Wire Derating Calculator
Enter table ampacity, ambient temperature and conductor count to see the effective ampacity after NEC temperature and fill derating.
Last 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.
Formula
I_derated = I_table × F_temp × F_fill
| Symbol | Meaning | Unit |
|---|---|---|
F_temp | Temperature correction (NEC 310.15(B)(1)) | — |
F_fill | Fill adjustment (NEC 310.15(C)(1)) | — |
I_derated | Effective ampacity | A |
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.
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.