300 kcmil Wire Specs
Ampacity, geometry, resistance, conduit fill and maximum run length for 300 kcmil conductors in copper and aluminium.
Quick answer
300 kcmil copper carries about 289 A at 75 °C (245 A at 60 °C, 326 A at 90 °C). The bare conductor is 0.5477 in (13.91 mm) and 300,000 circular mils, with 0.0346 Ω per 1000 ft. It pairs with a 250 A breaker at 75 °C terminations.
Copper 75 °C
289A
Aluminium 75 °C
225A
Diameter
0.5477in
Circular mils
300,000
Full specification
| Size designation | 300 kcmil |
|---|---|
| AWG gauge number | n/a (kcmil size) |
| Circular mils | 300,000 |
| kcmil | 300.00 |
| Bare diameter (in) | 0.5477 |
| Bare diameter (mm) | 13.912 |
| Cross-section (mm²) | 152.00 |
| THHN/THWN-2 OD (in) | 0.766 |
| Insulated area (sq in) | 0.4608 |
| Copper resistance (Ω/1000 ft) | 0.0346 |
| Aluminium resistance (Ω/1000 ft) | 0.0567 |
| Max standard breaker (Cu, 75 °C) | 250 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 | 326 A | 298 A | 266 A | 230 A |
| 6 | 261 A | 238 A | 213 A | 184 A |
| 9 | 228 A | 208 A | 186 A | 161 A |
| 15 | 163 A | 149 A | 133 A | 115 A |
| 25 | 147 A | 134 A | 120 A | 104 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 289 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 300 kcmil copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 4,186 ft | 8,372 ft | 8,378 ft | 19,335 ft |
| 15 A | 2,790 ft | 5,581 ft | 5,585 ft | 12,890 ft |
| 20 A | 2,093 ft | 4,186 ft | 4,189 ft | 9,667 ft |
| 30 A | 1,395 ft | 2,790 ft | 2,792 ft | 6,445 ft |
| 40 A | 1,046 ft | 2,093 ft | 2,094 ft | 4,833 ft |
| 50 A | 837 ft | 1,674 ft | 1,675 ft | 3,867 ft |
How many 300 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 | 2 | 5 | 7 | 10 | 12 |
| IMC | — | — | 1 | 1 | 1 | 3 | 4 | 6 | 9 | 11 |
| RMC | — | — | 1 | 1 | 1 | 2 | 4 | 6 | 8 | 11 |
| PVC Sch 40 | — | — | — | 1 | 1 | 2 | 4 | 6 | 8 | 10 |
| PVC Sch 80 | — | — | — | 1 | 1 | 1 | 3 | 5 | 7 | 9 |
Neighbouring sizes
Frequently asked
- How many amps can 300 kcmil wire carry?
- 300 kcmil copper is estimated at 245 A at 60 °C, 289 A at 75 °C and 326 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 225 A at 75 °C.
- What breaker size goes with 300 kcmil copper?
- The largest standard overcurrent device that 300 kcmil copper supports at 75 °C is 250 A. For a continuous load, the load itself must not exceed 80% of that rating.
- What is the diameter of 300 kcmil wire?
- The bare conductor is 0.5477 inches (13.912 mm) in diameter, which is 300,000 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.766 inches.
- What is the resistance of 300 kcmil wire?
- About 0.0346 ohms per 1000 feet for copper and 0.0567 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 300 kcmil wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 300 kcmil copper stays under 3% voltage drop for about 2093 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Materials for this size
Materials for this size
Wire, cable and terminations for 300 kcmil.
- THHN/THWN-2 300 kcmil copper300 kcmil copperBuilding wire in the size this page covers (300 kcmil copper).Check price on Amazon →
- NM-B cable 300 kcmil300 kcmilRomex-type cable for dry indoor branch circuits at this size.Check price on Amazon →
- Wire connectors for 300 kcmil300 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.