What is kcmil and How is it Used?
kcmil = thousands of circular mils, the US unit for conductor cross-sectional area above 4/0 AWG. Used for 250, 300, 400, 500, 750, and 1000-size feeders.
kcmil is the US unit for conductor cross-sectional area at sizes larger than 4/0 AWG. The acronym stands for thousand circular mils.
Where it's used in NEC
NEC sizes conductors as AWG up to 4/0 (211,600 CM), then switches to kcmil:
| Size | Area (CM) | Area (in²) |
|---|---|---|
| 4/0 AWG | 211,600 | 0.166 |
| 250 kcmil | 250,000 | 0.196 |
| 300 kcmil | 300,000 | 0.236 |
| 350 kcmil | 350,000 | 0.275 |
| 400 kcmil | 400,000 | 0.314 |
| 500 kcmil | 500,000 | 0.393 |
| 600 kcmil | 600,000 | 0.471 |
| 750 kcmil | 750,000 | 0.589 |
| 1000 kcmil | 1,000,000 | 0.785 |
What does "circular mil" mean?
1 mil = 0.001 inch (a unit used in the wire industry). The "circular mil" is the area of a circle with a 1-mil diameter. For a round conductor of diameter D mils:
Area in CM = D²
So a conductor 500 mils in diameter has 250,000 CM = 250 kcmil of conductor area.
kcmil → in² for NEC Table 5 fill
NEC Chapter 9 Table 5 lists THHN areas in in² for fill purposes. The conversion from kcmil to in² for round bare conductor:
in² = kcmil × π / 4 / 1,000,000
But THHN/THWN-2 has insulation that adds to the cross-section. NEC Table 5 gives the insulated dimension:
| kcmil | THHN in² (with insulation) |
|---|---|
| 250 | 0.3970 |
| 350 | 0.5242 |
| 500 | 0.7073 |
| 750 | 1.0496 |
| 1000 | 1.3478 |
These are what feed the wire fill chart and the conduit fill calculator.
Common applications
- 250 kcmil: 200A service entrance feeders, often aluminum
- 350 kcmil: 300A or large industrial sub-panel feeders
- 500 kcmil: 400A service entrance (single-conductor approach)
- 600 kcmil: 400A copper service, large motor circuits
- 750 kcmil: Service entrance for buildings up to 600A
- 1000 kcmil: Large feeders, parallel installations rare beyond this