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Voltage Drop Calculator

Find the voltage drop in copper or aluminum wire from the wire size, length, current and voltage, and see which wire sizes keep the drop under 3%.

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ItemValue

Voltage drop by wire size

SizeDrop (V)Drop (%)
Resistance is worked out for solid conductors at 75°C from the AWG size and the conductor’s resistivity, which closely matches the DC values in NEC Chapter 9, Table 8. Stranded wire is slightly higher. This ignores reactance, which matters for large AC conductors. Wire size must also meet ampacity and code rules, so check with a licensed electrician.

Quick Answer

Single-phase voltage drop = 2 × length × current × resistance per foot. A 100 ft run of 12 AWG copper carrying 16 amps at 120 V drops about 6.18 V, or 5.15%. Many electricians aim for 3% or less on a branch circuit.

Last updated: 2 November 2026 ·How we calculate this: the voltage drop formula with AWG wire resistance

How This Voltage Drop Calculator Works

Choose copper or aluminum, the wire size, and single-phase or three-phase. Enter the source voltage, the load current and the one-way length of the run. The calculator finds the wire's resistance, the voltage lost in the wire, and the voltage left at the load. A table shows the drop for every size from 14 AWG to 4/0, so you can pick a size that stays under 3%.

Quick explanation: long runs and high current lose voltage in the wire.

  • Longer wire: more drop.
  • More current: more drop.
  • Bigger wire (smaller AWG number): less drop.

How to Use the Voltage Drop Calculator

  1. Choose copper or aluminum and the wire size.
  2. Choose single-phase (or DC) or three-phase.
  3. Enter the source voltage and the load current in amps.
  4. Enter the one-way length from the source to the load.
  5. Press Calculate and compare the drop for each wire size.

Core Formulas

  • Single-phase or DC: VD = 2 × L × I × R ÷ 1,000
  • Three-phase: VD = 1.732 × L × I × R ÷ 1,000
  • Percent drop: VD ÷ source voltage × 100
  • Wire diameter: 0.005 in × 92^((36 − AWG) ÷ 39)

L is the one-way length in feet, I is the current in amps and R is the resistance in ohms per 1,000 ft. The 2 counts the wire out and back.

Key Terms

  • Voltage drop: the voltage lost along a wire because of its resistance.
  • AWG: American Wire Gauge. Smaller numbers mean thicker wire.
  • Ampacity: the maximum current a conductor can carry safely.
  • Resistance: opposition to current flow, measured in ohms.
  • Three-phase: a power system with three alternating currents, common in commercial buildings.

Worked Example

A 240 V, 30 A circuit runs 150 ft using 10 AWG copper.

  • Resistance of 10 AWG copper at 75°C ≈ 1.215 ohms per 1,000 ft
  • VD = 2 × 150 × 30 × 1.215 ÷ 1,000 = 10.93 V
  • Percent drop = 10.93 ÷ 240 = 4.56%, so a larger wire is worth considering

Real-Life Examples

  • A detached garage: 20 A at 120 V over 100 ft on 12 AWG copper drops 6.4%. 10 AWG cuts it to 4.0%.
  • A well pump: 10 A at 240 V over 250 ft on 10 AWG drops 2.5%.
  • Low-voltage lights: 5 A at 12 V DC over 50 ft on 12 AWG drops 8.0%, which is a lot for 12 V systems.
  • Three-phase motor: 40 A at 480 V over 200 ft on 8 AWG copper drops 2.2%.

Voltage Drop vs Ampacity

  • Ampacity: how much current a wire can safely carry without overheating. Set by code tables.
  • Voltage drop: how much voltage is lost over the length of the run.
  • Both matter: a wire can be safe for the current but still too small for a long run.

Wire Resistance by Size

Calculated from AWG size and resistivity. Stranded wire is a little higher.

SizeCopper (ohms)Aluminum (ohms)
14 AWG3.0715.064
12 AWG1.9323.185
10 AWG1.2152.003
8 AWG0.7641.260
6 AWG0.4800.792
4 AWG0.3020.498
3 AWG0.2400.395
2 AWG0.1900.313
1 AWG0.1510.248
1/0 AWG0.1200.197
2/0 AWG0.0950.156
3/0 AWG0.0750.124
4/0 AWG0.0600.098

Ways to Reduce Voltage Drop

  • Use a larger wire: each step down in AWG number lowers resistance by about 20%.
  • Shorten the run: move the panel or load closer if you can.
  • Use a higher voltage: 240 V carries the same power with half the current of 120 V.
  • Choose copper: it has lower resistance than aluminum of the same size.

Common Mistakes to Avoid

  • Entering the round-trip length. Enter one-way length. The formula doubles it.
  • Ignoring ampacity. Voltage drop does not replace code rules for wire size.
  • Using the breaker size for current. Use the real load current if you know it.
  • Forgetting aluminum is different. Aluminum needs a larger size for the same drop.
  • Ignoring low-voltage systems. A few volts lost on 12 V is a large percent.

Frequently Asked Questions

Many electricians aim for 3% or less on a branch circuit and 5% or less in total, as NEC informational notes suggest.

For single-phase: 2 × one-way length × current × resistance per foot. For three-phase, use 1.732 instead of 2.

Yes. Doubling the length doubles the drop.

It depends on the current and voltage. Enter them here and check the size table for 3% or less.

Copper has about 60% of aluminum's resistance, so it drops less for the same size.

The current flows out and back, so the formula doubles the one-way length for you.

Motors can run hot, lights can dim and electronics may not work properly.

The volts lost do not, but the percent does. The same drop is a smaller percent of 240 V than of 120 V.

Three-phase uses √3 (1.732) instead of 2, so the drop is lower for the same current.

It estimates voltage drop only. Always check ampacity, temperature ratings and local codes.

Slightly, a few percent more than solid wire of the same size.

Yes. Choose single-phase or DC and enter the DC voltage.

References & Sources

SourceUsed ForLink
Wikipedia, American wire gauge Wire diameters and the AWG formula. https://en.wikipedia.org/wiki/American_wire_gauge
Wikipedia, Voltage drop How resistance causes voltage drop. https://en.wikipedia.org/wiki/Voltage_drop