Ampacity from base through correction, adjustment and the terminal cap.
Terminal cap applied. Dashed line = actual load served.
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Banded correction; gaps show ambients with no NEC factor (invalid).
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Base and final usable (correction × adjustment × terminal cap) for both materials.
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Same conductor, 60/75/90°C columns, each with its own terminal cap applied.
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Final usable ampacity across covered sizes at current conditions. Unavailable sizes are skipped.
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Discrete NEC bands (Table 310.15(B)(1)(1)) — step function, not a smooth curve.
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Conductor count adjustment per Table 310.15(C)(1).
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Every number below is traceable to a table lookup or a code section.
Use the sidebar inputs to design a conductor that satisfies every requirement. The system grades your design in real time against the NEC engine.
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- Correction factor accounts for ambient other than 30°C — a discrete band lookup (Table 310.15(B)(1)(1) in the 2020/2023 editions). Higher ambient → lower factor; a dash means the conductor may not be used at that temperature. The NEC also permits calculating the factor with the equation under 310.15(B); linear interpolation of the table is not a method.
- Adjustment factor accounts for mutual heating when more than three current-carrying conductors share a raceway/cable (Table 310.15(C)(1)).
- They multiply. Two 0.8 factors give 0.64, not 0.6.
- The 90°C column may be used for correction/adjustment, but the final result is capped at the terminal-temperature column (110.14(C)).
- Continuous-load rule: size the base/terminal ampacity for noncontinuous load + 125% of continuous load, unless a verified 100%-rated assembly/OCPD exception applies.
- Going from 3 to 10 current-carrying conductors cuts you to 50% before temperature even enters.
- Final usable ampacity and the 240.4(D) small-conductor OCPD ceiling are different numbers — don't conflate them.
- Interpolating the correction-factor table. NEC table bands are discrete. 31°C and 35°C use the same factor. (The permitted alternative is the correction equation, not interpolation.)
- Assuming ≤100 A defaults to 75°C. Per 110.14(C)(1)(a) the default is the 60°C column unless the equipment/conductors are listed/identified for 75°C.
- Forgetting the neutral. A 2-wire circuit neutral and a harmonic/nonlinear neutral both count; a balanced wye/MWBC neutral carrying only imbalance does not.
- Treating final usable ampacity as the allowable OCPD. 240.4(D) can limit the OCPD below the thermal ampacity.
Conductor ampacity is limited by the lowest-rated termination. For circuits 100 A or less (or terminals marked for 14 AWG–1 AWG), the default is the 60°C column, unless the equipment and conductors are listed/identified for 75°C.
Above 100 A (or marked for conductors larger than 1 AWG), use the 75°C column unless otherwise marked. You may derate from the 90°C column, then cap the result at the terminal column value.
Click the answer you believe is correct.
Rendered from the selected edition's data. References are paraphrased for education; the adopted edition controls.
These cases are computed by the live engine on every load and asserted against independently hand-calculated NEC results (verified against NFPA 70-2023 Tables 310.16, 310.17, 310.15(B)(1)(1), 310.15(C)(1)). A red row means the engine regressed.
Ampacity tables and factor tables are transcribed from published NFPA 70-2023 tables (Table 310.16 — conductors in raceway/cable/earth; Table 310.17 — single conductors in free air; Table 310.15(B)(1)(1) — ambient correction; Table 310.15(C)(1) — adjustment). The 2020 edition shares identical values and section numbering for these tables; the edition selector changes only the rooftop clearance threshold (7/8 in. in 2020, 3/4 in. in 2023) and the displayed table designations (Table 310.4(A) vs 310.4(1)). See reviews/conductor-derating-explorer2-review.md in this repository for the verification record.