Twelve AWG has hotter copper cells above its selected small-conductor ceiling. Reading a hotter cell as permission to load the branch to that cell can overlook the separate overcurrent limit. The ceiling stays on the ordinary overcurrent device. The hotter cell can still be the place correction starts, before a terminal or cable cap pulls the result back down.
Aluminum is listed at twelve gauge, with its own lower cells and its own selected ceiling. A copper habit does not transfer to that metal. The equipment’s conductor-material marking and the specified torque decide whether aluminum is eligible at all. Voltage drop on a 120 V run is still calculated from the stranded resistance, not from the breaker slogan.
The example current of 20 A equals the copper listing at 60°C. The prefilled calculator uses 120 V, 60 ft one way, 60°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 20 A and aluminum 15 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 12 AWG is listed at 15 A (60°C), 20 A (75°C), and 25 A (90°C), against copper 20 A, 25 A, and 30 A. Stranded resistance at 75°C is 3.25 Ω/kft aluminum and 1.98 Ω/kft copper. The same label is not the same conductor. Some equipment permits copper only.
Table 8 also lists solid copper 1.93 Ω/kft and solid aluminum 3.18 Ω/kft beside stranded copper 1.98 Ω/kft and stranded aluminum 3.25 Ω/kft. The calculator’s sizing path uses the stranded column.
Table 8 lists 7 strands and 6530 circular mils for 12 AWG. The rounded nominal area is 3.31 mm². Bare stranded diameter is 0.092 in, which excludes insulation and is not conduit fill.
| Listing | Copper | Aluminum |
|---|---|---|
| 60°C ampacity (A) | 20 | 15 |
| 75°C ampacity (A) | 25 | 20 |
| 90°C ampacity (A) | 30 | 25 |
| Stranded Ω/kft | 1.98 | 3.25 |
| Solid Ω/kft | 1.93 | 3.18 |
| Selected 240.4(D) ceiling (A) | 20 | 15 |
| Circular mils | 6530 | |
| Rounded mm² | 3.31 | |
| Strands | 7 | |
| Bare diameter (in) | 0.092 | |
For 12 AWG itself, not for whatever size the calculator recommends, Table 8 stranded copper resistance of 1.98 Ω/kft at 75°C gives 4.75 V (3.96% of 120 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 3.25 Ω/kft give 7.80 V (6.50%). 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 14 AWG to 12 AWG, the 75°C copper listing goes from 20 A to 25 A. Stranded copper resistance at 75°C goes from 3.14 Ω/kft to 1.98 Ω/kft.
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.
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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