Wire Size Calculator (Instant AWG from Amps & Feet)

AWG screen at a 3% drop limit

Same screen as the calculator at 75°C, power factor 1.00, one conductor set, and a 3% limit. Single-phase uses the 2× factor. The two 480 V rows are three-phase. A longer run or a lower voltage steps the gauge up even when the ampacity column would still allow a smaller wire.

Planning look-up only. It does not apply ambient correction, bundling, or a full Chapter 9 impedance row. Enter the same amps, voltage, and length above if the run is not in this table.

Wire size formula

This screen sizes AWG from amps and feet. For kW, power factor, and mm² on a three-phase feeder, use the 3-phase cable size calculator. More conductor tools are on the protection calculators hub.

Single-phase and DC: CM = (2 × K × I × L) ÷ Vd

Three-phase: CM = (√3 × K × I × L) ÷ Vd

K is 12.9 for copper and 21.2 for aluminum at 75°C. I is amps, L is one-way feet, and Vd is the allowed volts. At 3%, Vd is 0.03 times the source voltage. The screen then takes the smallest AWG that meets that circular-mil target and the selected ampacity column. Other temperatures scale K by (T + T0) / (75 + T0). T0 is 234.5 for copper and 228.1 for aluminum.

What each input does

Voltage. Use the source voltage the percent is based on: 120, 240, or 480 for common AC circuits, or the battery voltage for DC. The 120 / 240 / 480 buttons only fill the voltage field. A higher voltage allows a smaller wire for the same amps and feet, because the same volt drop is a smaller percent.

Current. Enter the load in amps, not kW. For one motor, check the 1.25 box so the screen uses 1.25 times that entry. For several motors, combine them first: 1.25 times the largest motor, plus the other motors, and leave the box off.

Length. One-way distance, not the round trip. Feet is the default. Meters are converted to feet before the K formula (1 m = 3.2808 ft). The 2× or √3 factor already accounts for the return path or the three-phase circuit.

System. DC and single-phase use the 2× factor. Three-phase uses √3, about 0.866 times the single-phase circular mils for the same drop. Single-phase branch circuits use three conductors counting the equipment ground. Three-phase circuits use four. This page sizes the circuit conductors, not the equipment-grounding conductor. That job stays on the ground wire size calculator.

Drop limit. 3% is the default for a branch or other critical run. Informational notes often cite about 3% on the branch and 5% combined feeder plus branch. On DC, critical loads stay near 3%. Noncritical DC loads are often planned closer to 10%. Those are planning targets, not a promise that every authority having jurisdiction enforces them.

Temperature column. 60°C, 75°C, and 90°C change both K and the ampacity used to reject a gauge. 14, 12, and 10 AWG still cannot be protected above 15 A, 20 A, and 30 A. Parallel sets in Advanced split the current across identical conductors and add their circular mils for the drop.

Three answers, one screen

Minimum wire size is the default. It walks AWG from small to large and stops at the first copper size and the first aluminum size that pass both the drop and the ampacity column.

Maximum one-way length keeps a gauge you already have and reports how far that copper and that aluminum can go before the drop limit. If the gauge cannot carry the current, the line says so instead of printing a distance.

Voltage drop for a known gauge reports volts and percent on that AWG. Use it to check a size, then open the voltage drop calculator if you need the longer NEC and IEC write-up. kW feeder sizing stays on the 3-phase cable size calculator.

Example 1 — 20 A at 120 V, 100 ft

Single-phase, 75°C, 3%. Allowed drop is 3.6 V. 10 AWG copper is about 4.1%, so the screen steps to 8 AWG copper (3.13 V, 2.6%). Aluminum is 6 AWG (3.23 V, 2.7%). On the maximum-length answer, 8 AWG copper reaches about 115 ft before it crosses 3%. Switch to the known-gauge answer on 8 AWG at 100 ft and the copper line reads 3.13 V, 2.6%.

Example 2 — motors and the 1.25 factor

Three pumps on 480 V three-phase draw 40 A, 14 A, and 3 A from the motor tables. The feeder current is (40 × 1.25) + 14 + 3 = 67 A. Enter 67 A, leave the single-motor box off, set three-phase, 100 ft, 75°C, and 3%. Ampacity governs because the drop is small at 480 V: 4 AWG copper (85 A column, about 0.75%) and 3 AWG aluminum (75 A column, about 0.97%). A single 40 A motor would be entered as 40 A with the box checked, which sizes at 50 A.

DC runs, engine spaces, and the fuse line

DC uses the same 2× formula as single-phase. Low voltage makes the percent large. 15 A, 12 V, 20 ft, 3% allows only 0.36 V, so copper lands on 6 AWG (about 2.5%) and aluminum on 4 AWG. At 75°C the 6 AWG copper column is 65 A, and the fuse line prints 60 A, the largest standard rating at or under that column. The fuse line is a ceiling for the conductor. It is not a list of ABYC fuse types or a product pick.

Critical DC loads stay at 3%. Noncritical runs can be opened toward 10% in the drop field. Engine space, on a DC system only, forces the ampacity check onto the 60°C column because the space is hotter. Example: 45 A, 12 V, 5 ft, 75°C is 8 AWG copper on drop and ampacity. The same run with engine space checked steps to 6 AWG copper, because 8 AWG at 60°C is only 40 A. Confirm the marine ampacity table for the insulation you actually buy. Method notes: voltage drop calculation guide.

Common circuit wire size

Branch and DC runs at 75°C and 3%, one set, power factor 1.00. Match the row, then still run the calculator if the route is longer than the length shown. The fuse ceiling applies only to the DC row.

After the gauge is chosen, size the breaker on the breaker size calculator and the equipment ground on the ground wire size calculator. A 120 V run that looks fine at 50 ft often needs the next gauge at 100 ft. Compare the first two rows of the table above.

Wire size calculator questions