Wire size for a long run

Two different questions

Sizing a conductor answers two questions that people run together. Will this wire survive the current? is ampacity: heat, insulation rating, how the cable is installed, how many others share the raceway. Will the load still see enough volts? is voltage drop, and it depends on one thing ampacity does not care about at all - how far away the load is.

On a 15 ft kitchen circuit the first question wins and nobody thinks about the second. Run 100 ft to a detached garage and it flips: the wire that comfortably carries 16 A leaves the far end short of volts, the motor starts hot, the LED driver hums, and nothing about it looks like a wiring fault.

What 3% means in volts

The NEC does not make voltage drop a rule for most branch circuits. Informational Notes to 210.19(A) and 215.2(A)(1) recommend no more than 3% on the branch circuit and 5% for feeder and branch combined; your AHJ, or a specific article such as 647.4(D), can make a tighter figure mandatory. It is guidance with teeth, because the equipment at the end has its own tolerance.

Turn the percentage into volts before you argue about it. On a 120 V circuit, 3% is 3.6 V. That is the entire budget for the trip out and back - 200 ft of copper for a 100 ft run.

The 100 ft case, worked

A 16 A load at the end of a 100 ft, 120 V single-phase copper run needs 10 AWG to stay inside 3%. At 10 AWG the drop is 3.2 V, or 2.7%. Twelve gauge carries 16 A all day and still fails here: the drop lands above 4%, which is fine for a receptacle you plug a drill into and poor for anything with a motor or a power supply.

Two things make that number move fast. Doubling the length doubles the drop. Halving the current halves it. And 240 V instead of 120 V is the biggest lever of all - the same watts at twice the voltage means half the current and twice the allowance in volts, so the drop percentage falls by a factor of four. If a long run to an outbuilding is on the table, ask whether it can be 240 V before you price heavier copper.

What the calculator does not check

It sizes for voltage drop and stops there. It does not know your installation method, your ambient temperature, how many conductors share the raceway, or the terminal temperature rating of the device at each end - all of which decide ampacity, and none of which are the same on two jobs. Nor does upsizing for drop entitle you to a bigger breaker: 240.4(D) still limits 14 AWG to 15 A and 12 AWG to 20 A, because the breaker protects the smallest conductor in the circuit.

Size for the drop, check the ampacity against the table that applies to how you are actually installing it, and take the larger of the two.