Wire Size Calculator

Find the correct AWG wire gauge for a circuit based on amperage, one-way distance, system voltage, conductor material, and allowable voltage drop.

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Wire size tool Determine the correct AWG wire gauge from current, distance, voltage, material, and acceptable voltage drop.
Enter values Provide amperage, distance, and voltage drop to find the recommended wire gauge.

Wiring

Wire size calculator: find the right AWG gauge for any circuit

Determine the correct American Wire Gauge (AWG) conductor size for a circuit based on load current, one-way wire run distance, system voltage, conductor material, and the maximum acceptable voltage drop percentage. The result includes the recommended gauge, actual voltage drop, wire resistance, and ampacity rating.

How the wire size formula works

The calculator uses the circular-mil method to find the minimum conductor cross-section that keeps voltage drop within the specified limit. It multiplies the resistivity constant of the conductor material by twice the one-way distance (to account for the full circuit length) and the load current, then divides by the allowable voltage drop in volts.

Once the minimum circular-mil area is known, the calculator looks up the smallest standard AWG gauge whose area meets or exceeds that requirement.

Required CM = (2 x K x I x D) / V_drop

K is the resistivity constant (12.9 for copper, 21.2 for aluminum), I is current in amps, D is one-way distance in feet, and V_drop is the allowable voltage drop in volts (system voltage times the drop percentage).

Actual V_drop = (2 x K x I x D) / CM_wire

Calculates the real voltage drop using the circular-mil area of the selected wire gauge.

Choosing between copper and aluminum

Copper has a lower resistivity (K = 12.9) than aluminum (K = 21.2), so it carries the same current with a smaller gauge and lower voltage drop. Aluminum is lighter and cheaper per foot, but requires a larger gauge for the same performance.

Most residential branch circuits use copper. Aluminum is common in larger feeder runs and service entrance cables where the weight and cost savings justify the larger conductor size.

Voltage drop limits and code considerations

The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for the combination of feeder and branch circuit. These are recommendations rather than hard limits, but most electricians and inspectors treat the 3% guideline as standard practice.

Higher voltage systems (240 V, 480 V) tolerate longer wire runs at the same drop percentage because the allowable drop in volts is proportionally larger.

What this calculator does not cover

This tool sizes wire for voltage drop only. It does not account for conduit fill, ambient temperature derating, bundling adjustments, or overcurrent protection sizing. The ampacity ratings shown are NEC 60 C column values for general reference and may differ from the rating required by your installation conditions.

Always verify the final wire size against local electrical codes and NEC tables 310.16 through 310.21 for the specific installation environment.

Frequently asked questions

What AWG wire size do I need for a 20 amp circuit?

It depends on distance. For a short run under 50 feet at 120 V with copper, 12 AWG is typically sufficient. For longer runs, voltage drop increases and you may need 10 AWG or larger. Enter your specific distance and voltage to get an accurate recommendation.

 What is the 3% voltage drop rule?

The NEC recommends that voltage drop on a branch circuit should not exceed 3% of the system voltage. For a 120 V circuit, that means no more than 3.6 V of drop. Keeping within this limit ensures appliances receive enough voltage to operate efficiently and safely.

 Can I use aluminum wire instead of copper?

Yes, but aluminum has higher resistivity so you will need a larger gauge to achieve the same voltage drop performance. Aluminum connections also require anti-oxidant compound and approved connectors rated for aluminum. Check local codes for any restrictions on aluminum wiring in your jurisdiction.

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