What is Wire Ampacity?
Ampacity is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating, per NEC Table 310.16.
Ampacity is the current-carrying capacity of a conductor — how many amperes it can safely carry continuously without its insulation overheating. The NEC defines it formally in Article 100 and tabulates values in Table 310.16 for the common insulated conductors used in raceways or cables.
Three temperature columns in Table 310.16
Each conductor gets three ampacity ratings, one per insulation temperature class:
| Column | Common insulations |
|---|---|
| 60°C | TW, UF |
| 75°C | RHW, THW, THWN, USE, ZW, XHHW |
| 90°C | THHN, THWN-2, XHHW-2, RHH, RHW-2 |
A 12 AWG copper conductor has ampacity of:
- 20A at 60°C
- 25A at 75°C
- 30A at 90°C
You don't simply use the 90°C value. NEC 110.14(C)(1) requires using the column matching the lowest temperature rating in the circuit — typically the terminal temperature rating of the breaker and equipment lugs.
NEC 110.14(C)(1) — terminal temperature rules
| Circuit ampere rating | Terminal temp default | Wire column |
|---|---|---|
| ≤ 100A | 60°C | use 60°C column |
| > 100A | 75°C | use 75°C column |
Exception: if equipment is listed for higher temperature terminals (90°C lugs on a breaker, for example), you may use the higher column. Most residential equipment is 75°C-listed at best.
Adjustment factors (NEC 310.15(C)(1))
When more than 3 current-carrying conductors share a raceway >24" long:
| # of CCCs | Adjustment |
|---|---|
| 4–6 | × 0.80 |
| 7–9 | × 0.70 |
| 10–20 | × 0.50 |
| 21–30 | × 0.45 |
| 31–40 | × 0.40 |
| 41+ | × 0.35 |
Apply this multiplier to the base ampacity from Table 310.16 to get the adjusted ampacity.
Correction factors (NEC 310.15(B)(1))
When the ambient temperature differs from 30°C (86°F), apply a temperature correction. For 90°C THHN:
| Ambient | Correction |
|---|---|
| 21–25°C | × 1.04 |
| 26–30°C | × 1.00 |
| 31–35°C | × 0.96 |
| 36–40°C | × 0.91 |
| 41–45°C | × 0.87 |
| 46–50°C | × 0.82 |
Worked example
12 AWG THHN, 5 current-carrying conductors in conduit, ambient 35°C, 75°C terminals:
- Base ampacity (90°C column): 30A
- Adjust for 5 CCCs: 30 × 0.80 = 24A
- Correct for 35°C ambient: 24 × 0.96 = 23.04A
- Compare to 75°C terminal cap: 25A (12 AWG @ 75°C)
- Final ampacity: min(23.04, 25) = 23.04A → round to 20A breaker (next standard size below)
The cap step matters — even if adjustment+correction gives a higher value, you can't exceed the column matching your terminal rating.
What ampacity is NOT
- It is not the same as fill percentage (Chapter 9 conduit fill). Fill and ampacity are independent constraints.
- It is not the same as breaker rating. NEC 240.4 governs how to size overcurrent protection vs ampacity — they often differ.
- It is not maximum momentary current. Ampacity is for continuous current (>3 hours). Short-time inrush is governed separately.