Wire Derating — Temperature Correction & Adjustment Factors
Ampacity tables assume 30°C air and no more than three loaded conductors together. When either assumption fails, multiply the table ampacity by the factors below.
More than three conductors: 4–6 = 80%, 7–9 = 70%, 10–20 = 50%. Hot locations: at 36–40°C (96–104°F), 90°C wire is multiplied by 0.91 and 75°C wire by 0.88.
Ampacity & derating calculator
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Ambient temperature correction factors
| Ambient °C | Ambient °F | 60°C wire | 75°C wire | 90°C wire |
|---|---|---|---|---|
| 10 or less | 50 or less | 1.29 | 1.20 | 1.15 |
| 11–15 | 51–59 | 1.22 | 1.15 | 1.12 |
| 16–20 | 60–68 | 1.15 | 1.11 | 1.08 |
| 21–25 | 69–77 | 1.08 | 1.05 | 1.04 |
| 26–30 | 78–86 | 1.00 | 1.00 | 1.00 |
| 31–35 | 87–95 | 0.91 | 0.94 | 0.96 |
| 36–40 | 96–104 | 0.82 | 0.88 | 0.91 |
| 41–45 | 105–113 | 0.71 | 0.82 | 0.87 |
| 46–50 | 114–122 | 0.58 | 0.75 | 0.82 |
| 51–55 | 123–131 | 0.41 | 0.67 | 0.76 |
| 56–60 | 132–140 | — | 0.58 | 0.71 |
| 61–65 | 141–149 | — | 0.47 | 0.65 |
| 66–70 | 150–158 | — | 0.33 | 0.58 |
| 71–75 | 159–167 | — | — | 0.50 |
| 76–80 | 168–176 | — | — | 0.41 |
| 81–85 | 177–185 | — | — | 0.29 |
Adjustment factors for conductor count
| Current-carrying conductors | Percent of table ampacity |
|---|---|
| 4–6 | 80% |
| 7–9 | 70% |
| 10–20 | 50% |
| 21–30 | 45% |
| 31–40 | 40% |
| 41 and above | 35% |
| Copper THHN | 90°C value | 4–6 (80%) | 7–9 (70%) | 10–20 (50%) | 21–30 (45%) | 75°C cap |
|---|---|---|---|---|---|---|
| 14 AWG | 25 | 20 | 17.5 | 12.5 | 11.25 | 20 |
| 12 AWG | 30 | 24 | 21 | 15 | 13.5 | 25 |
| 10 AWG | 40 | 32 | 28 | 20 | 18 | 35 |
| 8 AWG | 55 | 44 | 38.5 | 27.5 | 24.75 | 50 |
| 6 AWG | 75 | 60 | 52.5 | 37.5 | 33.75 | 65 |
| 4 AWG | 95 | 76 | 66.5 | 47.5 | 42.75 | 85 |
| 2 AWG | 130 | 104 | 91 | 65 | 58.5 | 115 |
| 1/0 AWG | 170 | 136 | 119 | 85 | 76.5 | 150 |
How to derate, step by step
- Take the table ampacity from the column matching the wire’s insulation rating (90°C for THHN/THWN-2).
- Multiply by the ambient correction factor if the surroundings are not 26–30°C.
- Multiply by the adjustment factor if more than three current-carrying conductors are grouped.
- Compare with the ampacity in the termination-temperature column (usually 75°C). The lower number is the allowable ampacity.
Worked example. Six 10 AWG THHN current-carrying conductors in conduit through a 45°C (113°F) attic. Start at 40 A (90°C column). Temperature: × 0.87 = 34.8 A. Count: × 0.80 = 27.8 A. The 75°C cap is 35 A, so the allowable ampacity is 27.8 A — no longer enough for a 30 A circuit. Going to 8 AWG gives 55 × 0.87 × 0.80 = 38.3 A.
Rooftops: raceways and cables in direct sunlight closer than 7/8 inch above a roof must have 33°C (60°F) added to the outdoor design temperature before choosing the correction factor. XHHW-2 is exempt from the adder.
Frequently asked questions
When do I have to derate wire?
In two situations: when the ambient temperature is above 30°C (86°F) — attics, rooftops, boiler rooms — and when more than three current-carrying conductors share a raceway or cable, or cables are bundled without spacing for more than 24 inches. If both apply, multiply both factors.
Can I derate from the 90°C column?
Yes. If the insulation is rated 90°C (THHN, THWN-2, XHHW-2), correction and adjustment factors are applied to the 90°C ampacity. The result must still not exceed the ampacity for the termination temperature rating, normally the 75°C column.
Does the neutral count as a current-carrying conductor?
A neutral that carries only the unbalanced current of a multiwire circuit does not count. It does count on a 3-wire circuit taken from a 4-wire wye system, and on 4-wire wye circuits where most of the load is nonlinear (electronic ballasts, LED drivers, computers).
Do ground wires count for derating?
No. Equipment grounding and bonding conductors are never counted as current-carrying. They do count toward conduit fill.
How many 12 AWG THHN circuits can share a conduit and stay on 20 A breakers?
Up to nine current-carrying conductors: 30 A × 70% = 21 A. At ten, the 50% factor drops 12 AWG to 15 A, so you would need 10 AWG (40 A × 50% = 20 A).
Is there a derating exemption for short conduit?
Yes. Adjustment factors do not apply to conductors in a raceway nipple 24 inches or shorter.
NECTables.com is an independent reference and is not affiliated with, endorsed by, or a substitute for the National Fire Protection Association (NFPA). NEC® and National Electrical Code® are registered trademarks of the NFPA. Values shown are commonly published engineering data and independent calculations based on the methods of NFPA 70; they are not a reproduction of the Code. Always confirm against the edition adopted in your jurisdiction, including local amendments, and consult a licensed electrician or engineer before doing electrical work.