Wire Size Calculator
Finds the smallest table AWG that meets simplified ampacity and your voltage-drop limit.
Limitation: Educational estimate only.
How to Use the Wire Size Calculator
Enter the fields below, then review the educational estimate.
Enter load amps and source voltage.
Small-gauge limits
In US practice, 14/12/10 AWG are often further limited to 15/20/30 A by terminal rules—even if a table lists higher ampacity.
What This Calculator Estimates
Walks AWG 14 through 4/0, requiring both simplified 75 °C-style ampacity and your drop limit.
Worked Examples
15 A, 120 V, 40 ft, 3% copper
Inputs
Result
First size that clears ampacity and ≤3% drop.
Safety first
De-energize circuits before measuring. When in doubt, stop and consult a licensed electrician.
Frequently Asked Questions
Still have questions about this calculation?
Try the CalculatorFormulas & Assumptions
Transparent educational arithmetic. Not NEC design software or a wiring permit.
Formula
Scientific Background
Ampacity and Ω/kft tables are simplified educational summaries inspired by common handbook columns—not official NEC tables. Ambient temperature, bundling, insulation type, and termination temperature limits are omitted.
Sources & review
Wire Size Calculator documents the references behind its formulas and assumptions. Always treat results as educational estimates, not personalized professional advice.
Last reviewed
July 22, 2026
Reference
physics.nist.gov
https://physics.nist.gov/cuu/Constants/index.htmlReference
nist.gov
https://www.nist.gov/pml/owm/wire-gauge-and-current-carrying-capacityFree Calculator Hub methodology
Free Calculator Hub
/methodology
See our methodology and corrections policy.