Underground Solar Wire Sizing, Voltage Drop & PVC Conduit Fill Calculator
Running power 100 to 1,000 feet from a ground-mount solar array, barn, or solar well pump requires balancing NEC Chapter 9 Table 8/9 voltage drop, NEC 250.122(B) proportional EGC ground-wire upsizing, and NEC Chapter 9 Table 1 (40% conduit fill). Compare 450V DC string trenches against 240V AC feeders below.
Underground Solar Trench Wire Sizing, Conduit Fill & Voltage Drop Calculator
Calculate exact AWG/kcmil wire gauge, NEC 250.122(B) proportional ground wire upsizing, Schedule 40/80 PVC conduit trade size, and Copper vs. Aluminum trench cost.
1. Array Power, Voltage Architecture & Trench Length
Smallest wire gauge & lowest trench cost! Sends 450V DC from ground array to hybrid inverter at the house.
Total wire area: 0.106 sq. in. (4 circuit wires + 1 EGC).
Copy NEC Conductor & Conduit Specification for Permit Plan Check
Paste this NEC Chapter 9 & Article 250.122(B) calculation block directly onto your electrical single-line diagram.
NEC CHAPTER 9 & ARTICLE 250/300 UNDERGROUND CONDUCTOR & CONDUIT SCHEDULE ======================================================================== System Load / Array Power: 12 kW (12,000 Watts) Circuit Configuration: High-Voltage Solar DC String (450V DC Vmp Nominal) (2 Parallel DC String Circuits) One-Way Trench Distance: 250 Feet | Target Max Voltage Drop: <= 2% 1. CONDUCTOR & EGC SIZING (NEC 310.16 & NEC 250.122(B)) - Operating Current per Circuit: 13.3 Amps (125% Continuous Design Current: 16.7 Amps) - Base Ampacity Minimum Gauge: 12 AWG (Copper) - Voltage-Drop Upsized Conductor Gauge: (4) x 10 AWG Copper 90°C Wet-Rated (THWN-2 / XHHW-2 / UL 4703 PV Wire) - Proportional Upsized EGC (NEC 250.122(B) [1.59x CM Ratio]): (1) x 10 AWG Equipment Grounding Conductor 2. VOLTAGE DROP & THERMAL LINE LOSSES (NEC CHAPTER 9 TABLE 8 / 9) - Actual One-Way Voltage Drop: 8.27 Volts (1.84% at Full Peak Output) - Peak Line Power Dissipation Loss: 220 Watts (~418 kWh/year thermal line loss) 3. PVC CONDUIT TRADE SIZE (NEC CHAPTER 9 TABLE 1 - 40% MAX FILL) - Total Conductor Cross-Sectional Area: 0.106 sq. in. (4 Circuit Conductors + 1 EGC) - Underground Trench Raceway (18" Min Cover per NEC Table 300.5): 3/4" Trade Size Schedule 40 PVC - Vertical Above-Grade Risers (NEC 300.5(D)(4) + 300.5(J) Expansion Joint): 3/4" Trade Size Schedule 80 PVC 4. CONDUCTOR MATERIAL COST COMPARISON (250 FT TRENCH) - Copper Option (10 AWG Cu): Est. $550 - Aluminum Option (8 AWG Al AA-8000): Est. $403
Why 450V DC Strings Beat 240V AC in Long Trenches
Sending 12 kW across a 350-foot trench at 240V AC requires 50 Amps (demanding expensive #1 AWG or 1/0 AWG wire). Sending that exact same 12 kW as two 450V DC strings drops current to just 13.3 Amps per string—allowing inexpensive #10 AWG or #8 AWG copper PV wire with under 1.5% voltage drop!
The NEC 250.122(B) Ground-Wire Upsizing Rule
Whenever you upsize phase or PV conductors in a long trench to compensate for voltage drop, NEC 250.122(B) mandates that you upsize the Equipment Grounding Conductor (EGC) by the exact same circular-mil ratio. Failing to upsize the ground wire is the #1 cause of failed trench inspections.
Schedule 40 vs. Schedule 80 PVC Risers
While Schedule 40 PVC is permitted 18 inches underground (NEC Table 300.5), NEC 300.5(D)(4) requires Schedule 80 Heavy-Wall PVC wherever the conduit emerges from the ground exposed to lawnmowers, string trimmers, or livestock—and Schedule 80 has a smaller internal diameter!