Wire Ampacity Guide: NEC 310.16 Complete Reference
What Is Ampacity?
Ampacity is the maximum current (amps) a conductor can carry continuously without exceeding its temperature rating. It depends on:
- Wire gauge (cross-sectional area)
- Insulation temperature rating (60°C, 75°C, 90°C)
- Ambient temperature (base: 30°C / 86°F)
- Number of conductors in a raceway or cable
- Conductor material (copper vs aluminum)
NEC Table 310.16 — Allowable Ampacities
| AWG/kcmil | Cu 60°C | Cu 75°C | Cu 90°C | Al 60°C | Al 75°C | Al 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 |
| 1/0 | 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 |
Conditions: 30°C ambient, 3 or fewer current-carrying conductors, copper/aluminum conductors.
Temperature Derating (NEC Table 310.15(B)(2)(a))
When ambient temperature exceeds 30°C, multiply ampacity by the correction factor:
| Ambient Temp | 60°C Insulation | 75°C Insulation | 90°C Insulation |
|---|---|---|---|
| 30°C (base) | 1.00 | 1.00 | 1.00 |
| 35°C | 0.91 | 0.94 | 0.96 |
| 40°C | 0.82 | 0.88 | 0.91 |
| 45°C | 0.71 | 0.82 | 0.87 |
| 50°C | 0.58 | 0.75 | 0.82 |
Conductor Bundling Derating (NEC 310.15(B)(3)(a))
| Current-Carrying Conductors | Derating Factor |
|---|---|
| 1–3 | 1.00 (no derating) |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
Note: Neutral conductors in balanced 3-phase circuits don't count. In 120/240V single-phase, the neutral IS current-carrying (carries unbalanced load).
Worked Example: Derating
Scenario: 6 THWN copper conductors (12 AWG) in a conduit in an attic at 50°C.
Step 1: Base ampacity (75°C): 25A
Step 2: Ambient temp correction (50°C, 75°C insulation): × 0.75 → 18.75A
Step 3: Bundling correction (6 conductors): × 0.80 → 15A
Result: Each 12 AWG conductor can only carry 15A. Install on a 15A breaker (even though 12 AWG normally allows 20A).
Common Ampacity Mistakes
- Using 90°C column for breakers: Even if the wire is rated 90°C (THHN), the breaker terminal is usually rated 75°C. Use the 75°C column for termination ampacity.
- Ignoring attic temperatures: Attics can reach 60–70°C in summer. This significantly derates wire ampacity.
- Not counting neutrals: In multiwire branch circuits (shared neutral), the neutral is current-carrying and counts for derating.
- Using NM cable ampacity for conduit: NM (Romex) has different ampacity rules than THWN in conduit. NM is limited to 60°C column.
Standards Reference
- NEC Table 310.16 — Allowable Ampacities
- NEC 310.15(B) — Ambient Temperature Correction
- NEC 310.15(B)(3)(a) — Conductor Bundling
- NEC Article 240 — Overcurrent Protection
Frequently Asked Questions
What is the ampacity of 12 AWG copper wire?
12 AWG copper wire has an ampacity of 25A at 75°C insulation (THWN) and 30A at 90°C (THHN). However, NEC 240.4(D) limits 12 AWG to 20A maximum overcurrent protection for residential branch circuits, regardless of the higher ampacity rating.
How does temperature affect wire ampacity?
Wire ampacity decreases as ambient temperature increases. At 40°C (104°F), 75°C-rated wire loses 12% of its ampacity. At 50°C (122°F), it loses 25%. In attics that reach 60–70°C, you may need to upsize wire significantly or use higher temperature-rated insulation.
What is the derating for 4 wires in a conduit?
NEC 310.15(B)(3)(a) requires 80% derating for 4–6 current-carrying conductors in a raceway. For example, 12 AWG rated 25A (75°C) becomes 20A with 4 conductors. This effectively limits 12 AWG to 20A (matching the NEC 240.4(D) limit).
Can I use the 90°C column for ampacity?
You can use the 90°C column for derating calculations (temperature and bundling), but the final ampacity must not exceed the 75°C column for termination at standard breaker and panel lugs. This is a two-step process: derate from 90°C, then compare to 75°C limits.
What's the difference between ampacity and overcurrent protection?
Ampacity is the wire's thermal capacity (how much current it can carry continuously). Overcurrent protection (breaker) is the maximum allowed by NEC. For 14 and 12 AWG, NEC 240.4(D) caps the breaker below the wire's ampacity for added safety.