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Electrical

Wire Size Calculator

The smallest conductor that keeps voltage drop in range.

Your measurementsEvery assumption editable01

Distance to the load. The formula accounts for the return path.

3% on a branch circuit is the widely used design target.

Conductor sizeCalculated live02

Result

8AWG

8 AWG holds the drop to 1.95%

Load
20 A at 240 V
Run length
150 ft one way
Allowed drop3% of 240 V
7.2 V
Circular mils required
10750
Smallest conductor that clears16510 circular mils
8 AWG
Actual drop at that size1.95%
4.69 V

This sizes for voltage drop only, which is a performance limit. Ampacity — whether a conductor can carry the current without overheating — is a separate safety question that depends on insulation rating, ambient temperature and how many conductors share a raceway. Both must be satisfied, and whichever demands more governs.

On short runs ampacity almost always governs and this will suggest something too small to be legal. On long runs voltage drop governs first, which is exactly when this calculation earns its keep.

Nothing here tells you what a code requires. Final conductor sizing and overcurrent protection belong with a licensed electrician working to the edition in force where you are.

Works backwards from an allowable drop to the conductor that achieves it, walking the standard gauges and taking the first that clears. This sizes for voltage drop — a performance limit — not ampacity, which is a separate safety question.

Why use this tool?What it does differently03

Why use this tool?

Solves backwards

From your allowable drop to the gauge, rather than checking one guess at a time.

Shows the actual drop

At the size it recommends, so you can see the headroom you have.

Copper or aluminum

Aluminum needs meaningfully more area for the same run.

Ampacity flagged, not assumed

This is drop only. Both limits must be met and the stricter governs.

How this worksThe method04

How this wire size calculator works

The required conductor area in circular mils is the voltage-drop formula rearranged: multiplier times resistivity times length times current, divided by the volts you are willing to lose.

Standard gauges are then walked from smallest upward and the first meeting that area is taken. Because AWG is a geometric progression, each gauge is computed from its definition rather than looked up.

On short runs ampacity governs and this will suggest something too small to be legal. On long runs voltage drop governs first — which is exactly when the calculation is worth doing.

How to use itStep by step05

How to use it

  1. Step 1: Enter the load and voltage

    Current at the far end, and the system voltage.

  2. Step 2: Measure the run one way

    The formula accounts for the return path.

  3. Step 3: Set your drop target

    3% on a branch circuit is the widely used figure.

  4. Step 4: Check ampacity separately

    This result is a floor for drop, not a code-compliant size.

Example usageWorked figures06

Example usage

A long 240 V circuit
20 A over 150 ft with a 3% target needs 10,750 circular mils. 8 AWG at 16,510 clears it comfortably, giving an actual drop of 4.69 V — 1.95%.
Why the next size down fails
10 AWG has 10,384 circular mils — just under the 10,750 required. It would drop 3.1%, which is over target.
The same run in aluminum
Resistivity rises from 12.9 to 21.2, so the requirement jumps to 17,667 circular mils and the answer moves to 6 AWG.
Frequently asked questionsCommon questions07

Frequently asked questions

What size wire do I need?

It depends on load, distance and your drop target. A 20 A load over 150 ft at 240 V needs 8 AWG copper to stay inside 3% — but ampacity rules must also be satisfied.

Does this give a code-compliant size?

No. It sizes for voltage drop only. Ampacity depends on insulation, ambient temperature and conductor bundling, and whichever limit is stricter governs.

When does voltage drop govern over ampacity?

On long runs. Short circuits are almost always limited by ampacity; past roughly a hundred feet, drop usually becomes the binding constraint.

Why does aluminum need a bigger conductor?

Higher resistivity — about 21.2 against copper’s 12.9. For the same run and current it needs roughly 64% more area.

Should I use 3% or 5%?

Three per cent on a branch circuit and five per cent overall including the feeder is the widely used design target. It is a recommendation about performance, not a hard limit.

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