Solar Voltage Drop Calculator
Free solar voltage drop calculator. Enter system voltage, current, wire length, and AWG to see drop in volts and percentage. NEC compliant.
Solar Voltage Drop Calculator
How to use this calculator
Enter four numbers:
- System voltage — 12 V, 24 V, 48 V for off-grid; 600 V or higher for grid-tied PV strings
- Current — the maximum amps the circuit will carry (look at panel specs or charge controller rating)
- One-way length — distance from source to load in feet (the calculator handles the round trip)
- Wire size in AWG — what you have or plan to install
The calculator returns the total voltage drop in volts and as a percentage of system voltage, plus a verdict on whether you’re within NEC recommendations.
Why voltage drop is the silent solar killer
Every wire has resistance. When current flows through that resistance, some of the voltage is “dropped” — converted to heat instead of reaching your loads.
On a 240 V grid-tied AC circuit, 3% drop is barely noticeable. On a 12 V DC solar circuit, 3% drop means your inverter sees 11.6 V instead of 12 V — enough to trigger low-voltage disconnect on cloudy days. On a 48 V battery bank with a 100 A inverter draw, 3% drop means 144 watts of heat in your wires under full load.
This is why DIY solar systems frequently underperform: undersized wires create bottlenecks that don’t show up on a multimeter at idle but eat power under load.
The formula
Voltage drop on DC circuits:
V_drop = 2 × Length(meters) × Resistance(Ω/m) × Current(A)The 2× accounts for the round trip (current down one conductor, back through the other). Wire resistance is looked up from AWG tables — the calculator uses standard 25°C copper values.
Resistance per 1000 ft (Ω/1000ft @ 25°C) for common AWG sizes:
| AWG | Ω/1000ft |
|---|---|
| 14 | 2.525 |
| 12 | 1.588 |
| 10 | 0.999 |
| 8 | 0.628 |
| 6 | 0.395 |
| 4 | 0.249 |
| 2 | 0.156 |
| 0 | 0.098 |
Each step up (12 → 10 → 8) drops resistance ~37%, which is why going up one wire size is usually enough to fix marginal voltage-drop problems.
When to size up
If your drop is over 3% and you can’t shorten the run:
- Go up one wire size (e.g. 10 AWG → 8 AWG)
- Or run the system at a higher voltage (12 V → 24 V → 48 V cuts amps and drop in half each step)
- Or add a parallel conductor (effectively halves resistance)
Higher voltage is usually the cheapest fix for long runs because copper costs scale brutally with diameter.
NEC code reference
The 2026 National Electrical Code (NEC) Article 690.8 governs PV circuit sizing. The 3% recommendation comes from the Informational Note in 690.8(B) — it’s a recommendation, not a hard requirement, but most inspectors expect compliance.
For circuits where you cannot meet 3% (e.g. very long runs to a distant battery shed), document the calculation, size conductors to handle the heat, and verify with the inverter’s minimum input voltage spec.