Solar Panel Wire Calculator

How It Works

Solar panel systems are particularly sensitive to voltage drop because they operate at low DC voltages (12V, 24V, or 48V). A 2% voltage drop on a 12V system is only 0.24V, requiring much thicker wire than an equivalent AC circuit at 120V. Every volt lost in the wire is energy your panels produced but your battery never receives.

The Formula

Variables

• K — Resistivity constant for copper (12.9) or aluminum (21.2)
• I — Maximum current in amps (use Isc x 1.25 for solar panels)
• D — One-way distance from panels to charge controller or inverter in feet
• V — System voltage (12, 24, or 48 VDC)
• Drop% — Maximum acceptable voltage drop (2% recommended for solar)

Example

A 24V system with 30A max current and 30 ft wire run at 2% max drop: Circular mils = (2 x 12.9 x 30 x 30) / (24 x 0.02) = 48,375 cmil. That requires 4 AWG wire (41,740 cmil is too small, so use 3 AWG at 52,620 cmil).

Tips

• Higher system voltage (48V vs 12V) dramatically reduces required wire size -- this is why 48V systems are preferred for long runs.
• Use Isc (short circuit current) x 1.25 as your design current, not Imp (maximum power current).
• PV wire (USE-2 or PV Wire) is rated for sunlight exposure and is required for outdoor runs on rooftops.
• Keep wire runs as short as possible -- every foot costs you energy. Mount the charge controller close to the battery bank.
• For ground-mounted arrays far from the house, consider running higher voltage DC or using a micro-inverter system to minimize wire losses.