Solar PV System Voltage Drop: DC and AC Considerations
Complete guide to voltage drop in solar photovoltaic systems. Covers DC string wiring, inverter connections, and AC output circuits for residential and commercial solar.
Voltage Drop in Solar PV Systems
Solar photovoltaic systems present unique voltage drop considerations because they combine DC circuits (from panels to inverters) with AC circuits (from inverters to the grid or loads). Each side of the system requires different calculation approaches, and both must be optimized to maximize energy harvest and system efficiency. Excessive voltage drop in a solar system directly reduces the power delivered to the grid or loads.
This guide covers voltage drop calculations for all parts of a solar PV system, from the DC string wiring to the AC interconnection, with practical examples for both residential and commercial scale installations.
DC Side Voltage Drop
The DC side of a solar system includes the wiring from individual panels within strings, home runs from string combiners to the inverter, and any DC disconnect switches. DC voltage drop is calculated using simple resistance-based formulas since there's no reactance or power factor to consider.
DC Voltage Drop Formula
For string wiring, use short-circuit current (Isc) as the design current per NEC 690.8
Industry best practice limits DC voltage drop to 1-2% for string wiring and 2-3% for the DC home run. Lower voltage drop means more power delivered to the inverter and higher system efficiency.
AC Side Voltage Drop
The AC output from inverters must connect to the building electrical system or utility grid. This AC circuit is calculated using standard single-phase or three-phase formulas depending on the inverter output.
Residential Systems
- • Typically 240V single-phase
- • Short runs to main panel
- • 2% max recommended
- • Combined with DC drop
Commercial Systems
- • Often 480V three-phase
- • Longer feeder runs possible
- • May require transformers
- • Multiple inverters paralleled
NEC 690 Requirements
NEC Article 690 governs solar PV installations. While specific voltage drop limits are not mandated, 690.8(A) requires that maximum circuit current be calculated as 125% of the module short-circuit current (Isc), affecting conductor sizing. The code also requires that conductors be sized per NEC Chapter 3 requirements.
Design Current Calculation
Maximum circuit current for PV source circuits: Imax = Isc × 1.25
For conductor sizing: Ampacity ≥ Imax × 1.25 = Isc × 1.56
Calculate Solar System Voltage Drop
Use our DC calculator for solar string wiring and home runs, then verify your AC interconnection with the standard voltage drop calculator. Optimize both sides for maximum system performance.
DC Solar Calculator