Eight AWG is the first size in this chart that the selected small-conductor column does not specially limit. Below this size that column was doing extra work. From here on, the calculator goes back to table ampacity, the terminal or cable cap, and voltage drop. Expecting a single slogan instead of three temperature cells will pick the wrong column whenever the terminals are marked differently from the insulation.
This is also the largest row in the embedded Table 8 that still prints a solid-conductor resistance beside the stranded value. The sizing path keeps the stranded number. Bare diameter in the same row is the stranded conductor only. It is not the insulated cable and it is not a conduit-fill dimension.
The example current of 40 A equals the copper listing at 60°C. The prefilled calculator uses 240 V, 80 ft one way, 75°C insulation, and 60°C terminals. Those are inputs, not a required circuit.
The selected NEC 240.4(D) column in this tool has no special ceiling for 8 AWG. A blank is not permission to omit overcurrent protection. Table ampacity, the terminal cap, and voltage drop still apply.
Aluminum 8 AWG is listed at 35 A (60°C), 40 A (75°C), and 45 A (90°C), against copper 40 A, 50 A, and 55 A. Stranded resistance at 75°C is 1.28 Ω/kft aluminum and 0.778 Ω/kft copper. The same label is not the same conductor. Some equipment permits copper only.
Table 8 also lists solid copper 0.764 Ω/kft and solid aluminum 1.26 Ω/kft beside stranded copper 0.778 Ω/kft and stranded aluminum 1.28 Ω/kft. The calculator’s sizing path uses the stranded column.
Table 8 lists 7 strands and 16510 circular mils for 8 AWG. The rounded nominal area is 8.367 mm². Bare stranded diameter is 0.146 in, which excludes insulation and is not conduit fill.
| Listing | Copper | Aluminum |
|---|---|---|
| 60°C ampacity (A) | 40 | 35 |
| 75°C ampacity (A) | 50 | 40 |
| 90°C ampacity (A) | 55 | 45 |
| Stranded Ω/kft | 0.778 | 1.28 |
| Solid Ω/kft | 0.764 | 1.26 |
| Selected 240.4(D) ceiling (A) | — | — |
| Circular mils | 16510 | |
| Rounded mm² | 8.367 | |
| Strands | 7 | |
| Bare diameter (in) | 0.146 | |
For 8 AWG itself, not for whatever size the calculator recommends, Table 8 stranded copper resistance of 0.778 Ω/kft at 75°C gives 4.98 V (2.07% of 240 V) using 2 × I × R × L / 1,000. Reactance is omitted. Displayed volts are rounded to two decimals. The same inputs with stranded aluminum resistance 1.28 Ω/kft give 8.19 V (3.41%). If that percentage is outside the prefilled drop limit, the calculator will name a different size. That is the voltage-drop check, not a second ampacity table.
From 10 AWG to 8 AWG, the 75°C copper listing goes from 35 A to 50 A. Stranded copper resistance at 75°C goes from 1.24 Ω/kft to 0.778 Ω/kft.
From 8 AWG to 6 AWG, the 75°C copper listing goes from 50 A to 65 A. Stranded copper resistance at 75°C goes from 0.778 Ω/kft to 0.491 Ω/kft.
Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.
Sources for these cells: existing Table 310.16 numerical cells in NFPA’s public development record, pp. 142–143 (proposed changes are not treated as adopted code) and NFPA 70 (2017), Chapter 9 Table 8, printed p. 689, in a third-party-hosted copy. The same publication supplies 110.14 (terminals and aluminum), 210.19/215.2 (continuous loads), 240.4(D) (small conductors), 250.24/250.32 (grounding), and 625.41/625.42 (EV loads). Schneider Electric’s voltage-drop formulas support the arithmetic; Table 8 AC results here omit reactance and assume unity power factor. Resistance and geometry are construction- and temperature-specific. Adopted editions and local amendments can differ.
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