- Then check the chosen size against the ampacity tables in your local code, including corrections for temperature and bundling. Example (US, NEC 690.8): PV source circuit conductors are sized for 125% of panel short circuit current for high irradiance, times another 125% for continuous duty, so 156% of Isc in total.
- Use fine stranded cable with correctly crimped lugs for battery and inverter connections. Parallel cables can be used if their combined area meets the requirement.
Safety: Undersized cable can overheat and start a fire, and poor crimps are a common cause of hot spots.
Sources
- Wiring Unlimited: DC wiring. Victron Energy, accessed 2026-11
- National Electrical Code Basics: Computing Voltage Drop in Branch Circuits and Feeders, Part 1. EE Power, accessed 2026-11
- AE 868: Conductor/Wire sizing. Penn State University, John A. Dutton e-Education Institute, accessed 2026-11