Formula
The ampacity size is the smallest conductor whose Table 310.16 value, times the ambient factor times the bundling factor, is at least the load. The overcurrent size is that same search with NEC 240.4(D) applied: 14 AWG copper stops at 15 A, 12 AWG copper at 20 A, 10 AWG copper at 30 A, 12 AWG aluminum at 15 A, 10 AWG aluminum at 25 A. Fourteen gauge aluminum is not offered. The voltage-drop size is the smallest conductor at or under 3% using the same K method as the voltage drop page: 2 × K × I × L / cmil, or √3 for three phase. K is 12.9 for copper and 21.2 for aluminum.
adjusted ampacity = table × ambient factor × bundling factor
VD = 2 · K · I · L / cmil
K = 12.9 (Cu) or 21.2 (Al)
Conditions, unless you change them: 75°C column, not more than three current-carrying conductors, 30°C ambient. Both factors are then 1. The ambient bands are the 75°C column of Table 310.15(B)(1), and the bundling bands are Table 310.15(C)(1). A continuous load is often sized at 125% for the breaker. This page does not apply that 125% for you. Terminals rated 60°C can force the 60°C column, which is not what is tabulated here. Confirm the installation with an electrician and the code adopted where the work is.
Worked examples
30 A copper, 240 V, 10 ft, 30°C, 3 conductors. Ampacity size and overcurrent size are both 10 AWG. The factor is 1. The run is short, so voltage drop does not govern.
20 A copper, 120 V, 100 ft, single phase. Ampacity size 14 AWG. Overcurrent size 12 AWG, because 240.4(D) limits 14 AWG to 15 A. The 3% note wants 8 AWG. Drop on the 12 AWG is 7.90 V.
The same 20 A run at 40°C. The 75°C ambient factor at 40°C is 0.88, so 20 A of table ampacity is no longer enough and the size steps up. The page shows the factor it used.
Reference
NEC 2023 Table 310.16, 75°C column, for the sizes through 4/0. Larger kcmil sizes are in the selector’s logic and not all printed here.
| Size | Copper 75°C | Aluminum 75°C |
|---|---|---|
| 14 AWG | 20 | — |
| 12 AWG | 25 | 20 |
| 10 AWG | 35 | 30 |
| 8 AWG | 50 | 40 |
| 6 AWG | 65 | 50 |
| 4 AWG | 85 | 65 |
| 3 AWG | 100 | 75 |
| 2 AWG | 115 | 90 |
| 1 AWG | 130 | 100 |
| 1/0 AWG | 150 | 120 |
| 2/0 AWG | 175 | 135 |
| 3/0 AWG | 200 | 155 |
| 4/0 AWG | 230 | 180 |
Questions
What wire carries 30 amps of copper at 75°C with no derating?
10 AWG. Table 310.16 gives 10 AWG copper 35 A at 75°C, and 240.4(D) allows a 30 A breaker on 10 AWG copper. 12 AWG is 25 A in the table and is capped at 20 A by 240.4(D).
Why is a 20 amp, 120 volt, 100 foot run not 14 AWG?
The 75°C column rates 14 AWG copper at 20 A, so the ampacity size is 14. NEC 240.4(D) limits the overcurrent device on 14 AWG copper to 15 A, so the breaker size is 12 AWG. At 100 feet the 3% voltage-drop note wants 8 AWG. The drop on that 12 AWG is 2 × 12.9 × 20 × 100 / 6,530 = 7.90 V, 6.58%. The 3% figure is advisory.
Do I have to use this size?
No. This is NEC 2023 Table 310.16, 75°C column, with no derating unless you type an ambient temperature or a conductor count. It is not a design. Confirm the adopted code, the terminal rating, and the installation with an electrician.
Sources
- NEC 2023 Table 310.16, 75°C column, copper and aluminum. Not more than three current-carrying conductors, 30°C ambient, unless you enter otherwise. The table was not re-read from the NFPA PDF for this page; confirm the adopted edition.
- Ambient: 75°C column of Table 310.15(B)(1). Bundling: Table 310.15(C)(1). Small-conductor caps: 240.4(D).
- Voltage drop reuses 2 K I L / cmil and √3 K I L / cmil, K = 12.9 copper and 21.2 aluminum, circular mils from Chapter 9 Table 8. The 3% figure is the informational note in 210.19(A), not a mandatory maximum.