Calculate voltage drop in volts and percent from wire gauge, current, and distance, with an NEC 3%/5% guideline check.
Enter your wire run details and click Calculate
This free voltage drop calculator estimates how much voltage is lost over a wire run based on the wire gauge, the current it carries, the distance it travels, and whether the circuit is copper or aluminum, single-phase or three-phase. It also checks your result against the commonly used 3%/5% voltage drop guidelines referenced in the National Electrical Code (NEC).
Every wire has electrical resistance that increases with length and decreases with cross-sectional area (circular mils) — thinner, longer runs lose more voltage as heat. The calculator looks up your selected AWG wire gauge's circular mil area, then applies the standard voltage drop formula using a resistivity constant (K) specific to copper or aluminum, your entered current, and the one-way distance (doubled for single-phase to account for the full circuit path, or scaled by √3 for three-phase). The result shows both the raw voltage drop in volts and the percentage of your source voltage it represents.
Electricians and electrical students sizing a wire run, DIYers planning a long extension circuit (like a garage or shed sub-panel feed), and anyone checking whether an existing wire run might be undersized for its length and load. It's a planning estimate — always verify final wire sizing against your local electrical code and, for real installations, have the work inspected or performed by a licensed electrician.
The standard electrical formulas this calculator uses
Single-phase circuits use a 2× multiplier because current travels out to the load and back through the return conductor, doubling the effective resistance path.
Three-phase circuits use 1.732 (√3) instead of 2, reflecting the different relationship between line and phase voltage in a balanced three-phase system.
Circular mils measure a wire's cross-sectional area — larger circular mil values mean thicker wire, lower resistance, and less voltage drop for the same current and distance.
From entering wire details to reading your result
Choose copper or aluminum, and the AWG wire size being used.
Fill in the load current, one-way distance, and whether the circuit is single-phase or three-phase.
Enter the circuit's supply voltage to calculate percent voltage drop.
See the voltage drop in volts and percent, plus a check against the common 3% and 5% guidelines.
A single-phase 120V circuit and a three-phase 240V circuit
Standard cross-sectional area used by this calculator
| AWG Size | Circular Mils |
|---|---|
| 20 AWG | 1,020 |
| 18 AWG | 1,620 |
| 16 AWG | 2,580 |
| 14 AWG | 4,110 |
| 12 AWG | 6,530 |
| 10 AWG | 10,380 |
| 8 AWG | 16,510 |
| 6 AWG | 26,240 |
| 4 AWG | 41,740 |
| 3 AWG | 52,620 |
| 2 AWG | 66,360 |
| 1 AWG | 83,690 |
| 1/0 AWG | 105,600 |
| 2/0 AWG | 133,100 |
| 3/0 AWG | 167,800 |
| 4/0 AWG | 211,600 |
A lower AWG number means a thicker wire — 4/0 AWG is much larger and carries far more current with far less voltage drop than 20 AWG. Always match wire ampacity (current-carrying capacity), not just voltage drop, to your circuit's breaker or fuse rating per your local electrical code.
Where an accurate voltage drop estimate matters
Size the wire run for a sub-panel feeding a detached structure a long distance from the main panel.
Check whether a long-run circuit (like a workshop or dock) needs a heavier wire gauge.
Verify three-phase feeder sizing for equipment located far from the electrical panel.
Check whether excessive voltage drop explains dim lights or underperforming equipment at the end of a long run.
Practice voltage drop calculations for electrician apprenticeship or licensing exam prep.
What this voltage drop calculator does well, and where it can't replace a licensed electrician
Size a wire run accurately
Summary: This voltage drop calculator estimates voltage loss in volts and percent for a wire run based on gauge, current, distance, material, and phase, with a check against common 3%/5% guidelines — free, instant, and useful for planning, though not a substitute for licensed electrical work. Pair it with the Ohm's Law Calculator or Resistor Calculator for related circuit work.
Common questions about calculating voltage drop
Official code and safety references — not a substitute for a licensed electrician
Other circuits and measurement tools