400 kcmil Wire — Ampacity, Conduit Fill & Dimensions
Everything the code tables say about 400 kcmil conductors on one page: ampacity by temperature rating, breaker limits, conduit fill, physical size, resistance and voltage-drop distances.
400 kcmil copper carries 280 A at 60°C, 335 A at 75°C and 380 A at 90°C.
400 kcmil ampacity
| Copper | Aluminum | |||||
|---|---|---|---|---|---|---|
| Installation | 60°C | 75°C | 90°C | 60°C | 75°C | 90°C |
| Raceway, cable or earth — Table 310.16 | 280 | 335 | 380 | 225 | 270 | 305 |
| Single conductor in free air — Table 310.17 | 455 | 545 | 615 | 355 | 425 | 480 |
Typical use: 335 A copper; 400 A services are normally built from parallel sets or 500–600 kcmil.
Use the column that matches the lowest temperature rating in the circuit — usually the 75°C terminations on breakers and lugs. The 90°C column is a starting point for derating, not a rating you can load the wire to. NM-B (Romex) cable is always limited to the 60°C column.
As a copper equipment grounding conductor, 400 kcmil is the minimum for circuits protected at up to 3000 A; in aluminum, up to 2000 A. See the grounding conductor table.
400 kcmil conduit fill
| Conduit | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" | 4" |
|---|---|---|---|---|---|---|---|---|---|
| EMT | — | — | — | 1 | 1 | 1 | 4 | 6 | 10 |
| PVC Schedule 40 | — | — | — | 1 | 1 | 1 | 3 | 5 | 8 |
| PVC Schedule 80 | — | — | — | 1 | 1 | 1 | 3 | 4 | 7 |
| Rigid Metal Conduit | — | — | — | 1 | 1 | 1 | 3 | 5 | 8 |
| IMC | — | — | — | 1 | 1 | 1 | 3 | 5 | 9 |
| Flexible Metal Conduit | — | — | — | 1 | 1 | 1 | 3 | 5 | 8 |
| Liquidtight Flexible Metal Conduit | — | — | — | 1 | 1 | 1 | 3 | 5 | 8 |
| ENT | — | — | — | 1 | 1 | 1 | — | — | — |
Mixing sizes or using another insulation? Use the conduit fill calculator.
400 kcmil dimensions
| Insulation | Diameter (in) | Diameter (mm) | Area (in²) | Area (mm²) |
|---|---|---|---|---|
| THHN, THWN, THWN-2 | 0.864 | 21.95 | 0.5863 | 378.3 |
| XHHW, XHHW-2, XHH | 0.858 | 21.79 | 0.5782 | 373.0 |
| TW, THW, THHW, THW-2 | 0.918 | 23.32 | 0.6619 | 427.0 |
| RHH, RHW, RHW-2 (with outer covering) | 1.048 | 26.62 | 0.8626 | 556.5 |
400 kcmil voltage drop
| Load | 120 V 1-phase | 240 V 1-phase | 208 V 3-phase | 480 V 3-phase |
|---|---|---|---|---|
| 125 A | 448 ft | 897 ft | 897 ft | 2071 ft |
| 150 A | 373 ft | 747 ft | 748 ft | 1726 ft |
| 200 A | 280 ft | 560 ft | 561 ft | 1294 ft |
| 250 A | 224 ft | 448 ft | 448 ft | 1035 ft |
| 300 A | 186 ft | 373 ft | 374 ft | 863 ft |
Based on 0.0321 Ω per 1000 ft for stranded copper at 75°C (0.0529 Ω for aluminum). For other loads and lengths use the voltage drop calculator.
All wire sizes
- 14 AWG
- 12 AWG
- 10 AWG
- 8 AWG
- 6 AWG
- 4 AWG
- 3 AWG
- 2 AWG
- 1 AWG
- 1/0 AWG
- 2/0 AWG
- 3/0 AWG
- 4/0 AWG
- 250 kcmil
- 300 kcmil
- 350 kcmil
- 400 kcmil
- 500 kcmil
- 600 kcmil
- 700 kcmil
- 750 kcmil
- 800 kcmil
- 900 kcmil
- 1000 kcmil
Frequently asked questions
How many amps can 400 kcmil wire carry?
400 kcmil copper is rated 280 A at 60°C, 335 A at 75°C and 380 A at 90°C when run in conduit or cable. Aluminum is rated 225, 270 and 305 A. Most terminations are rated 75°C, so the 75°C figure is the one normally used.
What size conduit do I need for 400 kcmil wire?
Three 400 kcmil THHN conductors need at least 2-1/2-inch EMT. Add the ground wire’s area and check the chart above for larger bundles.
What is the diameter of 400 kcmil wire?
The bare conductor is 400,000 circular mils (202.68 mm²). With THHN insulation the overall diameter is about 0.864 in (21.95 mm) and the area is 0.5863 in², which is the figure used for conduit fill.
How far can I run 400 kcmil wire?
For a 3% voltage drop on a 240 V single-phase circuit carrying 300 A, about 373 feet one way in copper. Halve that at 120 V. Lighter loads go proportionally farther.
NECTables.com is an independent reference and is not affiliated with, endorsed by, or a substitute for the National Fire Protection Association (NFPA). NEC® and National Electrical Code® are registered trademarks of the NFPA. Values shown are commonly published engineering data and independent calculations based on the methods of NFPA 70; they are not a reproduction of the Code. Always confirm against the edition adopted in your jurisdiction, including local amendments, and consult a licensed electrician or engineer before doing electrical work.