Enter the load current, one-way distance, system voltage and your maximum voltage-drop target to get the smallest AWG that satisfies both the voltage-drop limit and the NEC 310.16 ampacity for the load. On long runs the voltage-drop limit, not ampacity, usually drives the size up.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Conductor and overcurrent sizing must comply with the NEC edition adopted by your jurisdiction, including all temperature, bundling and termination-rating derating. Values reference NEC 310.16 / Chapter 9 (ed. 2020/2023) — confirm the current code with a licensed electrician.
Calculator
Required area
14,333 circular mils
Minimum wire size
8 AWG
Ampacity at 75°C
50 A
Formula
The voltage-drop limit needs at least CM = (2 × K × I × L) / (Vsource × %max), where %max is the decimal fraction (3% = 0.03). The tool then picks the smallest AWG whose circular mils meet that requirement AND whose NEC 310.16 ampacity (75°C column) is at least the load current. The larger of the two requirements wins.
Worked example
For 20 A over 100 ft on a 120 V copper circuit at a 3% limit: CM = (2 × 12.9 × 20 × 100) / (120 × 0.03) = 51,600 / 3.6 = 14,333 circular mils. Ampacity alone would allow 12 AWG, but 14,333 cmil forces 8 AWG (16,510 cmil, 50 A) to hold voltage drop under 3%. The voltage-drop target, not ampacity, set the size.
On a short run, 6 AWG copper (65 A at 75°C) covers 50 A; many use 8 AWG copper only where the 50 A is non-continuous and the run is short. Always re-check voltage drop on long runs and confirm the termination temperature rating, because that can lower the usable ampacity.
What size wire for a 100 ft run?
It depends on the current and voltage, because length drives voltage drop. Enter your amps and 120 or 240 V with a 3% target: at 120 V and 20 A, a 100 ft run needs 8 AWG copper for voltage drop even though ampacity would allow 12 AWG.
Does voltage drop or ampacity decide wire size?
Whichever requires the larger conductor. On short runs ampacity usually wins; on long runs voltage drop dominates and pushes the gauge up. This tool checks both and returns the larger required size.
Should I size for 60, 75 or 90°C?
This tool uses the 75°C column, which matches the rating of most modern terminations. Conductors rated 90°C can be derated from the 90°C column, but the final ampacity is still limited by the lowest-rated termination in the circuit (NEC 110.14(C)).
Why does aluminum need a larger gauge?
Aluminum has lower conductivity and lower ampacity per gauge, so for the same load and voltage-drop target it usually lands one to two AWG sizes larger than copper. Select aluminum above to size it directly.
Source: NEC Table 310.16 ampacity plus the voltage-drop formula (NEC Chapter 9, Table 8) · All sources