Voltage Drop Calculator
Voltage drop is how much voltage is lost along a wire run. Enter your system voltage, current, wire length, and size above, then press Calculate. You get the drop in volts, the percentage, the voltage at the load, and whether it stays under the recommended 3% limit.
Long cables, small wire gauges, and high current all push voltage drop up. Too much drop means dim lights, overheating cable, and motors that struggle to start. This tool handles single-phase and three-phase circuits, copper and aluminum, in feet or meters. Below you will find the exact formula for both phase types, a drop chart by wire size, and the AWG resistance table the calculator uses.
Voltage Drop Recommended Limits by Application
| Application | Recommended Maximum Drop |
|---|---|
| Lighting Circuits | 3% |
| Motors / HVAC Units | 3–5% |
| General Branch Circuits | 3% |
| Feeder Circuits | 5% |
| Long-Distance Outdoor Wiring | 5–8% |
In this guide, we’ll explain how to calculate voltage drop, formulas, examples, and how voltage drop works across resistors and long cable runs.
What Is Voltage Drop?
Voltage drop is the loss of voltage as current travels through a conductor. Every wire has resistance, and that resistance eats a small amount of voltage along the way. The longer the run and the smaller the wire, the bigger the drop. Most electrical codes recommend keeping it under 3% for branch circuits and 5% for feeders, so the equipment at the end still gets enough voltage to run properly.
Voltage Drop Formula
The formula depends on whether your circuit is single-phase or three-phase. Both use current, one-way cable length, and the conductor’s resistance per unit length.
Single-phase (2-wire)
Vdrop = 2 × L × I × R
The 2 accounts for the round trip, since current flows out and back along both conductors.
Three-phase (balanced)
Vdrop = √3 × L × I × R
√3 equals about 1.732. Three-phase uses one-way length, not round trip, because of how the phases share the return.
Where L is the one-way cable length, I is the current in amps, and R is the conductor resistance per unit length. The calculator applies the correct version automatically once you pick your phase.
How to Calculate Voltage Drop (Step by Step)
Take a 240V single-phase circuit with a 30A load, running 100 feet on copper 10 AWG wire.
- 10 AWG copper resistance: 0.999 Ω per 1000 ft
- Total resistance (round trip): (0.999 ÷ 1000) × 100 × 2 = 0.1998 Ω
- Voltage drop: 30 × 0.1998 = 5.99 V
- Percentage: 5.99 ÷ 240 = 2.50%
That result sits just under the 3% limit, so 10 AWG works for this run. Drop below that and you would need a larger wire.
Voltage Drop Chart by Wire Size
This chart shows the approximate voltage drop for a 100 ft copper run at 20 amps, single-phase. Use it for a quick gut check before running the exact numbers in the calculator.
| Wire Size | Resistance (Ω/1000 ft) | Drop at 20A, 100 ft (V) |
|---|---|---|
| 14 AWG | 2.525 | 10.10 V |
| 12 AWG | 1.588 | 6.35 V |
| 10 AWG | 0.999 | 4.00 V |
| 8 AWG | 0.6282 | 2.51 V |
| 6 AWG | 0.3951 | 1.58 V |
| 4 AWG | 0.2485 | 0.99 V |
| 2 AWG | 0.1563 | 0.63 V |
Bigger wire has lower resistance, so the drop falls fast as you size up. Doubling the current or length roughly doubles the drop.
Copper vs Aluminum Wire Resistance (AWG Table)
These are the resistance values the calculator uses, in ohms per 1000 feet. Aluminum carries about 1.6 times the resistance of copper for the same size, so it drops more voltage over the same run.
| Wire Size | Copper (Ω/1000 ft) | Aluminum (Ω/1000 ft) |
|---|---|---|
| 14 AWG | 2.525 | 4.040 |
| 12 AWG | 1.588 | 2.541 |
| 10 AWG | 0.999 | 1.598 |
| 8 AWG | 0.6282 | 1.005 |
| 6 AWG | 0.3951 | 0.632 |
| 4 AWG | 0.2485 | 0.398 |
| 2 AWG | 0.1563 | 0.250 |
| 1 AWG | 0.1239 | 0.198 |
| 1/0 AWG | 0.0983 | 0.157 |
| 2/0 AWG | 0.0779 | 0.125 |
| 3/0 AWG | 0.0618 | 0.099 |
| 4/0 AWG | 0.0490 | 0.078 |
Three-Phase Voltage Drop
Three-phase drop uses the √3 factor and one-way length, not the round trip. Take a 208V three-phase circuit, 40A load, 150 ft on copper 6 AWG.
- 6 AWG copper resistance: 0.3951 Ω per 1000 ft
- One-way resistance: (0.3951 ÷ 1000) × 150 = 0.0593 Ω
- Voltage drop: 1.732 × 40 × 0.0593 = 4.11 V
- Percentage: 4.11 ÷ 208 = 1.97%
Switch the calculator’s phase to three-phase and it runs this math for you.
Voltage Drop Across a Resistor
For a single resistor, voltage drop is just Ohm’s Law: V = I × R. If 2 amps flow through a 10 Ω resistor, the drop is 2 × 10 = 20 volts. In a series circuit, current stays the same and each resistor drops its own share, so you add them up. In a parallel circuit, the voltage across each branch is the same while the current splits.
Allowable Voltage Drop Limits (NEC Guidance)
The National Electrical Code treats voltage drop as a recommendation, not a hard rule, but most installers follow these limits to keep equipment running safely.
| Application | Recommended Maximum Drop |
|---|---|
| Lighting circuits | 3% |
| General branch circuits | 3% |
| Motors and HVAC units | 3 to 5% |
| Feeder circuits | 5% |
| Combined feeder and branch | 5% total |
| Long outdoor runs | 5 to 8% |
NEC 210.19 and 215.2 reference a 3% branch and 5% total figure. Always follow your local code, since the exact requirement varies.
Common Voltage Drop Mistakes
- Using one-way length when the circuit needs round-trip length (single-phase)
- Mixing up copper and aluminum resistance values
- Ignoring motor startup current, which can be several times the running load
- Forgetting that temperature raises conductor resistance
- Running high loads through thin extension cords
Why Voltage Drop Matters
Keeping drop in check protects your equipment, cuts wasted energy, and keeps you code-compliant. Undersized wire on a long run overheats, wastes power as heat, and can leave motors and lights starved of voltage. Sizing the conductor correctly the first time is cheaper than pulling new cable later.
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Frequently Asked Questions
How do I calculate voltage drop?
Multiply current by the conductor’s total resistance. For single-phase, use Vdrop = 2 × L × I × R, where the 2 covers the round trip. For three-phase, use Vdrop = √3 × L × I × R with one-way length. The calculator above does both automatically.
What is the three-phase voltage drop formula?
Vdrop = √3 × L × I × R, where √3 is about 1.732, L is one-way cable length, I is current, and R is resistance per unit length. Three-phase uses one-way length, unlike single-phase which uses the round trip.
How much voltage drop is acceptable?
Most codes recommend keeping voltage drop under 3% for branch circuits and lighting, and under 5% for feeders. The combined feeder plus branch drop should stay at or below 5%. Long outdoor runs sometimes allow up to 8%.
How do I calculate voltage drop across a resistor?
Use Ohm’s Law: V = I × R. Multiply the current through the resistor by its resistance in ohms. A 2A current through a 10 Ω resistor drops 20 volts.
Does aluminum wire drop more voltage than copper?
Yes. Aluminum has about 1.6 times the resistance of copper for the same wire size, so it drops more voltage over the same length. That is why aluminum runs often need a larger gauge to hit the same drop target.