Reading PEC Table 3.10.2.6(B) — 60 °C Conductor Ampacities
THHN is a 90 °C conductor, but on most branch circuits you size it from the 60 °C column. The reason is the terminals, not the wire.
The termination temperature rule
Breakers, lugs, and equipment terminals have a temperature rating. Unless marked otherwise:
- Circuits rated 100 A or less, or conductors 14 mm² and smaller → use the 60 °C column.
- Circuits over 100 A → use the 75 °C column.
Equipment listed and marked for 75 °C terminations lets you use the 75 °C column for smaller circuits too — check the label.
Typical 60 °C ampacities (copper, 30 °C ambient)
| Size (mm²) | AWG | 60 °C ampacity |
|---|---|---|
| 2.0 | 14 | 15 |
| 3.5 | 12 | 20 |
| 5.5 | 10 | 30 |
| 8.0 | 8 | 40 |
| 14 | 6 | 55 |
Correction still uses the insulation column
Here is the subtlety: you select the final size from the 60 °C column, but you apply temperature and bundling correction factors to the ampacity of the conductor’s actual insulation rating (90 °C for THHN). Then you compare the corrected value back against the load, and separately confirm the 60 °C value still covers the load.
Worked example
24 A continuous load (30 A after 125%), 40 °C ambient. 90 °C correction factor ≈ 0.91.
5.5 mm²: 60 °C ampacity 30 A ≥ 30 A ✓ (just)
→ 5.5 mm² is acceptable; 8.0 mm² gives comfortable margin
EngEst Pro’s EE Tools apply the 60 °C termination rule for circuits at or below 100 A and the 75 °C column above it, while correcting from the conductor’s true insulation rating — so the size it returns is code-consistent.
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