NEC Article 310 Wire Sizing Guide
Complete reference guide to National Electrical Code ampacity tables and conductor sizing requirements
Understanding NEC Article 310
Article 310 of the National Electrical Code covers conductors for general wiring and is fundamental to proper wire sizing. This article provides the ampacity tables that form the basis of safe electrical installations.
Key Sections of Article 310
310.12 - Insulation
Specifies insulation types and temperature ratings. Common ratings include:
- 60°C: TW, UF
- 75°C: THW, THWN, USE
- 90°C: THHN, XHHW, USE-2
310.15 - Ampacity
Provides methods for determining conductor ampacity including:
- • Temperature correction factors
- • Adjustment factors for bundling
- • Installation method considerations
- • Ambient temperature effects
Table 310.16 - Allowable Ampacities
The most commonly referenced table for conductor ampacities. This table applies to not more than three current-carrying conductors in raceway, cable, or earth.
| AWG/kcmil | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 20 | 25 | - | - | - |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 150 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Temperature Correction Factors
When ambient temperature differs from 30°C (86°F), ampacities must be corrected using Table 310.15(B)(1):
For 75°C Rated Conductors
Practical Application
If you have a 20A circuit with 12 AWG copper wire (25A at 75°C) in a 40°C attic:
The corrected ampacity (20.5A) is still adequate for the 20A circuit.
Adjustment Factors for Multiple Conductors
When more than three current-carrying conductors are installed in the same raceway, cable, or trench, ampacity must be reduced per Table 310.15(B)(3)(a):
Important Notes
- • Neutral conductors carrying only unbalanced current are not counted
- • Equipment grounding conductors are not counted
- • Conductors that are not simultaneously energized are not counted
- • Both temperature correction and adjustment factors may apply to the same installation
Special Applications
Motor Circuits (Article 430)
- • Use 125% of motor full-load current for conductor sizing
- • Branch circuit conductors must have ampacity ≥ 125% of FLA
- • Feeder conductors use 125% of largest motor plus sum of others
Continuous Loads (Article 210)
- • Loads expected to operate for 3+ hours continuously
- • Conductor ampacity must be ≥ 125% of continuous load
- • Plus 100% of non-continuous loads
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