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Voltage drop

The other reason to go heavier

Ampacity keeps the cord from overheating. Voltage drop keeps the tool from starving: every foot of copper eats a little of the supply, and a motor fed 108 V instead of 120 V runs hot and slow. The fix is a heavier gauge, and the run length says how much heavier.

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The rule

The NEC (210.19(A), Informational Note No. 4) recommends keeping the drop on a branch circuit under 3 %. Our sizer holds that line and steps the gauge up until the run stays under it.

The arithmetic

Single phase (a 3-wire cord): drop = 2 × amps × R × feet ÷ 1,000. Three phase (4- and 5-wire): drop = 1.732 × amps × R × feet ÷ 1,000. R is the resistance of the gauge in ohms per 1,000 ft of copper at 75 °C:

Gauge2 current-carrying conductors (3-wire cord)3 current-carrying conductors (4- and 5-wire)Ω per 1,000 ft (copper, 75 °C)
18 AWG10 A7 A7.770
16 AWG13 A10 A4.890
14 AWG18 A15 A3.070
12 AWG25 A20 A1.930
10 AWG30 A25 A1.210
8 AWG40 A35 A0.764
6 AWG55 A45 A0.491
4 AWG70 A60 A0.308

Worked examples at 120 V

  • 12 A, 25 ft, 16 AWG: 2 × 12 × 4.89 × 25 ÷ 1000 = 2.9 V, about 2.4 %. Fine.
  • 15 A, 100 ft: 14 AWG carries the amps but loses 9.2 V (7.7 %); 12 AWG 5.8 V (4.8 %); 10 AWG 3.6 V (3.0 %, a hair over); 8 AWG 2.3 V (1.9 %). The sizer answers 8 AWG.
  • 20 A, 300 ft: even 4 AWG loses 3.7 V (3.1 %). Shorten the run, feed it at 240 V, or accept the drop knowingly.

Why higher voltage helps

The volts lost are the same; the percentage halves at 240 V and falls again at 480 V. That is why long three-phase runs are the easy case and long 120 V runs are the hard one.

The reels

Let the sizer do the arithmetic

Amps, feet, volts — it prints the drop on each gauge it considers and stops at the first one under the line.

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