NEC Circular Mils by Wire Size
Conductor cross-section areas in circular mils from NEC Chapter 9 Table 8, 14 AWG through 1000 kcmil. This is the column every voltage drop calculation stands on.
Need a sized answer, not a lookup? Open the free Voltage Drop Calculator. It reads this same dataset and shows every step.
| Size | Circular mils | Ω/kFT copper | Ω/kFT aluminum |
|---|---|---|---|
| 14 AWG | 4,110 | 3.139 | – |
| 12 AWG | 6,530 | 1.975 | 3.247 |
| 10 AWG | 10,380 | 1.243 | 2.042 |
| 8 AWG | 16,510 | 0.781 | 1.284 |
| 6 AWG | 26,240 | 0.492 | 0.808 |
| 4 AWG | 41,740 | 0.309 | 0.508 |
| 3 AWG | 52,620 | 0.245 | 0.403 |
| 2 AWG | 66,360 | 0.194 | 0.319 |
| 1 AWG | 83,690 | 0.154 | 0.253 |
| 1/0 AWG | 105,600 | 0.122 | 0.201 |
| 2/0 AWG | 133,100 | 0.097 | 0.159 |
| 3/0 AWG | 167,800 | 0.077 | 0.126 |
| 4/0 AWG | 211,600 | 0.061 | 0.100 |
| 250 kcmil | 250,000 | 0.052 | 0.085 |
| 300 kcmil | 300,000 | 0.043 | 0.071 |
| 350 kcmil | 350,000 | 0.037 | 0.061 |
| 400 kcmil | 400,000 | 0.032 | 0.053 |
| 500 kcmil | 500,000 | 0.026 | 0.042 |
| 600 kcmil | 600,000 | 0.021 | 0.035 |
| 700 kcmil | 700,000 | 0.018 | 0.030 |
| 750 kcmil | 750,000 | 0.017 | 0.028 |
| 1000 kcmil | 1,000,000 | 0.013 | 0.021 |
Circular mils per NEC Ch. 9 Table 8. Resistance columns are derived from the K-method the JobVolt voltage drop calculator uses (12.9 Cu, 21.2 Al Ω·cmil/ft at 75°C); Table 8 DC resistance values differ slightly.
What a circular mil is
A circular mil is the area of a circle one thousandth of an inch in diameter. It is the trade's unit for conductor cross-section because it makes the resistance arithmetic clean: resistance is K divided by circular mils, per foot. The kcmil sizes carry their area in the name; the AWG sizes need the table above.
The K-factor method
Voltage drop for a single-phase circuit is VD = (2 × K × I × L) / cmil, with 2 replaced by √3 for three phase. K is the industry constant of 12.9 for copper and 21.2 for aluminum at 75°C. The NEC does not print K; it is derived from the DC resistance column of Chapter 9 Table 8, the same table these circular-mil areas come from. The Ω/kFT columns above are that constant spread over 1000 feet, which is the form most field checks use.
Where the limit comes from
The NEC carries 3% on a branch circuit as an informational note to 210.19. It is a recommendation, not a requirement, but specs and inspectors treat it as the baseline, and some state energy codes make it binding. Size for ampacity first from the Table 310.16 ampacity chart, then check the drop. The free voltage drop calculator runs the formula with these exact areas, for single-phase and three-phase, copper or aluminum, and shows the arithmetic. Above 4/0 AWG, reactance starts to matter and the K method drifts conservative; the calculator flags that territory.
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