Single-phase and three-phase, copper and aluminum, 14 AWG through 500 kcmil. Enter your run and load — the meter answers instantly, tells you whether the run passes the NEC 3% recommendation, and names the smallest wire that does.
Part of The Talking Toolbox. Instead of you feeding the meter, the meter feeds you: endless generated job-site problems, graded on the spot. Build a streak of five and you've earned the wire.
Press New Problem to start the drill.
Streak: 0 · Best: 0
Every foot of conductor has resistance, and the current has to travel out to the load and back. Single-phase voltage drop is VD = 2 × K × I × L ÷ CM; three-phase replaces the 2 with 1.732. K is the metal's resistance constant (about 12.9 for copper, 21.2 for aluminum), I is your load in amps, L is one-way length in feet, and CM is the wire's cross-section in circular mils. Bigger wire means more circular mils, which means less drop — that's the whole game.
The NEC informational notes recommend keeping branch-circuit drop at or under 3%, and feeder plus branch combined at or under 5%. Motors that start hard, well pumps at the end of long runs, and detached buildings are where this bites in real life.
The NEC informational notes recommend no more than 3% on a branch circuit and no more than 5% across the feeder and branch combined. Some jurisdictions and job specifications make those limits mandatory — confirm with your local authority.
Resistance accumulates with every foot, out and back. A size that is fine at 40 feet can fail at 150 feet, which is why long runs to garages, well pumps, and gates so often get upsized.
No. Voltage drop and ampacity are two separate checks — a conductor must pass both. This tool handles the drop side; ampacity depends on insulation type, temperature, terminations, and conditions of use in the NEC tables.
No. It is a free educational estimating tool. Verify every result against the current NEC and your local inspector, and have licensed professionals perform electrical work.
Job-Site Math: The Sovereign Tradesman's Field Companion by Dr. Gene A. Constant teaches this exact math — worked examples, practice problems, and a full answer key — for $3.99 on Kindle. And GENO, the GSU AI tutor, has memorized the entire book: he'll walk you through any chapter out loud, 24/7, in 83 languages.
Get Job-Site Math — $3.99 Ask GENO to Explain ItEducational tool provided free by Global Sovereign University, a program of The Foundation for Global Instruction (501(c)(3)). Results are estimates using standard K-factor approximations. Final conductor sizing must be verified against the current National Electrical Code, local amendments, and a licensed electrician or inspector. Electrical work can kill — when in doubt, hire a professional.