Voltage Drop Calculator
Calculate voltage drop across any wire run — see drop percentage, power loss, and NEC compliance.
Free sizing tool
Voltage Drop Calculator
Calculate voltage drop across any wire run — see drop percentage, power loss, and NEC compliance.
Cable parameters
Acceptable voltage drop
AWG to mm² reference
| AWG | mm² | Max amps (Cu, 60°C) | Common use |
|---|---|---|---|
| 14 | 2.08 | 15 A | Lighting circuits |
| 12 | 3.31 | 20 A | General outlets |
| 10 | 5.26 | 30 A | Small solar, dryer |
| 8 | 8.37 | 40 A | Solar string, range |
| 6 | 13.30 | 55 A | Sub-panel feed |
| 4 | 21.15 | 70 A | Battery bank |
| 2 | 33.63 | 95 A | Large battery bank |
| 1/0 | 53.49 | 125 A | Main service |
| 2/0 | 67.43 | 145 A | Inverter feed |
| 4/0 | 107.22 | 195 A | Heavy inverter feed |
Based on NEC ampacity tables for copper at 60°C. Always verify with local electrical codes and a licensed electrician.
How to Use This Voltage Drop Calculator
This voltage drop calculator determines the voltage lost across a wire or cable run due to electrical resistance. Enter the wire length (one way), the current in amps, the wire gauge (AWG), and the system voltage. The calculator shows the total voltage drop in volts, the drop as a percentage of source voltage, and the power lost as heat in the wire. It also recommends whether to upsize the wire to meet NEC code limits.
Voltage drop matters because every wire has resistance, and that resistance converts some of your electrical energy into waste heat. In a solar system, excessive voltage drop between panels and the charge controller means your panels produce power that never reaches the battery. Between the battery and inverter, voltage drop can cause the inverter to shut down on low-voltage protection even when the battery is adequately charged.
Acceptable Voltage Drop Limits
The National Electrical Code (NEC) recommends a maximum 3% voltage drop for branch circuits and 5% total from the source to the load. For solar systems, the industry standard is stricter: 2% for battery-to-inverter runs and 3% for panel-to-controller runs. Our calculator flags any result that exceeds these thresholds and recommends the correct wire gauge to stay within limits.
| Circuit | Max Drop | Why |
|---|---|---|
| Solar panel to charge controller | 3% | Higher tolerance — panels have wider voltage range |
| Charge controller to battery | 2% | Charging voltage must be precise for battery health |
| Battery to inverter | 2% | Inverter needs stable voltage — drop triggers low-voltage shutdown |
| AC branch circuit (NEC) | 3% | NEC 210.19(A) recommendation |
| Total source to load (NEC) | 5% | Combined feeder + branch circuit |
The Voltage Drop Formula
Voltage drop (V) = 2 × Length (meters) × Current (amps) × Resistance per meter. The “2” accounts for the round-trip distance (current flows out on the positive wire and returns on the negative). Resistance per meter depends on the wire gauge and material (copper or aluminum). As a percentage: Drop % = (Voltage drop ÷ Source voltage) × 100. Our calculator uses the standard NEC resistance values for copper and aluminum conductors at 75°C operating temperature.
For solar-specific cable sizing, see our Solar Cable Sizing Guide which covers all three cable runs in a PV system. For battery-to-inverter cable sizing, see our battery-to-inverter wire guide.
Frequently Asked Questions
What is acceptable voltage drop for a solar system?
3% for the panel-to-controller run and 2% for the battery-to-inverter run. Total system voltage drop should stay under 5%. Exceeding these limits wastes energy as heat in the cables and can cause equipment malfunction.
How do I reduce voltage drop?
Three ways: use thicker wire (lower AWG number), shorten the cable run, or increase the system voltage. Switching from 12V to 24V cuts the current in half for the same power, halving the voltage drop. This is why systems above 1.5 kW should use 24V or 48V.
Does voltage drop waste electricity?
Yes. Voltage drop converts electrical energy into heat in the wire. A 5% voltage drop means 5% of your power is wasted as cable heating. On a 1,000W system, that is 50 watts continuously lost — roughly 130 kWh per year, worth $15-30 in electricity.
Is voltage drop the same for AC and DC?
The basic formula is the same, but DC systems typically have higher current at lower voltage, making voltage drop a much bigger issue. A 1,000W load at 12V DC draws 83A; the same load at 240V AC draws only 4.2A. The DC system needs much thicker cable to keep the same percentage drop.