TradeToolsElectrical · in the TradeTools app
Wire Size Calculator
The conductor for a load, with the four factors that move the answer most often: continuous load at 125 %, ambient temperature correction, adjustment for the number of current-carrying conductors, and the termination temperature that decides the column.
NEC 310.16 · 310.15(B) · 110.14(C)
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Worked example
40 A continuous load in a 104 °F attic — 8 AWG or 6?
| Load | 40 A, continuous |
|---|---|
| Voltage / phase | 240 V, single-phase |
| Conductor | Copper, THHN/THWN-2 |
| Installation | In conduit, 1–3 current-carrying conductors |
| Ambient temperature | 104 °F |
| Termination rating | 75 °C |
| Conductor | 8 AWG |
|---|---|
| Overcurrent device | 50 A |
| Design current | 50.0 A |
| Table ampacity | 55 A (NEC Table 310.16, 90 °C column) |
| Temperature correction | 0.91 |
| Ampacity after derating | 50.1 A |
| Termination-limited ampacity | 50 A |
40 A × 125 % = 50 A. 8 AWG THHN starts at 55 A in the 90 °C column; the 104 °F correction of 0.91 brings it to 50.05 A. The 75 °C terminations cap the conductor at the 75 °C column, 50 A — exactly the design current. The terminals decide.
How it's calculated
The table behind the number
The load is multiplied by 125 % if continuous (210.19(A)(1)). The app starts from the insulation's temperature column in Table 310.16, applies the ambient correction from Table 310.15(B)(1)(1) and the adjustment factor for the number of current-carrying conductors from Table 310.15(C)(1), and then checks the result against the termination column required by 110.14(C). The smallest conductor that passes both checks is the answer, with the standard overcurrent device from 240.6(A) and the small-conductor caps of 240.4(D).
- NEC 2023 Table 310.16Allowable ampacities, 60/75/90 °C columns, not more than three current-carrying conductors in raceway
- NEC 2023 Table 310.15(B)(1)(1)Ambient temperature correction factors based on 30 °C
- NEC 2023 Table 310.15(C)(1)Adjustment factors for more than three current-carrying conductors
- NEC 2023 210.19(A)(1)Branch-circuit conductors: 125 % of continuous load
- NEC 2023 110.14(C)Termination temperature limits
- NEC 2023 240.4(D) and 240.6(A)Small conductor caps and standard overcurrent device ratings
Questions
Before you trust the number
Why does the app use the 90 °C column and then the 75 °C column?
Derating starts from the conductor's own insulation rating — THHN is 90 °C — but the terminals in most equipment are rated 75 °C, so the derated value must also stay within the 75 °C column. Both checks have to pass.
Is 310.15(B)(2)(a) the same table?
That is the 2017 numbering. In NEC 2020 and 2023 the ambient correction table is Table 310.15(B)(1)(1) and the bundling adjustment is Table 310.15(C)(1). The math is unchanged; the citations are not.