Introduction to Solar Wire Sizing
Proper wire sizing is critical for solar installations to ensure safety, efficiency, and code compliance. Solar systems have unique requirements due to DC voltage, outdoor exposure, and high currents.
DC Side Wiring
DC wiring from panels to inverter requires special considerations:
- Panel to combiner box: PV wire (USE-2, PV-1000V)
- Combiner to inverter: PV wire or THWN-2 in conduit
- String voltage: Typically 300-600V DC
- Critical for efficiency: Every volt lost reduces output
AC Side Wiring
AC wiring from inverter to panel follows standard NEC rules:
- Inverter to panel: Standard building wire (THWN-2)
- Voltage: 120/240V split-phase or 208V 3-phase
- Same rules: Standard NEC ampacity and voltage drop
DC Wire Sizing Steps
Follow NEC 690.8 requirements for proper DC wire sizing.
NEC 690.8 Multipliers
Maximum Current = Isc × 1.25 × 1.25. First 1.25: continuous load factor. Second 1.25: irradiance adjustment. Total: 1.56× Isc.
PV Wire Requirements
Special wire types are required for outdoor solar installations:
- USE-2: Underground Service Entrance, rated 90°C wet
- PV Wire: 90°C wet, 150°C dry, sunlight resistant
- Voltage Rating: 600V or 1000V DC
- UV Resistance: Essential for rooftop exposure
Common Solar Wiring Mistakes
Avoid these common errors in solar installations.
Important
Standard THHN wire is NOT rated for outdoor PV use. Use only listed PV wire or USE-2 for DC circuits exposed to sunlight.
Conclusion
Proper wire sizing for solar installations requires attention to both DC and AC requirements, temperature derating, and voltage drop. Using the correct wire type and size ensures system safety, efficiency, and code compliance.
Use our Wire Gauge Calculator to calculate wire size for any solar circuit.