Free Wire Size Calculator – AWG Gauge by Amps and Voltage Drop

Choosing a Wire Gauge by Ampacity and Voltage Drop

Sizing a wire correctly means clearing two separate hurdles at once. First, the conductor must carry the load current safely without overheating — its ampacity. Second, it must deliver enough voltage to the load without losing too much along the way — its voltage drop. A wire can be fat enough to be safe yet still too thin for a long run, so this calculator tests both conditions and returns the smallest standard copper gauge that satisfies each. It works through sizes from 14 AWG up to 4/0 AWG using ampacity from NEC Table 310.16 (copper, 75°C column) and resistance from NEC Chapter 9, Table 8. You supply five things: the load current in amps, the one-way distance of the run, the system voltage, the maximum voltage drop you will accept as a percentage, and whether the circuit is single- or three-phase. The tool converts the distance to feet and, for each candidate gauge, computes the voltage drop with Vdrop = factor × I × R × distance. Here I is the current, R is the conductor's resistance per unit length, distance is the one-way length, and the factor is 2 for single-phase circuits (to count both the outgoing and returning conductors) or √3 for three-phase. It then reports the recommended gauge, the actual drop in volts, and that drop as a percentage of supply voltage. A concrete case makes it clear. Say you need to run 30 amps to a workshop sub-panel 50 feet away on a 120-volt single-phase circuit, holding voltage drop under 3 percent — that is 120 × 0.03 = 3.6 volts. Ampacity rules out 14 AWG (20 A) and 12 AWG (25 A) straight away. 10 AWG is rated 35 amps so it clears ampacity, but at 1.24 ohms per 1,000 feet its drop is 2 × 30 × 0.00124 × 50 = 3.72 volts, just over the 3.6-volt ceiling. So the calculator steps up to 8 AWG at 0.778 ohms per 1,000 feet, where the drop falls to 2.33 volts, or 1.95 percent. Run the same 30 amps on 240 volts instead and the allowance doubles to 7.2 volts, so 10 AWG passes comfortably at 1.55 percent — the same current, a different answer. This supports real decisions: feeding a garage, shed, or well pump; wiring solar or battery systems where long DC runs make drop critical; or confirming a planned cable will neither overheat nor starve the load. Keeping branch-circuit drop under 3 percent is a widely used rule of thumb, which the default reflects. Four situations show why the second check earns its keep. Someone feeding a detached garage 100 feet away with a 20-amp 120-volt circuit finds 12 AWG drops 7.92 volts, a hefty 6.6 percent, and needs to go thicker or raise the voltage. A homeowner adding a 15-amp lighting circuit 25 feet from the panel sees 14 AWG land at 2.35 volts, or 1.96 percent, and knows the cheapest cable is fine. An off-grid solar owner running 12-volt DC to a pump discovers that even a modest 30-foot run needs unusually thick cable, because 3 percent of 12 volts is only 0.36 volts. A well-pump installer sizing a long buried run uses the drop figure to decide between copper sizes before trenching. Because it relies on a specific standard reference, the tool refuses to guess beyond it. If your current exceeds the largest tabulated size, or no listed gauge can meet both limits on a very long run, it says the input is outside the supported range rather than returning a wrong value. This is a planning aid for copper conductors with 75°C terminations at 30°C ambient and no more than three current-carrying conductors, not a substitute for a full load calculation or your local code. Treat every result as an estimate. The single biggest mistake is skipping derating: the ampacity figures assume no more than three current-carrying conductors at 30°C ambient, so bundled cables, conduit packed with circuits, hot attics, and buried runs all reduce the safe current and push you to a thicker wire than the table alone suggests. Continuous loads normally get sized at 125 percent of the load current too. All mains wiring must follow your local electrical code and be installed or verified by a qualified licensed electrician — this tool plans the cable, it does not approve the installation. Everything runs locally in your browser.

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