Solar Wire Size & Voltage Drop Calculator
Determine required AWG wire gauge, DC voltage drop percentage, and power loss watts over long wire runs per NEC 690 standards.
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How Solar Wire Sizing & Voltage Drop Work
Solar wire sizing must satisfy two independent engineering criteria: thermal ampacity safety (preventing wire insulation from melting per NEC 690.8) and voltage drop efficiency (preventing energy harvest loss over long distances):
Max Allowable V_drop = System Voltage × (Target % / 100)
Required Circular Mils = (2 × K × Current × One-Way Distance) / Max V_drop
K-factor (Resistivity): Copper = 12.9 Ω·cmil/ft | Aluminum = 21.2 Ω·cmil/ft
Power Loss (Watts) = Current² × Total Loop Resistance
High-Voltage Strings vs Low-Voltage Batteries: A 380V string carrying 13.5A loses less than 2% voltage over 120 ft on standard 10 AWG wire. Conversely, a 12V battery circuit carrying 50A over just 20 ft would drop 12.5% on 10 AWG, requiring massive 2 AWG cable.
Frequently Asked Questions
Why is 2% voltage drop the recommended target for solar PV circuits?
While the NEC permits up to 3% branch circuit drop (and 5% total), solar systems generate revenue over a 25-year lifetime. A 1% extra voltage drop across a 10 kW system wastes roughly 150 kWh annually, which compounds to thousands of dollars in lost power production over time.
What is the difference between standard THHN building wire and PV Wire?
PV Wire (and USE-2) is specifically engineered for outdoor exposed rooftop conditions. It features thicker cross-linked polyethylene (XLPE) insulation rated for 90°C wet conditions, direct sunlight UV exposure, and 600V to 2,000V DC ratings.
How does NEC 690.8 apply the 125% continuous duty multiplier?
NEC 690.8 mandates that maximum solar circuit current must be calculated as 125% of the short circuit current rating (Isc × 1.25). Furthermore, conductor ampacity must be sized for an additional 125% continuous duty safety factor (1.25 × 1.25 = 1.56 × Isc).