Wire Gauge Calculator
Determine the minimum wire gauge needed to limit voltage drop in any electrical run. Ideal for electricians and solar installers sizing cables for long wiring runs.
About this calculator
Choosing the correct wire gauge ensures a circuit stays within a safe voltage drop, typically 3–5% for branch circuits. This calculator uses the formula: wire cross-sectional area ∝ (10.75 × current × distance) / voltage_drop, expressed in code as Math.round(10.75 × I × d / Vdrop), where 10.75 is a constant derived from the resistivity of copper and unit conversions for AWG calculations in feet and volts. Current (I) is measured in amperes, distance (d) is the one-way wire length in feet, and voltage_drop is the allowable drop expressed as a numeric percentage value (e.g., 3 for 3%). The result is rounded to the nearest standard AWG size. Heavier loads and longer runs demand lower AWG numbers (thicker wire). Using an undersized wire increases resistance, wastes energy as heat, and can create a fire hazard.
How to use
Suppose you are running a 20 A circuit over 50 ft of copper wire and want to keep voltage drop within 3%. Plug into the formula: Math.round(10.75 × 20 × 50 / 3) = Math.round(10750 / 3) = Math.round(3583.3) = 3584. This index value maps to an AWG gauge — the calculator resolves this to the appropriate wire size for your run. Enter your current load, wire length, and acceptable voltage drop percentage, and the calculator returns the minimum AWG gauge needed for a safe, efficient installation.
Frequently asked questions
What wire gauge do I need for a 20 amp circuit over a long run?
For a 20 A circuit, the minimum gauge under the NEC for a short run is 12 AWG, but longer runs require thicker wire to keep voltage drop under 3–5%. Use the formula to factor in your specific distance. For example, a 100 ft run at 20 A with a 3% drop on a 120 V circuit would require a heavier gauge than 12 AWG. Always consult local electrical codes and a licensed electrician for final installation decisions.
How does wire length affect voltage drop and gauge selection?
Voltage drop increases proportionally with wire length because longer conductors have more resistance. Doubling the run length doubles the resistance and therefore doubles the voltage drop for the same current. To compensate, you must increase the wire's cross-sectional area — meaning a lower AWG number. This is especially important in solar installations, outbuildings, and any long branch circuit where undersized wire would cause equipment to underperform or overheat.
What is the difference between AWG wire gauges and how does the numbering work?
AWG stands for American Wire Gauge, and its numbering system is counter-intuitive: lower numbers mean thicker wire. For example, 10 AWG wire is thicker and carries more current than 14 AWG. Each three-step decrease in AWG number approximately doubles the cross-sectional area of the wire. The gauge system originated in the number of drawing steps needed to produce the wire, with more steps producing a thinner wire and a higher number.