Ten gauge is the largest copper size inside this tool’s selected small-conductor group, which is why a one-number answer is a trap. The 60°C copper cell and the copper device ceiling land together. The hotter cells sit above that ceiling and do not lift it. Aluminum at this label is listed too, and its selected ceiling is not the copper ceiling. Treat those as different metals, not as a translation.
A 240 V example is prefilled so the same resistance model can be seen as a percentage of a higher source voltage. Percentage drop shrinks when voltage rises and the volt drop stays put. That arithmetic does not choose a cable construction, a wet-location marking, or a breaker. It only shows why “how many amps” and “how far” are different questions.
The example current of 30 A equals the copper listing at 60°C. The prefilled calculator uses 240 V, 100 ft one way, 75°C insulation, and 60°C terminals. Those are inputs, not a required circuit.
The selected NEC 240.4(D) ceilings in this tool are copper 30 A and aluminum 25 A. Those are ordinary overcurrent-device limits, not extra ampacity, and they are not a complete breaker-selection procedure. Equipment exceptions are not checked.
Aluminum 10 AWG is listed at 25 A (60°C), 30 A (75°C), and 35 A (90°C), against copper 30 A, 35 A, and 40 A. Stranded resistance at 75°C is 2.04 Ω/kft aluminum and 1.24 Ω/kft copper. The same label is not the same conductor. Some equipment permits copper only.
Table 8 also lists solid copper 1.21 Ω/kft and solid aluminum 2 Ω/kft beside stranded copper 1.24 Ω/kft and stranded aluminum 2.04 Ω/kft. The calculator’s sizing path uses the stranded column.
Table 8 lists 7 strands and 10380 circular mils for 10 AWG. The rounded nominal area is 5.261 mm². Bare stranded diameter is 0.116 in, which excludes insulation and is not conduit fill.
| Listing | Copper | Aluminum |
|---|---|---|
| 60°C ampacity (A) | 30 | 25 |
| 75°C ampacity (A) | 35 | 30 |
| 90°C ampacity (A) | 40 | 35 |
| Stranded Ω/kft | 1.24 | 2.04 |
| Solid Ω/kft | 1.21 | 2 |
| Selected 240.4(D) ceiling (A) | 30 | 25 |
| Circular mils | 10380 | |
| Rounded mm² | 5.261 | |
| Strands | 7 | |
| Bare diameter (in) | 0.116 | |
For 10 AWG itself, not for whatever size the calculator recommends, Table 8 stranded copper resistance of 1.24 Ω/kft at 75°C gives 7.44 V (3.10% 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 2.04 Ω/kft give 12.24 V (5.10%). 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 12 AWG to 10 AWG, the 75°C copper listing goes from 25 A to 35 A. Stranded copper resistance at 75°C goes from 1.98 Ω/kft to 1.24 Ω/kft.
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.
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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