300 kcmil Wire — Amp Rating, Ampacity & Diameter
Ampacity, geometry, resistance, conduit fill and maximum run length for 300 kcmil conductors in copper and aluminium.
Quick answer
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 |
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 |
300 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 | 245 A | 289 A | 326 A |
| Aluminium | 191 A | 225 A | 254 A |
The 300 kcmil copper ampacity figure you use on a real circuit is the 75 °C column, 289 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, 326 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 (225 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 300 kcmil with the sizes immediately above and below it before sizing a circuit.
300 kcmil amp rating, diameter and metric equivalent
The amp rating for 300 kcmil wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 245 A on a 60 °C conductor, 289 A at 75 °C and 326 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, 300 kcmil is 0.5477 in (13.91 mm) in bare diameter, which is 300,000 circular mils. The cross sectional area works out to 152.01 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 300 kcmil actually gets used
300 kcmil serves feeders in the 240 to 285 A copper range and around 230 A in aluminium. It appears on 250 to 300 A commercial services, generator and automatic transfer switch conductors, and larger motor feeders.
This size is often the result of a voltage-drop calculation rather than an ampacity calculation. On long industrial runs the conductor is upsized purely to hold percentage drop at the equipment terminals, and a conductor selected for drop is frequently one or two sizes above what the current alone would require.
Because 300 kcmil is bulky and stiff, the raceway route matters as much as the size. Pull boxes at bend intervals, larger sweep radii on underground elbows and 3 to 4 inch raceway are all typical, and the labour to install one 300 kcmil set can exceed the labour for two parallel smaller sets.
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 is the ampacity of 300 kcmil copper?
- Use 289 A for 300 kcmil copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 245 A instead when the conductor is inside NM-B cable, and treat 326 A as the derating base only. 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.
Related charts & calculators
- Allen Wrench Size ChartHex key sizes with SAE and metric cross-reference.
- 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.