500 kcmil Wire — Amp Rating, Ampacity & Diameter
Ampacity, geometry, resistance, conduit fill and maximum run length for 500 kcmil conductors in copper and aluminium.
Quick answer
Copper 75 °C
403A
Aluminium 75 °C
314A
Diameter
0.7071in
Circular mils
500,000
Full specification
| Size designation | 500 kcmil |
|---|---|
| AWG gauge number | n/a (kcmil size) |
| Circular mils | 500,000 |
| kcmil | 500.00 |
| Bare diameter (in) | 0.7071 |
| Bare diameter (mm) | 17.960 |
| Cross-section (mm²) | 253.35 |
| THHN/THWN-2 OD (in) | 0.949 |
| Insulated area (sq in) | 0.7073 |
| Copper resistance (Ω/1000 ft) | 0.0207 |
| Aluminium resistance (Ω/1000 ft) | 0.0340 |
| Max standard breaker (Cu, 75 °C) | 400 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 | 454 A | 415 A | 370 A | 321 A |
| 6 | 363 A | 332 A | 296 A | 257 A |
| 9 | 318 A | 290 A | 259 A | 225 A |
| 15 | 227 A | 207 A | 185 A | 160 A |
| 25 | 204 A | 187 A | 167 A | 144 A |
Source & method: The governing value is also capped by the termination rating, commonly 75 °C, which for this size is 403 A. Run your own conditions.
Maximum run length at 3% voltage drop
One-way distance for 500 kcmil copper before drop exceeds 3%.
| Load | 120 V 1Ø | 240 V 1Ø | 208 V 3Ø | 480 V 3Ø |
|---|---|---|---|---|
| 10 A | 6,976 ft | 13,953 ft | 13,964 ft | 32,225 ft |
| 15 A | 4,651 ft | 9,302 ft | 9,309 ft | 21,483 ft |
| 20 A | 3,488 ft | 6,976 ft | 6,982 ft | 16,112 ft |
| 30 A | 2,325 ft | 4,651 ft | 4,654 ft | 10,741 ft |
| 40 A | 1,744 ft | 3,488 ft | 3,491 ft | 8,056 ft |
| 50 A | 1,395 ft | 2,790 ft | 2,792 ft | 6,445 ft |
How many 500 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 | 6 | 8 |
| IMC | — | — | — | 1 | 1 | 1 | 2 | 4 | 5 | 7 |
| RMC | — | — | — | 1 | 1 | 1 | 2 | 4 | 5 | 7 |
| PVC Sch 40 | — | — | — | 1 | 1 | 1 | 2 | 4 | 5 | 7 |
| PVC Sch 80 | — | — | — | — | 1 | 1 | 1 | 3 | 4 | 6 |
500 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 | 341 A | 403 A | 454 A |
| Aluminium | 266 A | 314 A | 354 A |
The 500 kcmil copper ampacity figure you use on a real circuit is the 75 °C column, 403 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, 454 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 (314 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 500 kcmil with the sizes immediately above and below it before sizing a circuit.
Circuits that use 500 kcmil
Breaker ratings whose smallest copper conductor at 75 °C terminations is 500 kcmil.
500 kcmil amp rating, diameter and metric equivalent
The amp rating for 500 kcmil wire depends on the insulation temperature rating, not on the conductor alone. Copper ampacity is about 341 A on a 60 °C conductor, 403 A at 75 °C and 454 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, 500 kcmil is 0.7071 in (17.96 mm) in bare diameter, which is 500,000 circular mils. The cross sectional area works out to 253.35 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 500 kcmil actually gets used
500 kcmil is the standard large-feeder conductor of commercial construction: roughly 380 A copper and 310 A aluminium. Sets of 500 kcmil aluminium are the routine choice for 400 A services, main switchboard feeds and large transformer secondaries.
It is the largest size that most contractors will install as a single conductor per phase before going parallel. Above 500 kcmil the ampacity gained per dollar and per pound falls sharply, so a 600 A or 800 A service is normally built as two or three parallel 400 or 500 kcmil sets rather than one enormous conductor.
Practical constraints at 500 kcmil are almost entirely mechanical: sidewall pressure limits on bends, pulling tension, conductor support in vertical runs, and the number of hydraulic crimps needed at each end. Each parallel set also needs its own equipment grounding provision sized to the circuit.
Frequently asked
- How many amps can 500 kcmil wire carry?
- 500 kcmil copper is estimated at 341 A at 60 °C, 403 A at 75 °C and 454 A at 90 °C, at 30 °C ambient with no more than three current-carrying conductors. Aluminium of the same size is about 314 A at 75 °C.
- What is the ampacity of 500 kcmil copper?
- Use 403 A for 500 kcmil copper on equipment with 75 °C terminations, which covers most breakers and panel lugs. Read 341 A instead when the conductor is inside NM-B cable, and treat 454 A as the derating base only. Aluminium of the same size is about 314 A at 75 °C.
- What breaker size goes with 500 kcmil copper?
- The largest standard overcurrent device that 500 kcmil copper supports at 75 °C is 400 A. For a continuous load, the load itself must not exceed 80% of that rating.
- What is the diameter of 500 kcmil wire?
- The bare conductor is 0.7071 inches (17.960 mm) in diameter, which is 500,000 circular mils. With THHN/THWN-2 insulation the nominal outside diameter is about 0.949 inches.
- What is the resistance of 500 kcmil wire?
- About 0.0207 ohms per 1000 feet for copper and 0.0340 ohms per 1000 feet for aluminium, at 75 °C.
- How far can I run 500 kcmil wire?
- Distance depends on load and voltage. At 20 A on a 120 V single-phase circuit, 500 kcmil copper stays under 3% voltage drop for about 3488 feet one-way. Doubling the voltage roughly doubles the allowable distance.
Related charts & calculators
- 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.
- Wire Gauge ChartAWG diameter, circular mils, resistance and amp capacity in one table.