Wire planning

Voltage Drop Calculator

V_drop = 2 × L × R × I / 1000

Voltage Drop Calculator

Determine the voltage drop for a wire run.

Live Result
Formula-backed — instant professional result
System Type
VOLTAGE DROP
0 V
Also equals 0 %
End Voltage 0 V
Formula used V_drop = 2 × L × R × I / 1000 The primary result is shown in V using the formula displayed above. Calculates voltage drop based on NEC Chapter 9 Table 8.

This calculator is an educational planning estimate. Verify safety-critical work with equipment nameplate data, local electrical code, and a qualified professional.

Calculate voltage drop for any wire run. Enter current, distance, and conductor size to check compliance with 3% and 5% limits.

Wire and voltage drop planning

Conductor run visual

Calculate voltage drop for any wire run. Enter current, distance, and conductor size to check compliance with 3% and 5% limits.

AC/DC single-phase formula-first page-specific visual

Voltage Drop Calculator — Wire Length & Gauge | Ampstowatt Guide

Conductor planning map

Current, distance, conductor material, and voltage drop

Wire and voltage-drop pages rank better when the content shows the physical run: source, conductor length, voltage loss, and load.

Formula Voltage drop = 2 x K x I x D / circular mils
Voltage Drop Calculator — Wire Length & Gauge | Ampstowatt technical circuit diagram A premium electrical diagram showing the main values and conversion flow for Voltage Drop Calculator — Wire Length & Gauge | Ampstowatt. Source Current One-way distance Copper / aluminum Load % voltage drop
Voltage Drop Calculator — Wire Length & Gauge | Ampstowatt Conductor planning map with real formula values and planning checkpoints.

Quick answer

Use this calculator to estimate voltage lost across a wire run before the load.

Voltage drop depends on current, one-way distance, conductor material, wire size, and system voltage. A small voltage loss can matter more in 12V and 24V systems than in 120V or 240V circuits.

Voltage drop = 2 x K x I x D / circular mils

Inputs to check before calculating

  • Use one-way conductor distance unless the calculator specifically asks for round-trip length.
  • Choose copper or aluminum to match the installed conductor.
  • Enter the expected load current, not only breaker rating.
  • Compare the percentage drop with the tolerance of the equipment.

Example: long wire run planning

Enter current, one-way distance, system voltage, conductor material, and wire size. The calculator estimates voltage lost in the run so you can compare it with a practical voltage-drop limit before choosing cable size.

Best use cases

  • Copper and aluminum
  • Distance-aware
  • Percent drop result
  • single-phase AC load planning with clear unit labels.

Common mistake to avoid

Do not ignore distance on low-voltage DC circuits; long runs can lose useful power even when the amp value looks small.

How to use the result

Use the result to compare conductor sizes, then verify ampacity, temperature, terminals, and code rules.

Accuracy and safety note

Calculator output is a planning estimate. Final breaker sizing, wire gauge, derating, voltage-drop limits, equipment protection, and code compliance should be verified with manufacturer documentation and a qualified electrician or engineer.

For the broad formula, reference table, and breaker planning context, start with the main Amps to Watts Calculator.

Technical Diagram

Voltage Drop Calculator Formula Map

Vdrop = 2 x A x R x L

R = conductor resistance, L = one-way length. Use the calculator on this page to change the inputs and verify the result before sizing equipment.

Wire, Safety & Power Quality Tools

Start with the main conversion tool

After calculating watts and amps, use these pages to check conductor sizing, voltage loss, power factor, and high-current equipment planning.Amps to Watts Calculator

Methodology, Review Notes, and Sources

How this calculator works

The calculator applies Watt's law for DC and resistive loads, then adds power factor for AC loads and the 1.732 multiplier for balanced three-phase line-to-line calculations.

  • DC and resistive loads: W = A x V
  • Single-phase AC: W = A x V x PF
  • Three-phase AC: W = 1.732 x A x V x PF

Editorial review

Last reviewed: June 26, 2026. Ampstowatt.com is built around the amps to watts conversion — our core specialty. Content is maintained by the Ampstowatt editorial team and checked for formula consistency, unit labels, calculator behavior, and safety wording.

The page is an educational planning reference, not a licensed electrical design or inspection service.

Reference sources

FAQ

Voltage Drop FAQ

Fast answers before you use the calculator or compare formulas.

% What is acceptable voltage drop?

The NEC recommends a maximum of 3% voltage drop for branch circuits and 5% total (feeder + branch combined) per informational note to NEC 210.19. For sensitive electronics and LED lighting, aim for 2% or less. Low-voltage DC systems (12V, 24V) are especially sensitive — even 0.5V drop is significant.

CALC How do I calculate voltage drop for a wire run?

Voltage drop = (2 × K × I × D) ÷ CM, where K is the resistivity constant (10.75 for copper, 17.7 for aluminum), I is current in amps, D is one-way distance in feet, and CM is the circular mil area of the conductor. The calculator automates this for standard AWG sizes.

AC Does voltage drop differ for DC and AC circuits?

The basic resistance-based voltage drop formula is the same. However, AC circuits can have additional impedance from inductance in conduit or cable geometry, especially at higher frequencies or in large conductors. For standard residential and commercial wiring, the DC formula is accurate enough for planning.

12V Why is voltage drop worse at low voltage?

A 1V drop on a 12V circuit is an 8.3% loss, but on a 120V circuit it is only 0.8%. The same wire, same current, same distance produces the same absolute voltage drop, but the percentage impact is 10× worse at 12V. This is why low-voltage DC installations require significantly larger wire.

DIST How does wire length affect voltage drop?

Voltage drop increases linearly with distance. Doubling the wire length doubles the drop. The formula uses one-way distance and multiplies by 2 to account for the return conductor. For DC circuits, this is literal. For AC, the return is through the neutral or other phase.

CU Copper vs aluminum — how does material affect voltage drop?

Aluminum has about 1.6× the resistivity of copper, so it drops more voltage for the same gauge. To match copper performance, use aluminum wire two AWG sizes larger (e.g., 2 AWG aluminum instead of 4 AWG copper). Aluminum is cheaper per foot but requires larger conduit and specific connectors rated for aluminum.