Ampacity Correction Factors for Ambient Temperature and Bundling
The ampacity table assumes a 30 °C ambient and no more than three current-carrying conductors. Real installations are often hotter and more crowded, and both effects reduce the usable current.
Temperature correction
Above 30 °C ambient, multiply the table ampacity by the correction factor for the conductor’s insulation rating:
| Ambient | 60 °C conductor | 75 °C conductor | 90 °C conductor |
|---|---|---|---|
| 31–35 °C | 0.91 | 0.94 | 0.96 |
| 36–40 °C | 0.82 | 0.88 | 0.91 |
| 41–45 °C | 0.71 | 0.82 | 0.87 |
| 46–50 °C | 0.58 | 0.75 | 0.82 |
Rooftop conduit in direct sun gets an additional ambient adder — often 20–30 °C on top of the design air temperature.
Conductor bundling adjustment
| Current-carrying conductors | Adjustment factor |
|---|---|
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
The neutral of a balanced three-phase circuit is not counted; a neutral carrying harmonic current is.
The two factors multiply
Example: 22 mm² THHN, table 90 °C ampacity 95 A, in a 42 °C attic with 6 current-carrying conductors:
The conductor that looked like 95 A on the table is really a 66 A conductor in that location.
EngEst Pro applies both correction factors automatically once you set the ambient temperature and conductor count for a run — the ampacity it checks against is the corrected value, not the raw table figure.
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