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 Wire Sizing
Panel to Combiner Box
- Calculate String Current: Panel Isc × 1.25 (NEC safety factor)
- Voltage Drop: Keep under 3% for optimal performance
- Temperature Rating: Use 90°C rated wire (PV wire)
- UV Protection: Must be sunlight resistant
Combiner to Inverter
- Sum all string currents
- Apply 1.25× safety factor
- Account for conduit fill if applicable
- Consider voltage drop over distance
AC Side Wire Sizing
Inverter to main panel follows standard electrical code:
- Based on inverter output current × 1.25
- Standard NEC ampacity tables apply
- THWN-2 or THHN wire typical
- Must match breaker size
Common Wire Sizes for Solar
Typical Solar Installations:
- 10 AWG: String wiring up to 30A
- 8 AWG: Combiner to inverter (30-40A)
- 6 AWG: Large arrays (40-50A)
- 4 AWG: AC output from inverter
Special Considerations
- Conduit Temperature: Rooftop conduits can exceed 150°F
- Grounding: Separate equipment and system grounding required
- Rapid Shutdown: Module-level shutdown may affect wire sizing
- Battery Systems: Require heavy gauge for high current
Step-by-Step Calculation Example
For a 5kW system with 300W panels:
- Panel Isc = 9.5A
- String current = 9.5A × 1.25 = 11.88A
- For 100ft run, use 10 AWG to minimize voltage drop
- Combiner to inverter: 4 strings × 11.88A = 47.5A → Use 6 AWG
- AC side: 5000W / 240V × 1.25 = 26A → Use 10 AWG with 30A breaker
Tools and Resources
Use our Wire Gauge Calculator and Voltage Drop Calculator for accurate solar wire sizing.