Wire Size Calculator

Use this free wire size calculator to estimate a common copper AWG size that stays within a chosen voltage-drop percentage.

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Research-backed assumptions Formula steps Examples included Private in-browser use

Source voltageSupply voltage before the wire run.Current ampsLoad current in amps.One-way length feetOne-way distance from the source to the load.Max voltage drop percentThe largest voltage-drop percentage you want the estimate to allow.Circuit typeChoose single/DC or three-phase so the voltage-drop factor matches the circuit type.

Estimated copper wire size12 AWG copper

15 A over 75 ft

Estimated drop: 3.573 V
Percent drop: 2.9775%
Estimated load voltage: 116.427 V
Resistance table value: 1.588 ohms / 1000 ft

This is not a code-complete wire sizing tool. Ampacity, insulation rating, terminals, raceway, temperature, material, and local code must be checked separately.

Formula steps

Try common copper AWG sizes from smaller to larger.
Estimate voltage drop for each size with the chosen circuit type.
Return the first size that stays within the maximum voltage-drop percentage.

How to use the wire size calculator

Enter source voltage, current, one-way length, maximum voltage drop percent, and circuit type.
Press Estimate wire size to find the first common copper AWG size that fits the voltage-drop target.
Read the estimated voltage drop and load voltage before trusting the recommendation.
Check ampacity, insulation, raceway, terminals, temperature, and local code separately.

Common uses

Estimate copper AWG size for a voltage-drop target.

Compare branch-circuit and longer-run examples.

See estimated voltage drop and load voltage together.

Learn why current and length affect conductor choice.

Examples

120 V branch 120 V, 15 A, 75 ft, max 3%

12 AWG copper estimate

240 V run 240 V, 30 A, 100 ft, max 3%

Estimated AWG size

Three-phase run 208 V, 20 A, 150 ft, max 3%

Estimated AWG size

Frequently asked questions

Plain-language answers about when to use the tool, what it does with your inputs, what to double-check, and how privacy works.

When should I use the Wire Size Calculator?

Use it when your task matches one of these common needs: Estimate copper AWG size for a voltage-drop target. Compare branch-circuit and longer-run examples. It works best when you already know the values, dates, units, or settings the page asks for.

What is the Wire Size Calculator doing with my inputs?

In plain language: The calculator tests common copper AWG sizes and returns the first size whose estimated voltage drop is within the selected percentage. The examples on the page are there so you can compare your inputs with a filled-out calculation before copying the answer.

What do the main Wire Size Calculator inputs mean?

Source voltage: voltage before the wire run loses voltage. Current amps: load current for the voltage-drop estimate. One-way length: distance from source to load. Max voltage drop: the target percentage the estimate tries to stay under.

How should I read the Wire Size Calculator answer?

Read the main answer first, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.

What should I double-check before trusting the answer?

This is not a code-complete wire sizing tool. Ampacity, insulation rating, terminals, raceway, temperature, material, and local code must be checked separately. Also check that you used the right unit, date, scale, or mode because small input changes can change the result.

Is this the same as an electrical code wire-size chart?

No. This estimates voltage drop only. Real wire sizing also needs ampacity, conductor insulation, raceway fill, terminals, temperature, material, and local code rules.

Why can a long run need a larger wire?

Longer wire has more resistance. More resistance causes more voltage drop, so increasing wire size can reduce the estimated voltage lost along the run.

Does the site save what I enter?

No. The calculator runs in your browser tab. Your recent answers stay only on the page while you use it, and they are not sent to a server.