Solar wiring involves both DC circuits (panels to inverter or charge controller) and AC circuits (inverter to panel/grid). DC wire sizing is especially critical because solar panels can be far from the inverter, creating long runs where voltage drop matters.
Panel-to-combiner box: typically 10-12 AWG USE-2 or PV wire. String wiring uses the panel's maximum series fuse rating. DC home runs to the inverter may need 6-2 AWG depending on distance and current. NEC 690.8 requires PV source conductors to carry 156% of the short-circuit current (Isc × 1.25 for irradiance, × 1.25 again for continuous duty).
Battery cables carry the highest currents in an off-grid system. A 48V battery bank powering a 5kW inverter draws over 100A — requiring 2/0 or larger cable. Keep battery cable runs as short as possible. Use welding cable or marine battery cable for flexibility and durability.
The NEC sets no voltage-drop limit for PV circuits, but 1–2% is common practice to protect energy harvest. A 100-foot one-way run of 10 AWG copper at 8A and 48V drops about 2.0V (4.1%) using NEC Table 8 resistance, so that run would call for 6 AWG (about 1.6%). At higher currents or longer distances, upgrade to a larger gauge.
USE-2 and PV Wire (PV1-F) are rated for direct sunlight exposure and are required for outdoor panel wiring. THWN-2 in conduit is acceptable for interior runs. For underground, use UF-B or direct-burial rated cable.
Disclaimer: This guide is for informational purposes only. Always consult a licensed electrician and follow local building codes for actual installations. Wire sizing depends on specific conditions including ambient temperature, conduit fill, and derating factors.
Solar panel string wire must have an ampacity of at least 156% of the module short-circuit current (Isc × 1.25 × 1.25, NEC 690.8(A) and (B)). Most residential panels with 10–11A Isc use 10 AWG PV wire or USE-2. For runs over 50 feet, upsize to 8 AWG to keep voltage drop under 1% (recommended for DC solar circuits).
The charge controller to battery wire size depends on the controller's output current. A 40A MPPT controller needs at least 8 AWG copper. A 60A controller needs 4 AWG (60A × 1.25 = 75A). Keep this run as short as possible (under 6 feet) to minimize voltage drop and fire risk.
NEC 690 requires PV wire or USE-2 (Underground Service Entrance) cable for exposed solar array wiring. These are rated for UV exposure, outdoor temperature extremes, and wet locations. Standard THHN or NM-B cable is NOT permitted for exposed solar DC wiring.
The NEC has no voltage-drop requirement for PV circuits; keeping DC wiring under 1–2% is industry best practice to maximize energy harvest. Calculate it with the string’s operating voltage and current (Vmp and Imp), not the open-circuit voltage, to find the minimum wire gauge.