How to use the ampacity calculator
- Select the wire size and material.
- Pick the temperature rating of the insulation (60°C, 75°C or 90°C).
- Enter the ambient temperature where the wire runs — attics and rooftops are often far above 30°C.
- Enter how many current-carrying conductors share the raceway or cable.
Ampacity derating formula
Adjusted ampacity = Table 310.16 ampacity × temperature correction factor × adjustment factor
Worked example
Six 8 AWG copper THWN-2 conductors (90°C) in a conduit through a 40°C space. Start at 55 A (90°C column). The temperature factor for 36–40°C is 0.91, and six conductors get 80%: 55 × 0.91 × 0.80 = 40.0 A. Because the terminals are 75°C, also check the 75°C value (50 A); the lower number, 40 A, governs.
Ambient temperature correction (Table 310.15(B)(1))
| Ambient °C | Ambient °F | 60°C | 75°C | 90°C |
|---|---|---|---|---|
| 10 or less | 50 or less | 1.29 | 1.2 | 1.15 |
| 11–15 | 52–59 | 1.22 | 1.15 | 1.12 |
| 16–20 | 61–68 | 1.15 | 1.11 | 1.08 |
| 21–25 | 70–77 | 1.08 | 1.05 | 1.04 |
| 26–30 | 79–86 | 1 | 1 | 1 |
| 31–35 | 88–95 | 0.91 | 0.94 | 0.96 |
| 36–40 | 97–104 | 0.82 | 0.88 | 0.91 |
| 41–45 | 106–113 | 0.71 | 0.82 | 0.87 |
| 46–50 | 115–122 | 0.58 | 0.75 | 0.82 |
| 51–55 | 124–131 | 0.41 | 0.67 | 0.76 |
| 56–60 | 133–140 | — | 0.58 | 0.71 |
| 61–65 | 142–149 | — | 0.47 | 0.65 |
| 66–70 | 151–158 | — | 0.33 | 0.58 |
| 71–75 | 160–167 | — | — | 0.5 |
| 76–80 | 169–176 | — | — | 0.41 |
| 81–85 | 178–185 | — | — | 0.29 |
Adjustment for more than three conductors (Table 310.15(C)(1))
| Current-carrying conductors | Percent of table value |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
Related calculations
Packing more conductors into one raceway triggers derating, so check the physical fit with the conduit fill calculator. To go straight from a load to a wire size, use the wire size calculator.