How the result is calculated
- 01
DC / single phase: VD = 2 x K x I x L / CM - 02
Balanced three phase: VD = square root of 3 x K x I x L / CM - 03
Voltage loss percent = VD / system voltage x 100
ELECTRICAL
Voltage Drop Calculator
Calculate voltage drop, percent loss and load voltage for AC, DC and three-phase circuits. Enter your measurements below and the estimate updates instantly.
For a balanced three-phase circuit, choose that circuit phase below or load the 480 V worked example.
Formula, validation and worked example reviewed November 12, 2026. See our calculation methodology.
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ELECTRICAL
US UNITSVoltage Drop
Enter line-to-line voltage and line current for balanced three phase. This fixed 75°C resistance model omits reactance and power-factor effects.
YOUR ESTIMATE
7.46 V drop3.11% voltage lossEstimated load voltage 232.5 V
Fixed resistance model at 75°C: K = 12.9 copper / 21.2 aluminum Ω·cmil/ft. No temperature correction.
One-way length; AC assumes unity power factor and neglects reactance. Three phase assumes balanced load, line current and line-to-line voltage.
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Formula and safety
Understand the voltage reaching the load.
This simplified resistance model uses K = 12.9 for copper and 21.2 for aluminum at 75°C, in ohm-circular-mils per foot. AC assumes unity power factor and ignores reactance; balanced three phase uses line current and line-to-line voltage. It is not a complete conductor design.
How do you calculate voltage drop?
Every metallic conductor has internal electrical resistance. As current flows through a run of copper or aluminum wire, part of the source voltage is lost as heat along the cable path. Using Ohm's law (V = I × R) and circular-mil resistivity at 75°C, the voltage lost across a two-wire circuit is:
VD=2 · K · I · LCM
(Single-phase / DC)% Drop=VDVsource · 100%
(Percentage loss)where:
- VD – Total voltage drop between the supply panel and the load terminals, measured in volts (need a target gauge instead? Use the wire size calculator);
- K – Conductor resistance constant at 75°C (K = 12.9 for copper and K = 21.2 for aluminum in Ω·CM/ft);
- I – Load current flowing through the circuit in amps (estimate from wattage with our watts to amps calculator);
- L – One-way wire distance in feet (the multiplier of 2 already accounts for the return conductor); and
- CM – Conductor cross-sectional area in circular mils from NEC Chapter 9, Table 8.
Three-phase AC voltage drop equation
In a balanced three-phase system, the return currents cancel in phase opposition, so the round-trip factor of 2 is replaced by √3:
VD3Φ=√3 · K · I · LCM
(Balanced 3-phase)Subtract VD from the source voltage (V_load = V_source − VD) to find the actual voltage delivered to the equipment terminals. Explore all related tools in our electrical calculators hub.
240 V, 30 A, 100 ft, 10 AWG copper
- Use 100 ft as the one-way length.
- 10 AWG has an area of 10,380 circular mils in this calculator.
- 2 x 12.9 x 30 x 100 / 10,380 = 7.46 V.
Quick reference chart
Copper vs. Aluminum Voltage Drop Reference (240V, 30A, 100 ft One-Way, 1Φ)
Calculated at 75°C (K = 12.9 Cu, 21.2 Al) alongside NEC Table 310.16 75°C copper ampacity reference.
| Wire Gauge (AWG) | Circular Mils (CM) | Copper Drop (240V/30A/100ft) | Aluminum Drop (240V/30A/100ft) |
|---|---|---|---|
| 12 AWG | 6,530 CM | 11.85 V (4.94%) | 19.48 V (8.12%) |
| 10 AWG | 10,380 CM | 7.46 V (3.11%) | 12.25 V (5.11%) |
| 8 AWG | 16,510 CM | 4.69 V (1.95%) | 7.70 V (3.21%) |
| 6 AWG | 26,240 CM | 2.95 V (1.23%) | 4.85 V (2.02%) |
| 4 AWG | 41,740 CM | 1.85 V (0.77%) | 3.05 V (1.27%) |
| 2 AWG | 66,360 CM | 1.17 V (0.49%) | 1.92 V (0.80%) |
Common mistakes to avoid
- Entering round-trip distance when the formula already includes the return path.
- Choosing single phase for a balanced three-phase circuit, or the reverse.
- Selecting copper while the installed conductor is aluminum.
- Treating low voltage drop as proof that conductor ampacity is adequate.
- Ignoring motor starting current, temperature, terminals, routing, or loads at multiple points.
Questions answered
Calculator FAQ
How do I calculate voltage drop for a wire run?+
Use current, one-way distance, conductor material and circular-mil area with the circuit's phase multiplier, then divide the result by system voltage to find percent loss.
Does this voltage drop calculator match Southwire calculations?+
Yes. It uses the standard Ohm's law circular-mil resistance formula with industry-standard 75°C constants (K = 12.9 for copper, K = 21.2 for aluminum), matching Southwire and Mike Holt engineering formulas for unity power factor.
Should I enter one-way or round-trip distance?+
Enter the one-way conductor length. The DC / single-phase formula already applies a factor of two.
Is voltage drop the same as ampacity?+
No. Voltage drop estimates delivered voltage; ampacity concerns safe current-carrying capacity. Both must be checked separately.
What voltage-drop percentage is acceptable?+
That depends on the equipment, design, installation, and applicable code. This tool does not declare a circuit code-compliant.
Why is the three-phase formula different?+
A balanced three-phase calculation uses a square-root-of-three multiplier instead of the single-phase factor of two.
How do I use this as a 3-phase voltage drop calculator?+
Select three phase, enter line-to-line voltage, line current and the one-way conductor length. The tool applies the square-root-of-three multiplier for a simplified balanced circuit.
Can this calculator choose my breaker or final wire size?+
No. It does not check overcurrent protection, conductor ampacity, derating, terminal limits, conduit fill, or local requirements.
What is the single-phase voltage-drop formula?+
For the simplified resistance method used here, single-phase voltage drop is 2 x K x current x one-way length divided by conductor circular-mil area.
Practical selection guide
3-phase and single-phase voltage drop calculator methods.
Choose the circuit type before relying on the result. The 3-phase voltage drop calculator method uses line-to-line voltage and the square-root-of-three multiplier; DC and single phase use the two-conductor path.
Single phase / DC
Enter one-way distance. The calculator applies the return-path factor of two and reports volts lost, percent loss and estimated load voltage.
3-phase voltage drop
Select three phase and use line-to-line voltage, line current and one-way distance. Unbalanced loads and reactive effects need a fuller design method.
Copper or aluminum
Select the installed conductor material. Its configured K value changes the resistance estimate and therefore the calculated voltage loss.
Southwire formula comparison
Matches standard industry calculations: VD = (2 × K × I × L) ÷ CM for single-phase and (√3 × K × I × L) ÷ CM for three-phase using 75°C conductor resistivity constants.
Maximum circuit length target
NEC guidelines (Informational Note 210.19(A)) recommend keeping branch circuit drop under 3% and total feeder + branch circuit drop under 5%.
Method references: Mike Holt voltage-drop formulas and 75°C constants · Southwire Voltage Drop Calculator