Wire size calculator: estimate AWG or kcmil size from current, run length
Estimate the conductor size needed to keep voltage drop within your chosen limit based on load current, one-way run distance, system voltage, conductor material, and allowable drop percentage. The result highlights the recommended AWG or kcmil size, the actual voltage drop, and the resistance implied by that selection.
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 or kcmil conductor size in its reference table whose area meets or exceeds that requirement.
Required CM = (2 x K x I x D) / V_drop
K is the conductor material constant used by the calculator's copper or aluminum model, I is current in amps, D is one-way distance in feet, and V_drop is the allowable voltage drop in volts.
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 targets and code considerations
Many designers use a low single-digit percentage target for branch-circuit voltage drop so equipment still receives close to its intended voltage under load. Higher-voltage systems tolerate longer runs at the same percentage target because the allowable drop in volts is proportionally larger.
This tool is best used as a first-pass voltage-drop planner. Final conductor selection still has to be checked against the electrical code and the installation details that affect allowable ampacity and insulation temperature limits.
What this calculator does not cover
This tool sizes wire for voltage drop only. It does not account for conduit fill, ambient-temperature derating, conductor bundling, insulation class, overcurrent protection, or jurisdiction-specific code amendments.
The internal lookup covers common AWG and kcmil sizes only. If your scenario falls outside that range, the result should be treated as a prompt to move to a fuller code-based conductor-sizing workflow rather than as a final answer.
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.
Why does the calculator ask for allowable voltage drop percentage?
Voltage drop percentage is the design target you are willing to accept between the source and the load. A lower percentage usually means a larger conductor and lower energy loss, while a higher percentage allows a smaller conductor but gives up more voltage along the run.
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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