Wire Amp Calculator
Calculate the usable ampacity of any conductor after temperature correction, bundling adjustment, and the 110.14(C) termination limit that most amp calculators skip.
How the calculation works
This calculator implements the NEC 2023 ampacity derating chain in four steps. Most online calculators skip Step 4, which often controls the final answer.
Step 1 reads the base ampacity from NEC Table 310.16 at the insulation temperature column. THHN at 90C gives the highest starting value.
Step 2 applies the temperature correction factor from NEC Table 310.15(B)(1)(1). When ambient temperature exceeds 30C, the base ampacity is multiplied by a factor less than 1.0. The correction factor uses the insulation temperature rating, not the termination rating.
Step 3 applies the bundling adjustment from NEC Table 310.15(C)(1). More than three current-carrying conductors in a raceway reduce ampacity. Equipment grounding conductors do not count. Neutral conductors that carry only unbalanced current do not count per 310.15(E)(1).
Step 4 applies the 110.14(C) termination temperature limit. For circuits rated 100A or less, the final ampacity cannot exceed the value from the 75C column of Table 310.16 unless the equipment is specifically rated and listed for higher temperatures. This is the step most calculators miss, and it frequently controls the answer.
Frequently asked questions
Why does the 110.14(C) termination limit matter?
NEC 110.14(C) requires that conductors be used within the temperature limitations of their terminations. Most circuit breakers, receptacles, and switches are rated for 75C. Even if your THHN wire is rated for 90C, the connection point limits what ampacity you can actually use. This is why "usable ampacity" often differs from the ampacity that appears in simplified charts.
When can I use the 90C column for final ampacity?
You can use the full 90C ampacity only when both ends of the circuit terminate on equipment that is listed and marked for 90C. This is uncommon in residential work but may apply in some industrial installations. The 90C column is always used as the starting point for derating calculations (Steps 2 and 3) per 310.15(B), even when the final ampacity is limited by Step 4.
How do I count current-carrying conductors?
Count all phase and neutral conductors that carry load current. Do not count equipment grounding conductors. Do not count a neutral that carries only unbalanced current from two ungrounded conductors (per 310.15(B)(5) exception). In a typical 120/240V circuit, the neutral carries only the unbalance and does not count. In a 3-phase 4-wire wye system with nonlinear loads, the neutral carries harmonic currents and must be counted.
Does this calculator work for NEC 2026?
NEC 2026 reorganized Article 310 numbering but did not change the fundamental ampacity values in Table 310.16 or the derating methodology. The calculation chain (base ampacity, temperature correction, bundling adjustment, termination limit) remains the same. WireRef will update table references when NEC 2026 is widely adopted. See NEC 2026 changes.
NEC 2023 references verified April 2026