Wire & voltage
Voltage Drop Calculator
Voltage drop equals current multiplied by total conductor-path resistance. With the default 120 V DC example, 20 A over 50 ft one way of 10 AWG copper drops about 2 V, or 1.7%, leaving about 118 V at the load. The method uses the standard AWG geometry relationship and the NIST-sourced material and unit references documented on the Sources page.
Calculate voltage drop in volts and percent, voltage at the load, conductor-path resistance, and I²R loss for DC, single-phase, or balanced three-phase circuits.
Calculated result
Calculated voltage drop
1.998 V · 1.665%
Within your editable planning target
- Delivered voltage
- 118.002 V
- Path resistance
- 0.09988 Ω
- Power loss
- 39.954 W
Planning result only; verify ampacity, protection, installation method, and local rules separately.
What this calculator returns
Read volts, percent drop, and delivered voltage together. Compare the percentage with your editable project target, then verify ampacity, protection, insulation, terminals, installation method, and local rules separately.
How to calculate voltage drop with the right inputs
Select the circuit type first, then enter source voltage, measured or documented load current, conductor material and AWG, and the one-way source-to-load distance. The calculator adds the applicable return-path factor.
- One-way distance
- Measure from source to load once. Do not double the distance before entering it.
- Circuit type
- DC and single-phase use a two-conductor resistive path; balanced three-phase uses the square-root-of-three relationship.
- Result check
- Review drop in volts, drop as a percentage of source voltage, voltage delivered to the load, and conductor power loss.
- Separate safety checks
- Voltage drop does not verify ampacity, overcurrent protection, insulation, termination ratings, or code compliance.
Formula and variables
The calculation runs entirely in your browser. Static formulas, definitions, examples, and tables remain readable without JavaScript.
Vdrop = I × Rpath; Rconductor = ρ × L / A; Vload = Vsource − Vdrop; loss = I² × Rpath. The path factor is 2 for DC and single-phase and √3 for balanced three-phase.
- Vdrop
- Voltage lost across the conductor path, in volts.
- I
- Current through the conductor, in amperes.
- R
- Electrical resistance of the modeled conductor path, in ohms.
- ρ
- Material resistivity at the modeled temperature, in ohm-metres.
- L
- Conductor length used by the selected path model.
- A
- Metal cross-sectional area of the conductor.
I is load current, ρ is conductor resistivity adjusted for temperature, L is the entered one-way distance, A is conductor area, and Rpath includes the circuit path factor.
Worked example
| Input | Value |
|---|---|
| System type | DC |
| Source voltage | 120 V |
| Load current | 20 A |
| Conductor material | Copper |
For a 120 V DC circuit drawing 20 A through 50 ft one-way of 10 AWG copper at 20°C, the round-trip resistive drop is about 2 V, or 1.7%, and the load sees about 118 V.
Reference table
| Circuit | Load and run | Conductor | Voltage drop | Voltage at load |
|---|---|---|---|---|
| 12 V DC | 10 A over 20 ft one way | 10 AWG copper | 0.40 V (3.33%) | 11.60 V |
| 120 V DC | 20 A over 50 ft one way | 10 AWG copper | 2.00 V (1.66%) | 118.00 V |
| 240 V single-phase | 30 A over 75 ft one way | 8 AWG copper | 2.83 V (1.18%) | 237.17 V |
| 480 V balanced three-phase | 50 A over 150 ft one way | 4 AWG copper | 3.23 V (0.67%) | 476.77 V |
Frequently asked questions
What voltage-drop percentage should I enter?
Use the target required by your project criteria. The default is an editable planning value, not a code-compliance threshold.
Why does a larger conductor reduce voltage drop?
A larger conductor has more cross-sectional area and therefore less resistance for the same material, length, and temperature.
How do I calculate voltage drop?
Multiply load current by total conductor-path resistance. Divide the resulting voltage drop by source voltage and multiply by 100 for percent drop; subtract the drop from source voltage to estimate voltage at the load.
Why is distance entered one way?
The calculator applies the return-path factor for the selected system, so enter only the measured source-to-load distance and do not double it.
Does the calculator show voltage at the load?
Yes. Delivered voltage equals source voltage minus the calculated resistive voltage drop.
Does voltage-drop calculation also verify wire ampacity?
No. Voltage drop and ampacity are separate checks. Verify current capacity, protection, conductor insulation, terminals, installation method, and governing rules independently.
Assumptions and limitations
- Resistive planning model; AC reactance, harmonics, inrush, and unbalance are not modeled.
- The default 3% is editable and is not a compliance determination.
- Conductor dimensions are mathematically derived from AWG.
Method and sources
Review the AWG reference, calculation methodology, and standards status for the references most relevant to this calculation. The broader technical sources index records source scope and provenance. Content release .