Wire Size Calculator
Voltage drop ≈ 1.3% at 50 ft (target ≤ 3%)
Based on 75°C ampacity and the 3% voltage-drop guideline as of 2026. Conduit fill, temperature, and bundling can change the answer, a licensed electrician confirms final sizing.
What determines wire size
Two things size a conductor: ampacity (how much current the wire can carry without overheating) and voltage drop (how much power the run loses over distance). Short runs are sized by ampacity alone; long runs, detached garages, well pumps, EV charger circuits, often need a size or two larger to keep voltage drop under the 3% guideline.
Copper carries more per size than aluminum; aluminum wins on cost for large feeders. The calculator handles both, using standard 75°C ampacity ratings.
Why voltage drop matters
A circuit that loses 6% of its voltage runs appliances hot, dims lights under load, and wastes energy as heat in the wall. Motors and EV chargers are especially sensitive. The fix costs little at install time, one wire size, and is expensive to retrofit, which is why we calculate it on every long run.
A planning tool, not a permit
Real installs also weigh conduit fill, ambient temperature, bundling, and termination ratings, the details a licensed electrician carries in their head and the inspector checks. Use this to understand your project and sanity-check quotes; use a pro to build it.
Why electricians upsize beyond the minimum
The code minimum is a floor, not a recommendation. Professionals upsize for voltage drop on long runs (the 3% guideline this tool applies), for heat in bundled or conduit-packed routes, for aluminum's ampacity discount, and for the future: the feeder that's one size generous today makes the next decade's additions a wire pull instead of a re-trench.
Copper-versus-aluminum follows the same logic economically: aluminum feeders cost dramatically less at large sizes and perform beautifully with rated terminations, the reason nearly every modern service entrance uses them, while branch circuits stay copper for termination simplicity.
Reading your result against real installs
Sanity-check the output with the classics: 20A kitchen circuit at 30 feet → 12 AWG, textbook. 50A range at 20 feet → 6 AWG copper. 60A EV circuit at 80 feet → the ampacity table says 6, voltage drop pushes to 4, exactly the upsize this calculator exists to catch. 100A detached-garage feeder at 120 feet → 1 AWG copper or 2/0 aluminum, and suddenly the aluminum price advantage matters.
When your result surprises you, distance is almost always why. Long runs are where DIY wire purchases go wrong and where the calculator earns its keep.
The cost of getting wire size wrong
Undersizing announces itself badly: warm cable, voltage sag at the far end, and an inspection failure that means pulling new wire through the same route, paying the labor twice. Oversizing wastes less dramatically but real money: copper prices scale steeply with gauge, and a feeder one size beyond need can add hundreds on a long run.
The calculator's job is the goldilocks answer; the electrician's judgment covers what tables can't see, conduit fill, ambient heat, termination ratings. Together they produce the run that passes inspection the first time and never gets thought about again.
Copper theft-proofing your project budget
Wire is the volatile line in electrical quotes, copper commodity prices swing meaningfully year to year, and a 150-foot feeder carries real exposure. Two hedges work: aluminum for large feeders (stable pricing, universal in modern services), and locking quotes promptly once sizing is settled rather than shelf-sitting an estimate through a price cycle.
Either way, sizing precision protects the budget: knowing the run needs 2/0 aluminum rather than guessing at 4/0 copper is the difference this tool exists to make before anyone visits a supply house. And if you're comparing contractor quotes, the wire spec is a quiet quality signal: bids that name conductor size and material are bids that did the math.
Frequently asked questions
Typically 6 AWG copper at 75°C for short runs, upsized on long runs to control voltage drop. Enter your actual distance above; length changes the answer more than people expect.
Commonly 6 AWG copper for shorter runs, 4 AWG as distance grows. EV charging is a continuous load, so the circuit is sized with the 125% rule and voltage drop matters doubly.
Yes, modern aluminum (AA-8000 series) is standard for large feeders like sub-panels, at roughly one-to-two sizes larger than copper. Terminations must be aluminum-rated and properly prepared.
Ampacity minimums hold at any length, shortness only relaxes the voltage-drop upsizing. Never go below the ampacity table for the breaker; that floor is the fire-safety line.
Indirectly, conduit fill and bundling can trigger derating that effectively demands larger conductors, and THWN-rated wire in conduit differs from NM cable rules. It's the layer where a licensed electrician's judgment finalizes the calculator's starting point.
Typically 3 AWG copper or 1 AWG aluminum at short distances, with voltage drop pushing sizes up on long runs to detached buildings. Feeders also carry grounding-conductor requirements the calculator's output pairs with; it's a spec worth confirming with the installer before purchasing, feeder mistakes are the costliest wire errors precisely because the runs are long and the amperage is high.