Two different things decide wire size, and most calculators only check one. Ampacity keeps the conductor from overheating. Voltage drop keeps the thing on the far end working properly. On a short run ampacity wins; past about fifty feet, voltage drop usually takes over and asks for a bigger wire than the breaker alone would suggest.
Working from watts instead? Divide by volts. A 4,500 W water heater at 240 V is 18.75 A. Enter the actual draw — the continuous-load multiplier is applied below, not by you.
NEC 210.19(A) and 210.20(A) require a continuous load to be sized at 125%. A 48 A EV charger is therefore a 60 A circuit, not a 50 A one — this is the rule people most often skip.
This matters more than people expect. NM-B has 90 °C conductors inside it, but NEC 334.80 says you must use the 60 °C ampacity column anyway.
Measure the actual path the cable takes — up the wall, across the joists, back down — not the straight line between the two points. The calculator doubles it for the return leg.
Count hots and neutrals that carry current. The ground does not count, and neither does the neutral of a straight 240 V circuit that has no neutral load.
Why the breaker does not tell you the wire size
The familiar pairs — 14 gauge on 15 amps, 12 on 20, 10 on 30 — come from NEC 240.4(D), the small-conductor rule. It caps the overcurrent device on 14, 12 and 10 AWG below what the ampacity table would otherwise allow. Ten gauge copper is good for 35 amps in the 75 °C column, but you will never see it on a 35 amp breaker in a house, because 240.4(D) stops at 30.
Above 10 AWG the cap goes away and the table takes over, which is where people start guessing. That is also where guessing gets expensive.
The temperature column is the part everyone misses
Every conductor appears three times in Table 310.16, once for each insulation rating, and the numbers differ a lot. Six gauge copper is 55, 65 or 75 amps depending on which column you are entitled to use.
Two separate rules decide that. The first is the cable itself: NM-B has 90 °C conductors inside the jacket, but 334.80 requires you to use the 60 °C ampacity. The second is what the wire lands on. Under 110.14(C), the breaker lug and the device terminal have their own temperature ratings, and the circuit is limited by the lowest-rated part in it. Modern breakers are generally listed for 75 °C, older equipment often is not.
The 90 °C column is still useful, but only as a starting point for derating. You correct down from it for heat and bundling, then hold the answer to the lower ceiling. Starting from 90 and keeping the 90 is the error that gets wire run undersized.
Voltage drop is a recommendation, and it still matters
The NEC mentions 3% for branch circuits in an informational note. A note is not an enforceable requirement, and an inspector will not fail a run over it.
Your equipment will notice anyway. A universal motor at low voltage draws more current to make the same power, runs hotter and lasts less time. Anything with electronics may brown out under load. On a detached garage or a well two hundred feet out, voltage drop routinely decides the wire size and ampacity never gets a look in — which is why this calculator checks both and shows you which one won.
Where this stops
This sizes a single branch circuit of ordinary conductors at up to 4/0. It does not do service and feeder calculations under Article 220, parallel conductors, conduit fill, motor circuits with their own overload rules, continuous-load multipliers, or anything to do with your local amendments — and amendments are common in electrical work.
Use it to understand the sizing, sanity-check a quote, or work out whether the wire already in your wall is adequate for what you want to plug into it. Permits and inspections exist for a reason, and nothing here replaces either one.