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Voltage Drop Calculator

Calculate voltage drop for a wire run by AWG, length, and load. DC, single-phase, and three-phase, copper or aluminum, with a recommended wire size.

System type

Single-phase AC (hot and neutral). The current returns through the neutral, so the total resistance is 2 times the one-way length.

Inputs

Source voltage (V)

Nominal supply voltage at the start of the run.

Load current (A)

Steady-state current the load draws. For motors, use full-load amps; for EV chargers, the continuous rating.

One-way length

Distance from the source to the load. The return conductor (or sqrt(3) for three-phase) is added automatically.

Conductor material

The default for nearly all branch-circuit and low-voltage wiring. Lower resistance per AWG size and higher ampacity than aluminum.

Wire size

14 AWG is about 2.08 mm² in cross section. Resistance at 75 deg C: 3.07 Ω/1000 ft (copper)

Power factor

Cosine of the voltage-current angle, from 0 to 1. Use 1 for DC and resistive loads. Typical AC ranges: 0.95 for lighting and heating, 0.8 to 0.9 for motors.

Target maximum drop (%)

The threshold used by the recommended wire size and the comparison table. NEC informational guidance is 3% for branch circuits and 5% combined for feeders plus branches.

Quick presets

Drop by wire size

Same current, length, and system; only the conductor size changes. Rows highlighted in green meet the target. Click any row to load that size into the calculator.

Size	Cross section	Drop (V)	Drop (%)	End V
14 AWGcurrent	2.08 mm²	6.908 V	5.76%	113.1 V
12 AWG	3.31 mm²	4.342 V	3.62%	115.7 V
10 AWG	5.26 mm²	2.723 V	2.27%	117.3 V
8 AWG	8.37 mm²	1.719 V	1.43%	118.3 V
6 AWG	13.3 mm²	1.105 V	0.92%	118.9 V
4 AWG	21.2 mm²	0.693 V	0.58%	119.3 V
2 AWG	33.6 mm²	0.4365 V	0.36%	119.6 V
1/0 AWG	53.5 mm²	0.2745 V	0.23%	119.7 V
2/0 AWG	67.4 mm²	0.2176 V	0.18%	119.8 V
4/0 AWG	107 mm²	0.1368 V	0.11%	119.9 V
250 kcmil	127 mm²	0.1159 V	0.1%	119.9 V
500 kcmil	253 mm²	0.0581 V	0.05%	119.9 V

Formula reference

Standard voltage-drop relations using the R-only approximation, which matches NEC informational annex notes for typical branch and feeder circuits at 60 Hz.

DC and single-phase AC

Vdrop = 2 × L × R × I × PF / 1000

L is the one-way length in feet. The factor of 2 accounts for the round-trip path through hot and return.

Three-phase AC (line-to-line)

Vdrop = sqrt(3) × L × R × I × PF / 1000

Uses sqrt(3) instead of 2 because the three balanced conductors share the load and the drop is referenced line-to-line.

Drop percentage

drop% = (Vdrop / Vsource) × 100

NEC informational guidance: 3% for branch circuits and a combined 5% for feeders plus branches at the load.

Conductor power loss

Ploss = I² × R_total

Energy dissipated as heat in the run. Adds up fast at high current and long distances, even when the drop percentage looks small.

Practical voltage-drop notes

This calculator uses the R-only model (DC resistance at 75 deg C). For most branch, feeder, automotive, RV, marine, and solar-PV runs at 60 Hz, the reactance is small and the answer is close to within a few percent. For long conduit runs at high current or above 600 V, consult NEC Chapter 9 Table 9, which lists effective Z for typical raceway types.
Ampacity and voltage drop are separate constraints. The recommended size here only addresses voltage drop. Always cross-check against the ampacity table for the chosen insulation rating and the breaker size protecting the circuit.
Aluminum has about 60% the conductivity of copper. Expect to go one or two sizes larger to match the drop of an equivalent copper run, and follow the manufacturer guidance on antioxidant compound and torque values at terminations.
On AC circuits, the power factor lowers the drop when the load is partly inductive in the R-only model, but real conductor reactance can offset that. A power factor of 1 (resistive) is the most conservative input for most pre-build sizing decisions.
For 12 V, 24 V, and 48 V DC battery systems (RV, marine, off-grid solar), aim for a tighter drop target (1% to 3%) than NEC suggests for branch circuits, because the percentage of the system voltage lost is what matters to the equipment.
Output should be sanity-checked against local code. This tool does not replace an electrician, an engineering review, or the latest edition of the relevant electrical code for your jurisdiction.

How to use

Pick the system type at the top: DC, Single-phase AC, or Three-phase AC.
Click a voltage preset (12 V DC, 120 V AC, 240 V AC, 480 V three-phase, and more) or type the source voltage, then enter the load current in amps and the one-way run length in feet or meters.
Choose Copper or Aluminum, pick a wire size from 24 AWG through 1000 kcmil, and set the power factor for AC loads (1.0 for resistive, lower for motors).
Set the target maximum drop (3 percent by default for branch circuits, 5 percent combined for feeders plus branches).
Read the voltage drop, drop percentage, end voltage, conductor power loss, and the recommended minimum wire size. Click Apply on the recommendation, click Use on any row of the comparison table, or click a preset to load a real-world scenario.

About this tool

Voltage Drop Calculator computes how much voltage a wire run loses from the source to the load, so you can pick a wire size that keeps the equipment running properly and the conductor cool. Set the system type (DC, single-phase AC, or three-phase AC), enter the source voltage, load current, one-way length in feet or meters, conductor material (copper or aluminum), and the wire size from 24 AWG up through 1000 kcmil. The tool returns the voltage drop in volts and as a percentage of the source voltage, the end-of-run voltage available at the load, the total path resistance, and the power dissipated as heat in the conductor (I squared times R total). A recommended wire size panel walks the AWG ladder and reports the smallest size that keeps the drop under your target percentage (defaults to the 3 percent NEC informational guidance for branch circuits). A side-by-side comparison table shows the drop, percent, and end voltage for the most-used sizes (14 AWG through 500 kcmil) so you can see exactly which size is the right tradeoff. Built-in presets cover the scenarios people actually size for: 12 V solar-to-charge-controller runs, standard 120 V branch circuits, 240 V dryer feeds, 240 V EV charger circuits, 200 A residential service drops, 480 V three-phase motor feeders, and low-voltage LED strip runs. Resistance values are NEC Chapter 9 Table 8 figures for uncoated copper and aluminum at 75 degrees C and the formula uses the standard R-only approximation (Vdrop = 2 times L times R times I times PF for DC and single-phase, sqrt(3) times L times R times I times PF for three-phase) which matches how nearly every electrician and engineering reference does this calculation. Everything runs locally in your browser; the values you type stay on your device. Useful for electricians, makers, RV and marine wiring, solar installers, EV charger installers, low-voltage lighting designers, AV installers, and anyone planning a long wire run. This tool does not replace an electrician, an engineering review, or the latest edition of the relevant electrical code; always cross-check the wire size against the ampacity table for the chosen insulation rating.

Free to use. Works in your browser. No signup, no login.

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