Wire Ampacity Table - NEC 310.16 Reference Guide
Wire ampacity tables are essential reference tools for electrical professionals, providing the current-carrying capacity of conductors under various conditions. This comprehensive guide covers NEC Article 310.16 ampacity tables, temperature correction factors, and conductor bundling adjustments to ensure safe and code-compliant electrical installations.
Understanding Wire Ampacity
Ampacity is the maximum current a conductor can carry continuously under specific conditions without exceeding its temperature rating. The ampacity of a wire depends on several factors including conductor size, insulation type, ambient temperature, and installation method.
Key Factors Affecting Ampacity
- Conductor Size: Larger conductors have higher ampacity
- Insulation Rating: 60°C, 75°C, or 90°C temperature ratings
- Ambient Temperature: Higher ambient temperatures reduce ampacity
- Number of Conductors: Multiple conductors in a raceway require derating
- Installation Method: Conduit, cable tray, or direct burial affects heat dissipation
Important Note:
Always consult the latest NEC code book for official ampacity values. This guide provides general reference information and may not reflect the most recent code updates. Local amendments may also apply.
NEC 310.16 Ampacity Tables
NEC Article 310.16 (now 310.15(B)(16) in recent editions) provides ampacity tables for copper and aluminum conductors in raceway, cable, or direct burial at three temperature ratings: 60°C, 75°C, and 90°C.
Copper Conductor Ampacities (AWG)
| Wire Size | 60°C (140°F) | 75°C (167°F) | 90°C (194°F) |
|---|---|---|---|
| 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 | 110 |
| 2 AWG | 95 | 115 | 130 |
| 1 AWG | 110 | 130 | 150 |
| 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 |
Terminal Temperature Limitations
Critical Requirement - NEC 110.14(C):
Equipment terminal temperature ratings limit the ampacity you can use:
- Circuits rated 100A or less: Use 60°C column unless equipment is rated higher
- Circuits over 100A: Use 75°C column unless equipment is rated for 90°C
- The 90°C rating is primarily used for derating calculations, not for determining base ampacity
Temperature Correction Factors
When ambient temperature exceeds 30°C (86°F), ampacity must be derated using correction factors from NEC Table 310.15(B)(2)(a).
Temperature Correction Factors for 75°C Insulation
| Ambient Temperature | Correction Factor |
|---|---|
| 21-25°C (70-77°F) | 1.05 |
| 26-30°C (79-86°F) | 1.00 |
| 31-35°C (88-95°F) | 0.94 |
| 36-40°C (97-104°F) | 0.88 |
| 41-45°C (106-113°F) | 0.82 |
| 46-50°C (115-122°F) | 0.75 |
Conductor Bundling Adjustment
When more than three current-carrying conductors are in the same raceway or cable, apply adjustment factors per NEC 310.15(B)(3)(a).
| Number of Conductors | Adjustment Factor |
|---|---|
| 4-6 | 0.80 (80%) |
| 7-9 | 0.70 (70%) |
| 10-20 | 0.50 (50%) |
| 21-30 | 0.45 (45%) |
| 31-40 | 0.40 (40%) |
| 41+ | 0.35 (35%) |
Calculation Example:
12 AWG THHN copper conductor (75°C rating) in 40°C ambient with 5 current-carrying conductors:
- Base ampacity: 25A (75°C column)
- Temperature correction: 25A × 0.88 = 22A
- Bundling adjustment: 22A × 0.80 = 17.6A
- Final ampacity: 17.6A
Practical Application Tips
- Always use the lowest correction factor when multiple adjustments apply
- Check equipment terminal ratings before finalizing conductor size
- Consider voltage drop in addition to ampacity for long circuit runs
- Verify that grounding and neutral conductors don't count toward bundling in certain configurations
- Use continuous load factor (125%) for loads operating 3 hours or more