Wire Derating Calculator
Enter table ampacity, ambient temperature and conductor count to see the effective ampacity after NEC temperature and fill derating.
Open calculatorLast updated: 2026-09-15
How the calculation works
- Temperature correction scales ampacity for ambient above or below 30 °C using the NEC 310.15(B)(1) factors.
- Fill adjustment reduces ampacity when more than three current-carrying conductors share a raceway (310.15(C)(1)).
- Both factors compound — the effective ampacity is the product.
Worked example
Interpreting the result
Derating is where most DIY ampacity mistakes happen: a conduit through a hot attic with a dozen conductors can halve a conductor's rating. The design flow is: pick a conductor from the table, apply derating, and confirm the derated ampacity still exceeds the load (with the 125% continuous factor).
Assumptions
- 75 °C insulation column; 90 °C columns allow starting from a higher base.
- Uniform loading across conductors.
Limitations
- Rooftop installations add a separate adder (NEC 310.15(B)(2)).
- Cable trays and underground duct banks have their own ampacity tables.
Effective ampacity of a 30 A conductor by ambient temperature and number of conductors (A)
| Ambient temperature | 3 conductors | 4 conductors | 7 conductors | 10 conductors |
|---|---|---|---|---|
| 30 °C | 30.0 | 24.0 | 21.0 | 15.0 |
| 35 °C | 28.2 | 22.6 | 19.7 | 14.1 |
| 40 °C | 26.4 | 21.1 | 18.5 | 13.2 |
| 45 °C | 24.6 | 19.7 | 17.2 | 12.3 |
| 50 °C | 22.5 | 18.0 | 15.7 | 11.3 |
Effective ampacity = base ampacity × temperature factor × conductor-fill adjustment, using the factors built into the calculator. Check them against the code table for your insulation rating.
Worked scenario: six conductors in a 40 °C space
A conductor with a 30 A base rating keeps only 21.1 A after both adjustments: a temperature factor of 0.88 and a conductor-fill adjustment of 0.80. The two multiply, so heat and bundling together cost far more than either alone.
Why the rating drops
A conductor's limit is set by the temperature its insulation can take. Hot surroundings leave less room for the conductor's own heating, and conductors bundled together heat each other. Derating shrinks the allowed current to keep the insulation within its limit.
Limits of the result
- The correct factors depend on the insulation temperature rating and on the code edition you work to.
- Which conductors count toward the fill adjustment has its own rules (for example, neutral and grounding conductors).
- Terminal ratings can cap the usable current regardless of the derated figure.
Frequently asked questions
Do grounds count for derating?
No — equipment grounding conductors never count as current-carrying. Neutrals count unless they carry only unbalanced current of a 3-phase wye set.
Why does ambient temperature matter?
Conductor insulation has a maximum temperature. Ampacity tables assume 30 °C ambient; hotter surroundings leave less headroom for self-heating, so the allowable current drops.