3/0 AWG ampacity by temperature column
| Conductor | 60°C | 75°C | 90°C | 240.4(D) |
|---|---|---|---|---|
| Copper | 165 A175 A breaker | 200 A200 A breaker | 225 A225 A breaker | no cap |
| Aluminum | 130 A150 A breaker | 155 A175 A breaker | 175 A175 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 3/0 wire
- 3/0 AWG copper: 175 A on a 60°C column and 200 A on a 75°C column.
- 3/0 AWG aluminum: 150 A on a 60°C column and 175 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: 175 A on the 60°C column and 200 A on the 75°C column. Where an ampacity falls between standard ratings, 240.4(B) permits the next size up, so 175 A protects the 165 A 60°C ampacity. It relaxes the device rather than the conductor: 210.19(A)(1) still requires an ampacity at least equal to the calculated load. The allowance does not reach a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads, and it stops at an 800 A device. Aluminum at this size takes 150 A at 60°C and 175 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 3/0 wire is used for
- 200 A services and feeders with 75°C terminations
- The largest grounding electrode conductor 250.66 requires
- Equipment grounding conductor on 1200 A circuits
3/0 copper carries 165 A at 60°C and 200 A at 75°C, so it is the conductor that matches a 200 A device without any help from 240.4(B), and the one a 200 A service uses when 310.15(B)(7) is not available or when derating has already eaten the allowance. At 60°C it needs the next standard rating above 165 A, which is 175 A.
It is the largest grounding electrode conductor Table 250.66 ever requires: where the service conductors exceed 1100 kcmil copper or 1750 kcmil aluminum, the grounding electrode conductor is 3/0 copper or 250 kcmil aluminum, and no larger size appears in the table.
Table 250.122 makes 3/0 copper the equipment grounding conductor for a 1200 A circuit. Aluminum 3/0 is a 175 A conductor on a 75°C column, because its 155 A value there is not a standard rating and 240.4(B) permits the next one up, which happens to be 175 A.
Grounding with 3/0 AWG
As an equipment grounding conductor, 3/0 AWG is sized from the overcurrent device rather than from the phase conductors. Table 250.122 names 3/0 AWG copper on the 1200 A row and 3/0 AWG aluminum on the 800 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, 3/0 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 1100 kcmil, and aluminum where the service runs over 500 through 900 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 3/0 AWG copper at 0.0766 ohms per 1000 ft at 75°C and aluminum at 0.126 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 |
|---|---|---|---|
| 3/0 AWG copper at 165 A | 140 ft | 280 ft | 325 ft |
| 3/0 AWG aluminum at 130 A | 105 ft | 215 ft | 250 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 3/0 AWG sits
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 2/0 AWG copper | 145 A | 175 A | 195 A |
| 3/0 AWG copper | 165 A | 200 A | 225 A |
| 4/0 AWG copper | 195 A | 230 A | 260 A |
3/0 AWG questions
- How many amps can 3/0 wire handle?
- 3/0 AWG copper handles 165 A at 60°C, 200 A at 75°C, and 225 A at 90°C. No 240.4(D) cap applies at this size, so the breaker follows the column: 175 A at 60°C and 200 A at 75°C. Aluminum at the same size handles 130 A, 155 A, and 175 A.
- What size breaker for 3/0 wire?
- 175 A on the 60°C column and 200 A on the 75°C column. Where an ampacity falls between standard ratings, 240.4(B) permits the next size up, so 175 A protects the 165 A 60°C ampacity. It relaxes the device rather than the conductor, so a load at the full rating still needs the larger size. Aluminum takes 150 A at 60°C and 175 A at 75°C.
- What is 3/0 wire used for?
- 200 A services and feeders with 75°C terminations; the largest grounding electrode conductor 250.66 requires; equipment grounding conductor on 1200 A circuits. 3/0 copper carries 165 A at 60°C and 200 A at 75°C, so it is the conductor that matches a 200 A device without any help from 240.4(B), and the one a 200 A service uses when 310.15(B)(7) is not available or when derating has already eaten the allowance. At 60°C it needs the next standard rating above 165 A, which is 175 A.
- What size ground wire with 3/0 conductors?
- 3/0 AWG copper is the equipment grounding conductor for 1200 A circuits in Table 250.122, and 3/0 AWG aluminum is the ground for 800 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 3/0 a grounding electrode conductor?
- Yes, in one place. 3/0 AWG copper is the grounding electrode conductor where the service conductors are over 1100 kcmil, and 3/0 AWG aluminum is the grounding electrode conductor where the service conductors are over 500 through 900 kcmil.
- Is 3/0 AWG aluminum as good as copper?
- It carries less. In Table 310.16, 3/0 AWG aluminum is 130 A, 155 A, and 175 A against 165 A, 200 A, and 225 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.