Ampacity
A cord's ampacity is how much current it carries continuously without its insulation running too hot. For flexible cord the number comes from one table in the National Electrical Code, and the sizer on this site reads that table, nothing else.
NEC Table 400.5(A)(1), copper conductors, cords rated 60 °C to 105 °C, in a 30 °C room. The resistance column is NEC Chapter 9 Table 8 and feeds the voltage-drop rule.
| Gauge | 2 current-carrying conductors (3-wire cord) | 3 current-carrying conductors (4- and 5-wire) | Ω per 1,000 ft (copper, 75 °C) |
|---|---|---|---|
| 18 AWG | 10 A | 7 A | 7.770 |
| 16 AWG | 13 A | 10 A | 4.890 |
| 14 AWG | 18 A | 15 A | 3.070 |
| 12 AWG | 25 A | 20 A | 1.930 |
| 10 AWG | 30 A | 25 A | 1.210 |
| 8 AWG | 40 A | 35 A | 0.764 |
| 6 AWG | 55 A | 45 A | 0.491 |
| 4 AWG | 70 A | 60 A | 0.308 |
Count the conductors that carry current, not the conductors in the cord. A three-conductor cord — hot, neutral, ground — has two current-carrying conductors; the grounding conductor never counts. A four- or five-conductor three-phase cord has three. The second column runs lower because three heated conductors in one jacket cool each other less.
It takes your amps and walks down the column until a gauge carries them, then checks the drop over your run. A 12 A tool on a 3-wire cord lands on 16 AWG; a 15 A saw lands on 14 AWG; 25 A on 12 AWG. Nameplate amps, not the breaker size.
A cord in free air at 30 °C. Coiled on a reel under load, bundled with others, or in a hot room it carries less. If that is your case, size one gauge heavier — a config change on our side if the company adopts a derating policy, never a guess in the sizer.
The reels
Type the amps and the run; the sizer names the gauge and shows the reels in that cell by the foot and by the spool.
