400 kcmil Wire Specs
Ampacity, geometry, resistance, conduit fill and maximum run length for 400 kcmil conductors in copper and aluminium.
Quick answer
400 kcmil copper carries about 348 A at 75 °C (295 A at 60 °C, 393 A at 90 °C). The bare conductor is 0.6325 in (16.07 mm) and 400,000 circular mils, with 0.0259 Ω per 1000 ft. It pairs with a 300 A breaker at 75 °C terminations.
Copper 75 °C
348A
Aluminium 75 °C
272A
Diameter
0.6325in
Circular mils
400,000
Full specification
| Size designation | 400 kcmil |
|---|---|
| AWG gauge number | n/a (kcmil size) |
| Circular mils | 400,000 |
| kcmil | 400.00 |
| Bare diameter (in) | 0.6325 |
| Bare diameter (mm) | 16.065 |
| Cross-section (mm²) | 202.71 |
| THHN/THWN-2 OD (in) | 0.864 |
| Insulated area (sq in) | 0.5863 |
| Copper resistance (Ω/1000 ft) | 0.0259 |
| Aluminium resistance (Ω/1000 ft) | 0.0425 |
| Max standard breaker (Cu, 75 °C) | 300 A |
Published standard — source: Geometry computed from the ASTM B258 AWG formula; resistance from standard resistivityFormula estimate — source: Ampacity from the site model
Ampacity with derating
Estimated ampacity from the 90 °C column at various ambient temperatures and conductor counts.
| Conductors in raceway | 30 °C ambient | 40 °C ambient | 50 °C ambient | 60 °C ambient |
|---|---|---|---|---|
| 3 | 393 A | 359 A | 321 A | 278 A |
| 6 | 314 A | 287 A | 257 A | 222 A |
| 9 | 275 A | 251 A | 224 A | 194 A |
| 15 | 197 A | 179 A | 160 A | 139 A |
| 25 | 177 A | 161 A | 144 A | 125 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 348 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 400 kcmil copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 5,581 ft | 11,162 ft | 11,171 ft | 25,780 ft |
| 15 A | 3,720 ft | 7,441 ft | 7,447 ft | 17,186 ft |
| 20 A | 2,790 ft | 5,581 ft | 5,585 ft | 12,890 ft |
| 30 A | 1,860 ft | 3,720 ft | 3,723 ft | 8,593 ft |
| 40 A | 1,395 ft | 2,790 ft | 2,792 ft | 6,445 ft |
| 50 A | 1,116 ft | 2,232 ft | 2,234 ft | 5,156 ft |
How many 400 kcmil conductors fit in conduit
Maximum count of this size alone, at the fill percentage matching the resulting count.
| Conduit | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" | 3-1/2" | 4" |
|---|---|---|---|---|---|---|---|---|---|---|
| EMT | — | — | — | 1 | 1 | 1 | 3 | 6 | 7 | 10 |
| IMC | — | — | — | 1 | 1 | 1 | 3 | 5 | 7 | 9 |
| RMC | — | — | — | 1 | 1 | 1 | 3 | 5 | 6 | 8 |
| PVC Sch 40 | — | — | — | 1 | 1 | 1 | 3 | 4 | 6 | 8 |
| PVC Sch 80 | — | — | — | 1 | 1 | 1 | 2 | 4 | 5 | 7 |
Neighbouring sizes
Frequently asked
- How many amps can 400 kcmil wire carry?
- 400 kcmil copper is estimated at 295 A at 60 °C, 348 A at 75 °C and 393 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 272 A at 75 °C.
- What breaker size goes with 400 kcmil copper?
- The largest standard overcurrent device that 400 kcmil copper supports at 75 °C is 300 A. For a continuous load, the load itself must not exceed 80% of that rating.
- What is the diameter of 400 kcmil wire?
- The bare conductor is 0.6325 inches (16.065 mm) in diameter, which is 400,000 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.864 inches.
- What is the resistance of 400 kcmil wire?
- About 0.0259 ohms per 1000 feet for copper and 0.0425 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 400 kcmil wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 400 kcmil copper stays under 3% voltage drop for about 2790 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Materials for this size
Materials for this size
Wire, cable and terminations for 400 kcmil.
- THHN/THWN-2 400 kcmil copper400 kcmil copperBuilding wire in the size this page covers (400 kcmil copper).Check price on Amazon →
- NM-B cable 400 kcmil400 kcmilRomex-type cable for dry indoor branch circuits at this size.Check price on Amazon →
- Wire connectors for 400 kcmil400 kcmilPush-in or twist-on connectors rated for this conductor size.Check price on Amazon →
- Terminal lugs & anti-oxidantCopper-rated mechanical lugs for panel and disconnect terminations.Check price on Amazon →
Links go to Amazon search results, not a single listing — confirm ratings, insulation type and listing details before buying, and make sure the part is approved for your application.
Related charts & calculators
- Voltage Drop CalculatorPercent drop, end voltage and minimum conductor for any run length.
- Conduit Fill CalculatorFill percentage and maximum conductor count for EMT, RMC, IMC and PVC.
- Breaker Size CalculatorOvercurrent device rating and minimum conductor from load current.
- Wire Ampacity CalculatorDerated ampacity for ambient temperature and bundled conductors.
- Box Fill CalculatorVolume allowances for conductors, clamps, devices and grounds per NEC 314.16.
- Ampacity ChartEstimated allowable ampacity by conductor size, material and insulation rating.