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Voltage Drop Calculator
Calculate the electrical voltage drop and percentage loss for copper and aluminum conductors. Supports DC, single-phase AC, and three-phase AC systems with circuit checks and wire size comparisons per NFPA 70-2026.
Set your inputs and click Calculate.
Voltage drop by wire size
Compare drop across nearby wire sizes and click to select.
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Request a QuoteVoltage drop formulas
Voltage drop uses Ohm's law and the standard phase multipliers.
DC: VD = 2 × I × R_eff × (L ÷ 1000)
Single-phase AC: VD = 2 × I × R_eff × PF × (L ÷ 1000)
Three-phase AC: VD = 1.732 × I × R_eff × PF × (L ÷ 1000)
I is load current in amperes. L is one-way length in feet. R_eff is ohms per 1,000 ft divided by the number of parallel runs. R comes from NEC Chapter 9, Table 8 at 75°C. PF is power factor.
Conductor resistance reference chart
DC resistance in ohms per 1,000 feet at 75°C (167°F) from NEC Chapter 9, Table 8.
| Conductor size | Copper (Ω/1k ft) | Aluminum (Ω/1k ft) | Circular mils |
|---|---|---|---|
| 14 AWG | 3.14 | — | 4,110 |
| 12 AWG | 1.98 | 3.25 | 6,530 |
| 10 AWG | 1.24 | 2.04 | 10,380 |
| 8 AWG | 0.778 | 1.28 | 16,510 |
| 6 AWG | 0.491 | 0.808 | 26,240 |
| 4 AWG | 0.308 | 0.508 | 41,740 |
| 2 AWG | 0.194 | 0.319 | 66,360 |
| 1/0 AWG | 0.122 | 0.201 | 105,600 |
| 2/0 AWG | 0.0967 | 0.159 | 133,100 |
| 4/0 AWG | 0.0608 | 0.1 | 211,600 |
| 350 kcmil | 0.0367 | 0.0605 | 350,000 |
| 500 kcmil | 0.0276 | 0.0453 | 500,000 |
Source: NFPA 70-2026, Chapter 9, Table 8. Resistance values are for uncoated, stranded conductors at 75°C. The calculator uses the full size list, including sizes omitted from this summary chart.
How to reduce voltage drop in your circuits
If your voltage drop exceeds the recommended limits, there are several standard engineering methods to mitigate the loss and restore voltage stability at the load:
1. Increase conductor size
The most common and direct solution. Upsizing to a larger wire gauge (for example, from #6 AWG to #4 AWG) reduces the resistance per foot, lowering the voltage drop. Use the comparison chart to find the exact size that satisfies your limit.
2. Run parallel conductors
For high-current industrial circuits, running multiple parallel wires per phase (for example, two runs of 2/0 AWG instead of one run of 250 kcmil) splits the load current and halves the effective circuit resistance, substantially reducing drop.
3. Increase system voltage
For a given power level, increasing the system voltage (for example, from 120 V to 240 V, or 208 V to 480 V) proportionally reduces the current needed to deliver that power. Lower current results in a smaller voltage drop percentage.
Frequently asked questions
What is voltage drop and why does it occur?
Voltage drop is the decrease of electrical potential along the path of a current flowing in an electrical circuit. It occurs because all conductors have internal electrical resistance. As current flows through this resistance, some energy is lost as heat, resulting in a lower voltage at the load than at the source.
What is the maximum allowable voltage drop recommended by the NEC?
The National Electrical Code recommends a maximum voltage drop of 3% for branch circuits, and a maximum of 5% total for the combined feeder and branch circuits. These guidelines are recommendations, and maintaining them matters for equipment efficiency, longevity, and safety.
How does phase type affect voltage drop calculations?
Phase type determines the multiplier used in the voltage drop formula. For DC and single-phase AC circuits, current travels out and back, so the multiplier is 2. For balanced three-phase AC circuits, the multiplier is 1.732 (the square root of 3). Three-phase systems experience less voltage drop for the same load and wire size.
Does power factor affect voltage drop?
Yes, in AC systems. The power factor represents the phase difference between voltage and current. A lower power factor increases the current required to deliver the same real power, which increases the voltage drop. This calculator accounts for power factor in AC calculations. DC calculations use a power factor of 1.
How do parallel runs reduce voltage drop?
Running multiple conductors in parallel per phase divides the total current among the conductors and reduces the overall circuit resistance. Two parallel conductors per phase cut the circuit resistance in half, which reduces the voltage drop by 50%.
Why is aluminum wire more susceptible to voltage drop than copper?
Aluminum has a higher electrical resistivity than copper. For a given gauge and length, an aluminum conductor has higher resistance and more voltage drop. To achieve the same drop, aluminum conductors are typically one to two gauges larger than copper.
Disclaimer: This calculator is provided for educational and informational purposes, referencing standard formulas and NFPA 70-2026 tables. Results should be verified by a licensed electrical engineer or certified electrician before installation. Local codes and AHJ requirements may vary.