Wire Size Calculator: Copper Gauge for Voltage Drop

Why do lights dim at the end of a long extension cord? Thin wire has resistance, and that resistance eats voltage along the way. The Wire Size Calculator picks a copper wire gauge so the voltage drop stays under the limit you set.

It is meant for homeowners, solar and car audio hobbyists, and students. It checks voltage drop only, so a licensed electrician must still confirm the final wire choice.

Table of Contents

• What the Calculator Does
• How to Use It, Step by Step
• The Wire Size Formula
• Worked Example
• How to Read the Results
• Voltage Drop Versus Safe Current
• Limitations and Assumptions
• FAQs About the Wire Size Calculator

What the Calculator Does

The tool finds the smallest copper gauge in its list that meets your drop limit. Four inputs feed three results.

• Supply Voltage (Volts): 120 V, 240 V, 12 V DC, 24 V DC, or 480 V.
• Current Load (Amps): the current the wire carries.
• One-Way Distance (feet): the length from source to load, not the round trip.
• Max Allowed Voltage Drop (%): your limit, such as 3%.
• Recommended Copper Wire Gauge, Estimated Voltage Drop in volts and percent, and Minimum Required Size (Circular Mils).

The button is Determine Wire Gauge. The results also update as you type. The tool covers copper wire only, with no aluminum option. A yellow box on the page says it checks voltage drop only.

How to Use It, Step by Step

1. Choose a Supply Voltage. The starting choice is 120 V.
2. Enter Current Load (Amps). The starting value is 20.
3. Enter One-Way Distance (feet). The starting value is 75.
4. Enter Max Allowed Voltage Drop (%). The starting value is 3.0.
5. Press Determine Wire Gauge, or let it update, and read the Recommended Wire Gauge card.

The Wire Size Formula

The tool first finds the area of copper you need, in circular mils. It uses a fixed copper constant K of 12.9, which applies at 75 C. Then it picks the first gauge in its table with enough area.

• Allowed volts lost = supply voltage × (max drop % ÷ 100).
• Required circular mils = (2 × 12.9 × amps × one-way feet) ÷ allowed volts lost.
• The tool picks the first gauge whose circular mils meet or exceed that number.
• Estimated drop in volts = (2 × feet × amps × ohms per 1,000 feet) ÷ 1,000.
• Drop percent = estimated drop ÷ supply voltage × 100.
• For the 480 V option, the 2 in both drop formulas becomes 1.732.

The factor of 2 covers the trip out and the trip back. The 480 V option uses 1.732, the square root of 3, which is the balanced three-phase line-to-line factor. The gauge list is 14, 12, 10, 8, 6, 4, 2, 1/0, 2/0, and 4/0 AWG.

Worked Example

Carlos runs power to a workshop. The load is 20 amps at 120 volts, one way is 75 feet, and he allows a 3% drop.

A 240 volt circuit with 30 amps over 100 feet gives the same 10,750 circular mils. The tool again shows 8 AWG, with a 4.58 V drop, or 1.91%.

Now take a 480 volt three-phase run of 50 amps over 300 feet at 3%. Allowed loss is 14.4 V, and 1.732 × 12.9 × 50 × 300 ÷ 14.4 gives 23,274 CM. The tool shows 6 AWG, with a 12.76 V drop, or 2.66%.

How to Read the Results

Recommended Copper Wire Gauge is the smallest listed size that meets your drop limit. Lower AWG numbers mean thicker wire.

Estimated Voltage Drop uses the real resistance of the chosen gauge, so it sits below your limit. Minimum Required Size is the exact area before rounding up to a gauge. If no listed size works, the gauge card says Exceeds 4/0 AWG and the drop shows N/A.

Voltage Drop Versus Safe Current

Voltage drop and safe current are two separate checks. This tool runs only the first one. A wire can pass the drop test and still run too hot.

For example, 20 amps over 5 feet at 120 volts needs only 717 circular mils. The tool shows 14 AWG. Many electrical codes limit 14 AWG copper to 15 amp circuits, so ampacity rules still apply.

Bundled cables, hot spaces, insulation type, and conduit all change the safe current. Local electrical codes apply, and a licensed electrician should do or approve any wiring.

To see the drop on its own, try the voltage drop calculator. Use the Ohm's law calculator to link volts, amps, and ohms. The watt calculator turns watts into amps.

Sizing a whole power setup? The generator size calculator and the solar panel calculator cover the source. More tools sit in the math and science calculators hub.

Limitations and Assumptions

• Copper only, in the ten gauges listed. There are no 3 AWG, 1 AWG, or 3/0 sizes, and no aluminum.
• No ampacity, temperature, insulation, or conduit checks are made.
• The 480 V option assumes a balanced three-phase load.
• Amps must be above 0 and at most 5,000. Distance must be above 0 and at most 100,000 feet. The drop limit must be 1 to 10 percent.
• A blank, zero, negative, or out of range value shows a red message and hides the results.
• If the load needs more than 4/0 AWG, a red alert says no listed size works. Larger conductors or parallel runs may be needed, so ask an electrician.

FAQs About the Wire Size Calculator

This calculator gives general estimates only and is not professional advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.