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Wire size calculator: ampacity and voltage drop
Smallest wire for the load and the run.
Open the Wire Size toolFree. Runs in your browser, no account, and keeps working offline.
What it does
Enter the voltage, single or three phase, the load in amps, the one-way length of the run, and copper or aluminum. The calculator finds the smallest wire that carries the load by ampacity and the smallest that keeps voltage drop under your limit (3% is a common target), and gives you the larger of the two.
It shows which requirement set the size, the drop on that wire, the voltage at the load and the longest run that wire can go within the limit — handy when the run is near the edge.
Ampacity comes from NEC 2023 §310.16 at the terminal temperature you choose, with the §240.4(D) limits for small wire. Conditions that derate a conductor are not applied: verify with your inspector (AHJ).
Worked example
A 20 A load on 120 V single phase, copper, 60 °C terminals, keeping voltage drop to 3% or less: at 50, 75 and 150 ft one way.
- 50 ft: wire size
- 10 AWG (drop 2.1%)
- 75 ft: wire size
- 8 AWG (drop 2.0%)
- 150 ft: wire size
- 6 AWG (drop 2.5%)
Ampacity from NEC 2023 §310.16 and §240.4(D); verify with your inspector (AHJ).
Questions
What size wire for a 20 amp circuit 100 feet away?
It depends on the voltage and your drop limit. On short runs 12 AWG copper is common for 20 A; on long runs voltage drop pushes it up. Enter your run — the worked example below shows the size growing with distance.
What is an acceptable voltage drop?
A common design target is 3% on a branch circuit. It is a performance guideline, not a fixed rule everywhere; set the limit you design to.
Is the length one way or round trip?
One way: the distance from the panel to the load. The calculation accounts for both conductors.