400 kcmil Wire — Amp Rating, Ampacity & Diameter
Ampacity, geometry, resistance, conduit fill and maximum run length for 400 kcmil conductors in copper and aluminium.
Quick answer
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 |
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 |
400 kcmil copper ampacity vs aluminium ampacity
Ampacity by conductor material and insulation temperature rating, at 30 °C ambient with no more than three current-carrying conductors.
| Material | 60 °C (TW, NM-B) | 75 °C (THW, THWN, XHHW) | 90 °C (THHN, THWN-2, XHHW-2) |
|---|---|---|---|
| Copper | 295 A | 348 A | 393 A |
| Aluminium | 230 A | 272 A | 306 A |
The 400 kcmil copper ampacity figure you use on a real circuit is the 75 °C column, 348 A, because breakers and lugs on most equipment are listed for 75 °C even when the conductor insulation is rated 90 °C. The 90 °C value, 393 A, is the starting point for ambient and conductor-count derating only — you derate from 90 °C, then cap the answer at the termination rating.
Aluminium of the same size carries roughly 78% of the copper value (272 A at 75 °C), so an aluminium feeder is normally stepped up one to two sizes to match a copper circuit. NM-B cable is always read from the 60 °C column regardless of the temperature stamped on the conductor insulation.
Source & method: Values are model estimates from conductor geometry and resistivity — see methodology and verify against the NEC edition enforced locally.
Neighbouring sizes
Compare 400 kcmil with the sizes immediately above and below it before sizing a circuit.
400 kcmil amp rating, diameter and metric equivalent
The amp rating for 400 kcmil wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 295 A on a 60 °C conductor, 348 A at 75 °C and 393 A at 90 °C, all at 30 °C ambient with no more than three current-carrying conductors in the raceway. The carrying capacity drops from there in hot spaces or crowded conduit, and terminations rated 75 °C cap what you can actually use.
Geometrically, 400 kcmil is 0.6325 in (16.07 mm) in bare diameter, which is 400,000 circular mils. The cross sectional area works out to 202.68 mm², so the nearest IEC 60228 metric conductor is the next standard size at or above that figure — AWG sizes and mm² sizes never line up exactly, so round up rather than to the closest value.
American Wire Gauge (AWG) is a standard specification for the sizes of solid round wires used as electrical conductors, and the AWG number counts the drawing operations used to reach the diameter. That is why larger gauges are thinner wire and why standardized wire steps follow a fixed ratio: one AWG step changes diameter by about 12.2% and area by about 26%. Above 1 AWG the American standard continues 1/0 to 4/0, then switches to kcmil for anything larger.
Where 400 kcmil actually gets used
400 kcmil handles roughly 335 A in copper and 270 A in aluminium, and it typically appears on 350 A to 400 A service and feeder work: large commercial services, chiller feeds and main switchboard conductors.
At this size the entire design is dominated by terminations and geometry. Lug pads, bus stabs, bending space inside enclosures and the minimum radius of the conductor all constrain the layout, and the wire bending space required at the terminals frequently dictates a larger enclosure than the current would suggest.
Aluminium is the norm at 400 kcmil for cost and weight, especially on long runs where the weight of copper affects support spacing in cable tray and vertical risers. Copper survives where space is tight and the smaller conductor diameter for the same ampacity buys a smaller raceway.
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 is the ampacity of 400 kcmil copper?
- Use 348 A for 400 kcmil copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 295 A instead when the conductor is inside NM-B cable, and treat 393 A as the derating base only. 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.
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.