250 kcmil Wire — Amp Rating, Ampacity & Diameter
Ampacity, geometry, resistance, conduit fill and maximum run length for 250 kcmil conductors in copper and aluminium.
Quick answer
Copper 75 °C
257A
Aluminium 75 °C
200A
Diameter
0.5000in
Circular mils
250,000
Full specification
| Size designation | 250 kcmil |
|---|---|
| AWG gauge number | n/a (kcmil size) |
| Circular mils | 250,000 |
| kcmil | 250.00 |
| Bare diameter (in) | 0.5000 |
| Bare diameter (mm) | 12.700 |
| Cross-section (mm²) | 126.68 |
| THHN/THWN-2 OD (in) | 0.711 |
| Insulated area (sq in) | 0.3970 |
| Copper resistance (Ω/1000 ft) | 0.0415 |
| Aluminium resistance (Ω/1000 ft) | 0.0680 |
| 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 | 289 A | 264 A | 236 A | 204 A |
| 6 | 231 A | 211 A | 189 A | 163 A |
| 9 | 202 A | 185 A | 165 A | 143 A |
| 15 | 145 A | 132 A | 118 A | 102 A |
| 25 | 130 A | 119 A | 106 A | 92 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 257 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 250 kcmil copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 3,488 ft | 6,976 ft | 6,982 ft | 16,112 ft |
| 15 A | 2,325 ft | 4,651 ft | 4,654 ft | 10,741 ft |
| 20 A | 1,744 ft | 3,488 ft | 3,491 ft | 8,056 ft |
| 30 A | 1,162 ft | 2,325 ft | 2,327 ft | 5,370 ft |
| 40 A | 872 ft | 1,744 ft | 1,745 ft | 4,028 ft |
| 50 A | 697 ft | 1,395 ft | 1,396 ft | 3,222 ft |
How many 250 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 | 5 | 8 | 11 | 14 |
| IMC | — | — | 1 | 1 | 1 | 3 | 5 | 7 | 10 | 13 |
| RMC | — | — | 1 | 1 | 1 | 3 | 4 | 7 | 10 | 12 |
| PVC Sch 40 | — | — | 1 | 1 | 1 | 3 | 4 | 7 | 9 | 12 |
| PVC Sch 80 | — | — | — | 1 | 1 | 2 | 4 | 6 | 8 | 11 |
250 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 | 217 A | 257 A | 289 A |
| Aluminium | 170 A | 200 A | 226 A |
The 250 kcmil copper ampacity figure you use on a real circuit is the 75 °C column, 257 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, 289 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 (200 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 250 kcmil with the sizes immediately above and below it before sizing a circuit.
Circuits that use 250 kcmil
Breaker ratings whose smallest copper conductor at 75 °C terminations is 250 kcmil.
250 kcmil amp rating, diameter and metric equivalent
The amp rating for 250 kcmil wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 217 A on a 60 °C conductor, 257 A at 75 °C and 289 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, 250 kcmil is 0.5000 in (12.70 mm) in bare diameter, which is 250,000 circular mils. The cross sectional area works out to 126.67 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 250 kcmil actually gets used
250 kcmil is the first size past the AWG series and the smallest conductor normally specified by circular-mil area rather than gauge. Copper at this size handles roughly 255 A; aluminium lands near 205 A. Typical uses are 200 to 250 A commercial feeders, transformer secondaries and switchboard taps.
Diminishing returns become visible here. Because ampacity rises much more slowly than area, doubling copper area past 250 kcmil buys far less current than the same doubling did at 12 AWG. That is the fundamental reason engineers switch from one large conductor to parallel sets rather than continuing up the size ladder.
Installation practice changes too: 250 kcmil is almost never pulled by hand, requires a cable puller on any run with bends, and typically lands on compression lugs installed with a hydraulic crimper and a die matched to the conductor. Cable tray and larger PVC underground runs become common at this size.
Frequently asked
- How many amps can 250 kcmil wire carry?
- 250 kcmil copper is estimated at 217 A at 60 °C, 257 A at 75 °C and 289 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 200 A at 75 °C.
- What is the ampacity of 250 kcmil copper?
- Use 257 A for 250 kcmil copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 217 A instead when the conductor is inside NM-B cable, and treat 289 A as the derating base only. Aluminium of the same size is about 200 A at 75 °C.
- What breaker size goes with 250 kcmil copper?
- The largest standard overcurrent device that 250 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 250 kcmil wire?
- The bare conductor is 0.5000 inches (12.700 mm) in diameter, which is 250,000 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.711 inches.
- What is the resistance of 250 kcmil wire?
- About 0.0415 ohms per 1000 feet for copper and 0.0680 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 250 kcmil wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 250 kcmil copper stays under 3% voltage drop for about 1744 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Related charts & calculators
- 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.
- Wire Gauge ChartAWG diameter, circular mils, resistance and amp capacity in one table.
- Wire Size ChartWhat size wire for each breaker rating and typical circuit.
- kcmil Wire Size ChartMCM to AWG conversion, circular mils, resistance and ampacity for 250-750 kcmil.