Wire Size Calculator
Size a copper or aluminum conductor per NEC Table 310.16, with ambient temperature correction, conductor adjustment and terminal limits applied. Every step is shown with its code section, and the table opens right below with your row highlighted.
| SIZE | 60°C | 75°C | 90°C |
|---|---|---|---|
| 14 AWG | 15 | 20 | 25 |
| 12 AWG | 20 | 25 | 30 |
| 10 AWG | 30 | 35 | 40 |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 4 AWG | 70 | 85 | 95 |
| 3 AWG | 85 | 100 | 115 |
| 2 AWG | 95 | 115 | 130 |
| 1 AWG | 110 | 130 | 145 |
| 1/0 AWG | 125 | 150 | 170 |
| 2/0 AWG | 145 | 175 | 195 |
| 3/0 AWG | 165 | 200 | 225 |
| 4/0 AWG | 195 | 230 | 260 |
| 250 kcmil | 215 | 255 | 290 |
| 300 kcmil | 240 | 285 | 320 |
| 350 kcmil | 260 | 310 | 350 |
| 400 kcmil | 280 | 335 | 380 |
| 500 kcmil | 320 | 380 | 430 |
| 600 kcmil | 350 | 420 | 475 |
| 700 kcmil | 385 | 460 | 520 |
| 750 kcmil | 400 | 475 | 535 |
| 1000 kcmil | 455 | 545 | 615 |
Ampacities from NEC 2023 Table 310.16, 30°C ambient basis. Highlighted: your conductor size and insulation column.
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The JobVolt app runs the same verified engines offline, keeps every calculation in History, and turns them into inspector-ready PDF submittals on your letterhead.
How wire size is calculated per the NEC
The calculator starts from the base ampacity in Table 310.16 for your conductor material and insulation column. It applies the ambient temperature correction from Table 310.15(B)(1) and the adjustment factor for the number of current-carrying conductors from Table 310.15(C)(1). For continuous loads, terminations must support 125% of the load per 210.19(A), read against the terminal temperature column required by 110.14(C). The smallest size that passes every check wins, including the small-conductor caps of 240.4(D) and the NM cable limit of 334.80 when they apply.
Why the terminal column matters
A 90°C THHN conductor rarely gets to use its 90°C ampacity. Unless your equipment is marked otherwise, circuits rated 100 A or less are sized from the 60°C column per 110.14(C)(1)(a). The calculator handles that automatically and tells you which limit governed the result.
Prefer the raw numbers? The Table 310.16 ampacity chart is on the site too, rendered from the same verified dataset this calculator reads.