350 kcmil Wire — Amp Rating, Ampacity & Diameter
Ampacity, geometry, resistance, conduit fill and maximum run length for 350 kcmil conductors in copper and aluminium.
Quick answer
Copper 75 °C
319A
Aluminium 75 °C
249A
Diameter
0.5916in
Circular mils
350,000
Full specification
| Size designation | 350 kcmil |
|---|---|
| AWG gauge number | n/a (kcmil size) |
| Circular mils | 350,000 |
| kcmil | 350.00 |
| Bare diameter (in) | 0.5916 |
| Bare diameter (mm) | 15.027 |
| Cross-section (mm²) | 177.34 |
| THHN/THWN-2 OD (in) | 0.817 |
| Insulated area (sq in) | 0.5242 |
| Copper resistance (Ω/1000 ft) | 0.0296 |
| Aluminium resistance (Ω/1000 ft) | 0.0486 |
| Max standard breaker (Cu, 75 °C) | 300 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 | 360 A | 329 A | 294 A | 255 A |
| 6 | 288 A | 263 A | 235 A | 204 A |
| 9 | 252 A | 230 A | 206 A | 178 A |
| 15 | 180 A | 164 A | 147 A | 127 A |
| 25 | 162 A | 148 A | 132 A | 115 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 319 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 350 kcmil copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 4,883 ft | 9,767 ft | 9,774 ft | 22,557 ft |
| 15 A | 3,255 ft | 6,511 ft | 6,516 ft | 15,038 ft |
| 20 A | 2,441 ft | 4,883 ft | 4,887 ft | 11,278 ft |
| 30 A | 1,627 ft | 3,255 ft | 3,258 ft | 7,519 ft |
| 40 A | 1,220 ft | 2,441 ft | 2,443 ft | 5,639 ft |
| 50 A | 976 ft | 1,953 ft | 1,954 ft | 4,511 ft |
How many 350 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 | 4 | 6 | 8 | 11 |
| IMC | — | — | — | 1 | 1 | 2 | 3 | 6 | 8 | 10 |
| RMC | — | — | — | 1 | 1 | 2 | 3 | 5 | 7 | 9 |
| PVC Sch 40 | — | — | — | 1 | 1 | 1 | 3 | 5 | 7 | 9 |
| PVC Sch 80 | — | — | — | 1 | 1 | 1 | 3 | 4 | 6 | 8 |
350 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 | 271 A | 319 A | 360 A |
| Aluminium | 211 A | 249 A | 281 A |
The 350 kcmil copper ampacity figure you use on a real circuit is the 75 °C column, 319 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, 360 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 (249 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 350 kcmil with the sizes immediately above and below it before sizing a circuit.
Circuits that use 350 kcmil
Breaker ratings whose smallest copper conductor at 75 °C terminations is 350 kcmil.
350 kcmil amp rating, diameter and metric equivalent
The amp rating for 350 kcmil wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 271 A on a 60 °C conductor, 319 A at 75 °C and 360 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, 350 kcmil is 0.5916 in (15.03 mm) in bare diameter, which is 350,000 circular mils. The cross sectional area works out to 177.34 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 350 kcmil actually gets used
350 kcmil is one of the most commonly stocked large sizes, covering roughly 310 A in copper and 250 A in aluminium. It is a standard 300 A and 350 A feeder conductor, widely used on switchboards, large rooftop equipment and building risers.
Its popularity is partly a hardware fact: a great many mechanical and compression lugs, taps and insulated multi-tap connectors are listed with 350 kcmil at the top of their range, so designs gravitate toward it to keep termination hardware ordinary and readily available.
Where a design exceeds what one 350 kcmil set can do, the normal next step is a parallel set rather than 500 kcmil, because two 350 conductors per phase are easier to pull, easier to bend into a switchboard and often cheaper installed than a single much larger conductor.
Frequently asked
- How many amps can 350 kcmil wire carry?
- 350 kcmil copper is estimated at 271 A at 60 °C, 319 A at 75 °C and 360 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 249 A at 75 °C.
- What is the ampacity of 350 kcmil copper?
- Use 319 A for 350 kcmil copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 271 A instead when the conductor is inside NM-B cable, and treat 360 A as the derating base only. Aluminium of the same size is about 249 A at 75 °C.
- What breaker size goes with 350 kcmil copper?
- The largest standard overcurrent device that 350 kcmil copper supports at 75 °C is 300 A. For a continuous load, the load itself must not exceed 80% of that rating.
- What is the diameter of 350 kcmil wire?
- The bare conductor is 0.5916 inches (15.027 mm) in diameter, which is 350,000 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.817 inches.
- What is the resistance of 350 kcmil wire?
- About 0.0296 ohms per 1000 feet for copper and 0.0486 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 350 kcmil wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 350 kcmil copper stays under 3% voltage drop for about 2441 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Related charts & calculators
- 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.
- Conduit Fill ChartMaximum THHN conductors per trade size for every raceway type.