Voltage Drop Calculator
Calculate wire voltage drop, end voltage and power loss from current, length, gauge and material
Circuit inputs
Enter the one-way distance; the round trip is handled automatically.
Too high — exceeds the 5% combined recommendation. Use a larger conductor or shorter run.
Results use the resistivity method (resistance only; cable inductance is ignored). The 3% and 5% figures are NEC informational recommendations, not strict requirements — local codes, conductor temperature and installation conditions vary. Always verify with a qualified electrician.
About this tool
This voltage drop calculator works out how much voltage is lost along a wire run and how much voltage actually reaches the load. It uses the resistivity method: from the conductor material (copper or aluminum), its cross-sectional area, the run length and the current, it computes the conductor resistance and the resulting drop. You can enter the current directly in amps or give the load power in watts with a power factor, and choose a DC, single-phase or three-phase circuit. Conductor size can be picked as a standard AWG gauge or entered directly in mm². It is built for electricians, solar and battery installers, makers and students who need a quick, physically accurate answer.
How to use
- 1 Choose the circuit type (DC, single-phase or three-phase) and the conductor material (copper or aluminum).
- 2 Enter the source voltage, then give the load either as a current in amps or as power in watts with a power factor.
- 3 Enter the one-way run length in metres or feet, and select the conductor size as an AWG gauge or in mm².
- 4 Read the voltage drop, drop percentage, voltage at the load and power loss, and check the colored bar against the 3% / 5% guidance.
How it works
The conductor resistance is R = ρ·L / A, where ρ is the resistivity at 20 °C (copper 1.724×10⁻⁸ Ω·m, aluminum 2.82×10⁻⁸ Ω·m), L is the conductor length and A is the cross-sectional area. For DC and single-phase circuits the current flows down and back, so the total length is twice the one-way run: Vdrop = I·ρ·(2L)/A. For a balanced three-phase circuit the line-to-line drop uses the one-way length with a √3 factor: Vdrop = √3·I·ρ·L/A. AWG sizes are converted to area with the standard formula d = 0.127·92^((36−n)/39) mm and A = (π/4)·d², so AWG 12 is 3.31 mm² and 4/0 is 107.2 mm². The drop percentage is Vdrop divided by the source voltage, the voltage at the load is the source minus the drop, and the power lost in the wire is I²·R. Inductive reactance is ignored, which is the usual simplification for typical low- and medium-length runs.
Frequently asked questions
Do I enter one-way or round-trip wire length?
Enter the one-way distance from the source to the load. The calculator adds the return conductor automatically for DC and single-phase circuits (it uses twice the length), and applies the correct √3 factor with the one-way length for three-phase circuits.
What voltage drop percentage is acceptable?
A common guideline (NEC informational notes) recommends keeping the drop at or below 3% on a branch circuit and 5% combined for feeder plus branch. The bar turns green up to 3%, amber from 3% to 5%, and red above 5%. These are recommendations, not strict code requirements; local rules and conditions vary.
Should I use copper or aluminum, and which AWG?
Copper has lower resistivity, so for the same gauge it drops less voltage than aluminum; aluminum is cheaper and lighter but needs a larger size for the same drop. Pick a gauge so the drop stays in the green zone — increase the conductor cross-section (a lower AWG number) for longer runs or higher current.
Does this account for AC reactance and temperature?
No. It uses the resistive (DC-resistance) model with resistivity at 20 °C and ignores cable inductive reactance, which is a standard simplification for low- and medium-length runs. For very long AC runs, high frequencies or hot conductors, use manufacturer cable tables or a qualified electrician.
Related tools and uses
Pair this with the Ohm's law calculator for V, I, R and power, the resistor color code calculator for component values, and the series and parallel resistance calculator for circuit networks.