Voltage Drop and Wire Gauge Calculator

Find the voltage lost in a cable run, and the smallest AWG size that keeps the loss under your limit.

Circuit

Supply voltage (V)

Load current (A)

System

One-way length

Length unit

Conductor

Conductor temperature (°C)

Length is the distance from source to load, one way. The calculator counts the return conductor for DC and single-phase circuits. Use 75 °C for a conservative estimate of a loaded conductor, or 20 °C for the cold value.

Voltage drop for a chosen wire size

Wire size

Smallest wire size for a drop limit

Maximum drop (%)

Voltage drop only. This does not check ampacity, insulation rating, conduit fill, short-circuit withstand or code requirements. Size conductors for current capacity separately and follow the applicable electrical code.

How it works

Every conductor has resistance, so current flowing through it loses some voltage along the way. Resistance depends on the metal, the cross-section (AWG size) and the temperature. For a one-way run of length L, the current travels out and back in a DC or single-phase circuit, so the loop length is 2L. In a balanced three-phase circuit the drop between lines is √3 times the drop in one conductor.

DC / single-phase: V_drop = 2 × L × R × I / 1000
Three-phase: V_drop = √3 × L × R × I / 1000
R = ohms per 1000 ft at the conductor temperature

Resistance per 1000 ft is calculated from the AWG cross-section using 10.371 Ω·cmil/ft for copper (17.0 for aluminum) at 20 °C, then corrected for temperature with a coefficient of 0.00393 per °C for copper (0.00403 for aluminum). The 20 °C values match standard DC resistance tables for solid copper wire. This is a resistance-only calculation; it ignores reactance, which matters for large conductors on AC circuits and long runs.

Worked example

A 24 V DC load draws 10 A over a 100 ft run of 12 AWG copper at 75 °C.

12 AWG copper at 75 °C is about 1.93 Ω per 1000 ft.
Drop = 2 × 100 × 1.93 × 10 / 1000 = 3.86 V.
3.86 / 24 = 16.1%. Only about 20.1 V reaches the load, which is far too low for a 24 V device.

Asking for a 3% limit instead gives 4 AWG (about 2.5% drop). Low-voltage DC circuits are the ones most often caught out by voltage drop.

Common questions

What drop is acceptable?

A frequently quoted guideline is 3% on a branch circuit and 5% total from the source to the load. These figures are recommendations rather than strict rules in many codes, and some equipment, especially 24 V DC electronics, tolerates much less. Check your device's allowed supply range.

Why does temperature matter?

Copper resistance rises about 0.4% per degree. A conductor at 75 °C has roughly 21% more resistance than the same conductor at 20 °C.

Calculations are provided for convenience and education. Verify results against the applicable code, standards and manufacturer data before relying on them for design or installation.