Voltage Drop Calculator

Calculate the voltage drop, power loss, and NEC compliance for any wire size. Enter your AWG gauge, current, distance, and system voltage.

Circuit details

Wire gauge (AWG)?

Current?

One-way distance?

Conductor material?

System voltage?

Voltage drop results

1.199V drop (1.00%)

Voltage at load118.80 V

Power loss24.0 W

3% threshold (NEC branch)✓ Pass

5% threshold (NEC total)✓ Pass

Ampacity30A ✓ OK

Link copied to clipboard

How to Use This Calculator

Select your wire gauge and conductor material

Choose the AWG wire gauge you're planning to use (or already have installed). American Wire Gauge numbers run inversely to wire diameter — lower numbers mean thicker wire. AWG 14 is standard for 15A household circuits; AWG 4/0 is used for large battery bank cables. Select copper for standard solar wiring or aluminum for service entrance and large feeder conductors.

Enter current and distance

Enter the current in amps the circuit carries and the one-way distance in feet. The calculator doubles the distance automatically — current flows through both the positive (hot) conductor and the negative (neutral/return) conductor. Both must be sized correctly.

Enter system voltage

Enter your system voltage so the calculator can express voltage drop as a percentage. Percentage drop is what matters for compliance — 1V drop on a 12V system is 8.3% (terrible), but 1V on a 120V circuit is 0.8% (fine). DC solar systems at low voltage are especially sensitive.

Check the pass/fail thresholds

The result shows whether your wire passes the 3% NEC branch circuit and 5% NEC total system thresholds. If your wire fails, click through to the Wire Size Calculator to find the gauge that meets your target.

The Formula

Total circuit length = One-way distance × 2 Resistance = (Wire ohms/1,000ft × Total circuit length) ÷ 1,000 Voltage drop = Current × Resistance Voltage drop % = (Voltage drop ÷ System voltage) × 100 Power loss = Voltage drop × Current

Resistance values are from NEC Table 9 at 75°C conductor temperature. Higher temperatures increase resistance slightly — the actual drop at full load may be 5-10% higher than calculated at ambient temperature, which is another reason to add safety margin.

Voltage Drop Thresholds

NEC 210.19(A)(1) Informational Note3% max for branch circuits

NEC recommendation (total)5% max from service to outlet

Solar DC runs (best practice)1-2% for battery/inverter cables

Solar panel-to-controller runs2-3%

Low-voltage DC systems (12V/24V)1-3% strongly recommended

FAQ

What causes voltage drop in electrical wiring? ▼

Voltage drop is caused by the resistance of the wire itself. All conductors have resistance, and when current flows through resistance, some voltage is "used up" as heat (V = I × R). The voltage available at the load end is lower than the source voltage. Longer wires, thinner wires, and higher currents all increase voltage drop. Voltage drop converts electrical energy to heat — it's a direct efficiency loss from your solar system.

Does voltage drop affect solar panel output? ▼

Yes. Voltage drop in the panel-to-controller wiring reduces the voltage reaching the charge controller's MPPT input. This means the controller operates at a lower input voltage point, extracting less power. For MPPT controllers, the effect is smaller because they can adjust the operating point, but significant voltage drop still reduces array output. In PWM systems, string voltage drop directly reduces the charging voltage available at the battery, potentially preventing full charge at the top of the I-V curve.

Is voltage drop different for DC vs AC circuits? ▼

The resistance calculation is the same for DC and resistive AC loads. For AC circuits with reactive loads (motors, transformers, electronics), impedance (which includes reactance) replaces simple resistance, and the calculation becomes more complex. For practical solar system wiring — panel wiring, battery cables, and most branch circuits — the simple resistance formula is accurate enough. This calculator uses DC-appropriate resistance values from NEC Table 9.

Why is the 3% limit just a recommendation, not a code requirement? ▼

The NEC 3% voltage drop recommendation appears in an "Informational Note" — it's guidance for good design practice, not a mandatory code requirement. The actual code requirement is that conductors be sized for safe ampacity (preventing overheating and fire). Excessive voltage drop won't cause a fire — it just wastes energy and reduces equipment performance. In practice, AHJs (Authorities Having Jurisdiction) may enforce the 3% guideline on inspection for solar installations.

How much power am I wasting to voltage drop? ▼

Power loss = Voltage drop (V) × Current (A). A 3% voltage drop at 20A on a 120V circuit means 3.6V drop × 20A = 72W of heat generated in the wire. Over a year of 8-hour daily operation, that's 72W × 8h × 365 days = 210 kWh lost to wire resistance — at $0.15/kWh, about $31/year wasted. For high-current DC solar circuits operating continuously, minimizing wire resistance has a real economic payback.