What Wire Size Do I Need? NEC Ampacity + Voltage Drop, Demystified
If you've ever pulled out a wire chart, found the right ampacity, run the cable, and then watched something brown out at the far end, you've met the trap that bites every new electrician: wire size has two answers, not one.
A circuit needs wire that's big enough on both counts:
- Ampacity — the wire must carry the load without overheating. This is the number you find in the NEC table.
- Voltage drop — the wire must lose so little voltage along the way that the equipment at the far end still gets what it needs.
The bigger of the two answers wins. On short runs, ampacity almost always dominates and you can ignore voltage drop. On long runs, voltage drop sneaks up and you end up needing wire one or two sizes bigger than the ampacity table suggests.
Ampacity, in one paragraph
NEC Table 310.16 lists the maximum current a copper or aluminum conductor can carry continuously without exceeding its insulation's temperature rating. Common columns are 60°C (older TW insulation), 75°C (THW, RHW — the most common), and 90°C (THHN, XHHW). For a typical residential branch circuit you're reading the 75°C copper column. A #14 copper wire at 75°C carries 20 A. A #12 carries 25 A. A #10 carries 35 A. NEC 240.4(D) caps these for small conductors at 15 / 20 / 30 A respectively, regardless of what the temperature column says — so a #12 effectively maxes out at 20 A on a normal breaker.
Voltage drop, in one formula
Single-phase voltage drop in volts equals (2 × K × I × L) / cm, where K is 12.9 for copper or 21.2 for aluminum, I is the load current, L is the one-way length in feet, and cm is the wire's circular-mil area. For three-phase circuits, replace the leading 2 with the square root of 3.
The NEC informational notes recommend keeping voltage drop under 3% on a branch circuit and under 2% on a feeder, with the combined feeder + branch not exceeding 5%. These are guidelines, not code minimums — equipment manufacturers may set their own, especially for motors and electronics.
Why long runs are where this bites
A 20 A circuit at 240 V over 100 feet on #12 copper drops about 2 volts — under 1%, well within tolerance. The same circuit at 300 feet drops 6 volts — about 2.5%, right at the recommended limit. At 500 feet you're past 4%, and now your motor is starting at 230 V instead of 240, your LED driver's regulator is working harder, and your tool feels weak. Upsizing to #10 cuts the drop by about a third and brings the run back into spec. That's why "I just looked at the chart" isn't enough.
A worked example
Say you're running a 20 A 240 V single-phase circuit out to a detached garage 200 feet away. Copper, THHN insulation (90°C, but limited to the 75°C column at the breaker terminals — that's typical), 3% drop target.
By ampacity alone, 12 AWG copper at 25 A is fine for 20 A. But the voltage drop on 12 AWG over 200 ft works out to about 2 × 12.9 × 20 × 200 / 6530 = 15.8 V, which is 6.6% of 240 V. Way over the 3% target.
Step up to 10 AWG: voltage drop becomes about 2 × 12.9 × 20 × 200 / 10380 = 9.9 V, or 4.1%. Still over.
Step up to 8 AWG: voltage drop becomes about 6.3 V, or 2.6%. That's the right wire, even though ampacity-wise we could have used 12 AWG.
Skip the math
The Wire Size Calculator does this both ways at once. Enter your load amps, run length, voltage, phase, conductor material, and drop target — it returns the smallest size that satisfies both ampacity AND voltage drop, and tells you which of the two drove the recommendation. If even 600 kcmil isn't enough for your run, it says so explicitly so you know to split the load or rethink the routing.
The reference table inside the tool highlights the row it picked, so you can see at a glance how close you are to the next size up. Useful when a conduit is already crowded and you'd rather not upsize unnecessarily.
The short version
Pick the bigger of: ampacity-required size, voltage-drop-required size. Long runs make the second one matter. The chart on the wall covers ampacity. For voltage drop, run the formula or use a tool. Don't trust your gut on the 200-foot runs — they're exactly where intuition fails.