Building / DIY

Wire Gauge Calculator: AWG Wire Size by Amps & Distance

This wire gauge calculator finds the minimum AWG for copper or aluminum from amps, one-way distance, voltage, and an allowed voltage-drop percent.

Load (amps)

Current the load draws.

One-way distance (ft)

Distance from the source to the load, one way. The formula accounts for the return path.

System voltage

120 V is common on branch circuits. 12 V is common on low-voltage DC.

Allowed drop (%)

3% is a common target for branch circuits.

Supply

DCSingle-phase ACThree-phase AC

DC and single-phase AC use a factor of 2. Three-phase uses √3, about 1.732.

Conductor

CopperAluminum

K is 12.9 for copper and 21.2 for aluminum. Aluminum needs a larger size for the same drop.

Examples

The 100 ft example is single-phase copper. The 15 ft example is 12 V DC copper.

Minimum wire size

8 AWG

2.60% drop · 3.125 V · 116.875 V at the load

16,510 circular mils. 14,333.3 circular mils required for a 3% drop.

Within the 3% drop you entered. A larger AWG number is a thinner wire, so this is the smallest size in the table that meets the limit.

Allowed drop: 3% of 120 V = 3.6 V. Required CM = 2 × 12.9 × 20 × 100 / 3.6 = 14,333.3. Smallest size at or above that is 8 AWG (16,510 cmil). VD = 2 × 12.9 × 20 × 100 / 16,510 = 3.125 V (2.60%).

Voltage-drop sizing only. Ampacity, temperature ratings, and local code also apply. Consult a licensed electrician.

How to use

Enter the load in amps, the one-way distance in feet, and the system voltage.
Enter the allowed voltage-drop percent. 3% is a common branch-circuit target.
Choose copper or aluminum, and DC, single-phase AC, or three-phase AC.
Read the smallest AWG or kcmil size that stays within that drop, plus the drop that size produces.

Formula

Required circular mils = factor × K × I × L / VD. VD is system volts × allowed percent ÷ 100. The factor is 2 for DC and single-phase AC, and √3 (about 1.732) for three-phase. K is 12.9 for copper and 21.2 for aluminum. I is amps and L is one-way feet. The result is the smallest size in the table whose circular mils meet or exceed that number. Actual drop uses VD = factor × K × I × L / CM. The circular-mil table matches the voltage drop calculator.

Example. 20 A at 120 V, 100 ft one way, copper, single-phase, and a 3% allowed drop needs 14,333.3 circular mils. The smallest size is 8 AWG (16,510 cmil), which drops 3.125 V (2.60%).

Example. The same run in aluminum needs 6 AWG and drops 3.232 V (2.69%).

Example. 10 A at 12 V DC over 15 ft of copper, with 3% allowed, needs 8 AWG and drops 0.234 V (1.95%).

Voltage drop only

A bigger AWG number is a thinner wire. This page does not apply ampacity. In typical US residential copper wiring, 12 AWG is used on 20 A breakers and 14 AWG on 15 A, and a long run may need a larger size than that. Temperature ratings and local code still apply. Ask a licensed electrician before you buy wire.

Frequently asked questions

Does a bigger AWG number mean thicker wire?

No, the opposite. 10 AWG is thicker than 14 AWG.

What gauge for a 20 amp circuit?

In typical US residential copper wiring, 12 AWG is used on 20 A breakers and 14 AWG on 15 A. Long runs may need a larger size.

Why does distance matter?

Longer wire has more resistance, so voltage drop grows with length. Low-voltage DC systems are especially sensitive.

What voltage drop should I target?

3% is a common target for branch circuits; many 12 V DC builders aim for 3% or less on critical loads.

Is this code-compliant sizing?

It checks voltage drop only. Ampacity, temperature ratings, and local code also apply; consult a licensed electrician.