1/0 AWG ampacity by temperature column
| Conductor | 60°C | 75°C | 90°C | 240.4(D) |
|---|---|---|---|---|
| Copper | 125 A125 A breaker | 150 A150 A breaker | 170 A175 A breaker | no cap |
| Aluminum | 100 A100 A breaker | 120 A125 A breaker | 135 A150 A breaker | no cap |
Ampacities are NEC Table 310.16 values for a conductor at 30°C ambient in a raceway, cable, or direct burial with three or fewer current-carrying conductors. Ambient correction and conductor adjustment are applied to these numbers before the 240.4(D) cap is checked, never after it.
What size breaker for 1/0 wire
- 1/0 AWG copper: 125 A on a 60°C column and 150 A on a 75°C column.
- 1/0 AWG aluminum: 100 A on a 60°C column and 125 A on a 75°C column.
No small-conductor cap applies at this size, so the device follows the column the equipment is listed for: 125 A on the 60°C column and 150 A on the 75°C column. Aluminum at this size takes 100 A at 60°C and 125 A at 75°C, since 240.4(B) carries the device to the next standard rating where the ampacity falls between two. 334.80 and 340.80 hold NM-B and UF-B cable to the 60°C column whatever the terminals are marked, so a cable assembly is read from the left-hand column while conductors in raceway are read from the column the equipment is listed for.
What 1/0 wire is used for
- 125 A service-entrance conductors
- Grounding electrode conductors on large services
- Equipment grounding conductor on 800 A circuits
1/0 copper carries 125 A at 60°C and 150 A at 75°C. Both figures are standard ratings, so there is no next-size-up room in either column: a 1/0 conductor serves 125 A or 150 A and nothing between, and any correction or adjustment moves it to 2/0.
It is the grounding electrode conductor Table 250.66 calls for where the service conductors are over 350 through 600 kcmil copper, and the equipment grounding conductor Table 250.122 calls for on an 800 A circuit. In dwelling work it is the 60°C answer for a 125 A service, which is the rating a 100 A service is usually upgraded to.
Aluminum 1/0 is a 125 A conductor, because its 75°C ampacity of 120 A is not a standard rating and 240.4(B) permits the next one up. That is how a 125 A service is built with aluminum conductors one trade size below the copper answer, and why the aluminum service is not the smaller ampacity the table appears to show.
Grounding with 1/0 AWG
As an equipment grounding conductor, 1/0 AWG is sized from the overcurrent device rather than from the phase conductors. Table 250.122 names 1/0 AWG copper on the 800 A row and 1/0 AWG aluminum on the 500 A row. 250.122(B) raises the ground in the same proportion as the phase conductors when they are increased for voltage drop, and 250.122(A) caps it at the size of the conductors it runs with.
As a grounding electrode conductor, 1/0 AWG appears in Table 250.66, which is sized from the largest ungrounded service conductor rather than from the service rating: copper where the service runs over 350 through 600 kcmil, and aluminum where the service runs over 250 through 500 kcmil. The table value is what the service conductor calls for; the caps in 250.66(A) through (C) apply to the electrode connection itself.
Voltage drop and run limits
Chapter 9 Table 8 lists 1/0 AWG copper at 0.122 ohms per 1000 ft at 75°C and aluminum at 0.201 ohms per 1000 ft. Resistance is what a voltage-drop calculation multiplies by current and length, so a conductor twice as long drops twice the volts.
| Conductor | 120 V single phase | 240 V single phase | 240 V three phase |
|---|---|---|---|
| 1/0 AWG copper at 125 A | 115 ft | 235 ft | 270 ft |
| 1/0 AWG aluminum at 100 A | 85 ft | 175 ft | 205 ft |
NEC 210.19(A)(1) Informational Note 4 recommends keeping branch-circuit drop at or below 3%. The lengths above are floored to the nearest 5 ft so a listed length never exceeds the computed limit, and they leave reactance out, which makes a large conductor in steel raceway fall short. Halving the current doubles the distance.
Where 1/0 AWG sits
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 1 AWG copper | 110 A | 130 A | 145 A |
| 1/0 AWG copper | 125 A | 150 A | 170 A |
| 2/0 AWG copper | 145 A | 175 A | 195 A |
1/0 AWG questions
- How many amps can 1/0 wire handle?
- 1/0 AWG copper handles 125 A at 60°C, 150 A at 75°C, and 170 A at 90°C. No 240.4(D) cap applies at this size, so the breaker follows the column: 125 A at 60°C and 150 A at 75°C. Aluminum at the same size handles 100 A, 120 A, and 135 A.
- What size breaker for 1/0 wire?
- 125 A on the 60°C column and 150 A on the 75°C column. Aluminum takes 100 A at 60°C and 125 A at 75°C.
- What is 1/0 wire used for?
- 125 A service-entrance conductors; grounding electrode conductors on large services; equipment grounding conductor on 800 A circuits. 1/0 copper carries 125 A at 60°C and 150 A at 75°C. Both figures are standard ratings, so there is no next-size-up room in either column: a 1/0 conductor serves 125 A or 150 A and nothing between, and any correction or adjustment moves it to 2/0.
- What size ground wire with 1/0 conductors?
- 1/0 AWG copper is the equipment grounding conductor for 800 A circuits in Table 250.122, and 1/0 AWG aluminum is the ground for 500 A circuits. The ground is sized from the overcurrent device rather than from the phase conductors, and 250.122(B) raises it in the same proportion if the phase conductors are increased for voltage drop. It never has to be larger than the conductors it runs with.
- Is 1/0 a grounding electrode conductor?
- Yes, in one place. 1/0 AWG copper is the grounding electrode conductor where the service conductors are over 350 through 600 kcmil, and 1/0 AWG aluminum is the grounding electrode conductor where the service conductors are over 250 through 500 kcmil.
- Is 1/0 AWG aluminum as good as copper?
- It carries less. In Table 310.16, 1/0 AWG aluminum is 100 A, 120 A, and 135 A against 125 A, 150 A, and 170 A for copper, so a load that copper serves at this size usually needs one size larger in aluminum. The lugs and devices it lands on have to be listed for aluminum conductors.