300 kcmil Wire — Ampacity, Conduit Fill & Dimensions
Everything the code tables say about 300 kcmil conductors on one page: ampacity by temperature rating, breaker limits, conduit fill, physical size, resistance and voltage-drop distances.
300 kcmil copper carries 240 A at 60°C, 285 A at 75°C and 320 A at 90°C.
300 kcmil ampacity
| Copper | Aluminum | |||||
|---|---|---|---|---|---|---|
| Installation | 60°C | 75°C | 90°C | 60°C | 75°C | 90°C |
| Raceway, cable or earth — Table 310.16 | 240 | 285 | 320 | 195 | 230 | 260 |
| Single conductor in free air — Table 310.17 | 375 | 445 | 500 | 290 | 350 | 395 |
Typical use: 285 A copper; 300 kcmil aluminum is widely used for 250 A equipment.
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.
300 kcmil conduit fill
| Conduit | 1/2" | 3/4" | 1" | 1-1/4" | 1-1/2" | 2" | 2-1/2" | 3" | 4" |
|---|---|---|---|---|---|---|---|---|---|
| EMT | — | — | — | 1 | 1 | 3 | 5 | 7 | 13 |
| PVC Schedule 40 | — | — | — | 1 | 1 | 3 | 4 | 6 | 11 |
| PVC Schedule 80 | — | — | — | 1 | 1 | 1 | 3 | 5 | 9 |
| Rigid Metal Conduit | — | — | 1 | 1 | 1 | 3 | 4 | 6 | 11 |
| IMC | — | — | 1 | 1 | 1 | 3 | 4 | 7 | 12 |
| Flexible Metal Conduit | — | — | — | 1 | 1 | 3 | 4 | 6 | 11 |
| Liquidtight Flexible Metal Conduit | — | — | 1 | 1 | 1 | 3 | 4 | 6 | 11 |
| ENT | — | — | — | 1 | 1 | 3 | — | — | — |
Mixing sizes or using another insulation? Use the conduit fill calculator.
300 kcmil dimensions
| Insulation | Diameter (in) | Diameter (mm) | Area (in²) | Area (mm²) |
|---|---|---|---|---|
| THHN, THWN, THWN-2 | 0.766 | 19.46 | 0.4608 | 297.3 |
| XHHW, XHHW-2, XHH | 0.760 | 19.30 | 0.4536 | 292.6 |
| TW, THW, THHW, THW-2 | 0.820 | 20.83 | 0.5281 | 340.7 |
| RHH, RHW, RHW-2 (with outer covering) | 0.950 | 24.13 | 0.7088 | 457.3 |
300 kcmil voltage drop
| Load | 120 V 1-phase | 240 V 1-phase | 208 V 3-phase | 480 V 3-phase |
|---|---|---|---|---|
| 100 A | 419 ft | 839 ft | 839 ft | 1937 ft |
| 125 A | 335 ft | 671 ft | 671 ft | 1550 ft |
| 150 A | 279 ft | 559 ft | 559 ft | 1291 ft |
| 200 A | 209 ft | 419 ft | 419 ft | 968 ft |
| 250 A | 167 ft | 335 ft | 335 ft | 775 ft |
Based on 0.0429 Ω per 1000 ft for stranded copper at 75°C (0.0707 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 300 kcmil wire carry?
300 kcmil copper is rated 240 A at 60°C, 285 A at 75°C and 320 A at 90°C when run in conduit or cable. Aluminum is rated 195, 230 and 260 A. Most terminations are rated 75°C, so the 75°C figure is the one normally used.
What size conduit do I need for 300 kcmil wire?
Three 300 kcmil THHN conductors need at least 2-inch EMT. Add the ground wire’s area and check the chart above for larger bundles.
What is the diameter of 300 kcmil wire?
The bare conductor is 300,000 circular mils (152.01 mm²). With THHN insulation the overall diameter is about 0.766 in (19.46 mm) and the area is 0.4608 in², which is the figure used for conduit fill.
How far can I run 300 kcmil wire?
For a 3% voltage drop on a 240 V single-phase circuit carrying 250 A, about 335 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.