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.
Solar System Wire Types
DC Side Wiring
- 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
- 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
Step 1: Determine Short-Circuit Current (Isc)
Find the panel's Isc rating on the nameplate. For strings in parallel, multiply by number of strings.
Step 2: Apply 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
Step 3: Calculate Voltage Drop
For DC circuits, voltage drop should be under 2% to maintain system efficiency. Use formula:
VD = 2 × I × R × L / 1000
VD = Voltage drop (volts), I = Current (amps), R = Resistance (ohms/1000ft), L = One-way length (feet)
DC Wire Size Reference
| Current (A) | 50ft Run | 100ft Run | 150ft Run |
|---|---|---|---|
| 10A | 14 AWG | 12 AWG | 10 AWG |
| 15A | 12 AWG | 10 AWG | 8 AWG |
| 20A | 10 AWG | 8 AWG | 6 AWG |
| 30A | 8 AWG | 6 AWG | 4 AWG |
PV Wire Requirements
USE-2 and PV Wire
- 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
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.
AC Wire Sizing
AC wiring from inverter to panel follows standard NEC rules:
- Apply 125% continuous load factor to inverter output
- Size wire for ampacity per NEC Table 310.16
- Check voltage drop (3% max recommended)
- Use appropriate conduit and wire type for location
Grounding Requirements
- Equipment grounding conductor required
- Size per NEC Table 250.122 based on overcurrent device
- Grounding electrode conductor per NEC 250.166
- DC grounding separate from AC in most cases
Common Solar Wiring Mistakes
1. Undersizing DC Wire
DC voltage drop directly reduces system output and energy production. Size for 2% max drop.
2. Using Wrong Wire Type
Standard building wire degrades rapidly in UV exposure. Always use listed PV wire outdoors.
3. Ignoring Temperature Derating
Rooftop conduit can exceed 140°F. Apply temperature correction factors from NEC Table 310.15(B)(1)(1).
4. Improper Connections
Use only MC4 or listed connectors for DC connections. Never splice PV wire without proper connectors.
Sizing Tools
Use our tools to properly size solar system wiring:
- Wire Gauge Calculator - Calculate wire size for any circuit
- Voltage Drop Calculator - Verify acceptable voltage drop
- AWG Reference Chart - Wire specifications
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. Always work with a licensed electrician familiar with NEC Article 690 for solar installations.