3 AWG ampacity by temperature column
| Conductor | 60°C | 75°C | 90°C | 240.4(D) |
|---|---|---|---|---|
| Copper | 85 A90 A breaker | 100 A100 A breaker | 115 A125 A breaker | no cap |
| Aluminum | 65 A70 A breaker | 75 A80 A breaker | 85 A90 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 gauge wire
- 3 AWG copper: 90 A on a 60°C column and 100 A on a 75°C column.
- 3 AWG aluminum: 70 A on a 60°C column and 80 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: 90 A on the 60°C column and 100 A on the 75°C column. Where an ampacity falls between standard ratings, 240.4(B) permits the next size up, so 90 A protects the 85 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 70 A at 60°C and 80 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 gauge wire is used for
- 100 A feeders built on 75°C terminations
- Single-conductor pulls in raceway
- Equipment grounding conductor on 400 A circuits
Three gauge carries 85 A at 60°C and 100 A at 75°C, which makes it the smallest conductor that carries a 100 A feeder with nothing left over. That match cuts both ways: there is no room above it for ambient correction or for the adjustment that applies to more than three current-carrying conductors, so any derating moves the answer to 2 AWG or 1/0.
It is uncommon in residential work and rarely appears in cable assemblies, so it is bought as single conductors for raceway. That makes it a conduit size end to end: pulled individually, landed on lugs listed for the conductor material, and read from the 75°C column, which is the only column where it reaches 100 A at all.
Its other role is grounding. Table 250.122 makes 3 AWG copper the equipment grounding conductor for a 400 A circuit, the only row that names it, and Table 250.66 never lists it, so this is a phase conductor and an equipment ground rather than a grounding electrode conductor. Aluminum 3 AWG is a 70 A conductor on a 75°C column.
Grounding with 3 AWG
As an equipment grounding conductor, 3 AWG is sized from the overcurrent device rather than from the phase conductors. Table 250.122 names 3 AWG copper on the 400 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.
Table 250.66 never lists 3 AWG as a grounding electrode conductor, so it is used for phase conductors, neutrals, and equipment grounds rather than for the connection to earth.
Voltage drop and run limits
Chapter 9 Table 8 lists 3 AWG copper at 0.245 ohms per 1000 ft at 75°C and aluminum at 0.403 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 AWG copper at 85 A | 85 ft | 170 ft | 195 ft |
| 3 AWG aluminum at 65 A | 65 ft | 135 ft | 155 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 AWG sits
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 4 AWG copper | 70 A | 85 A | 95 A |
| 3 AWG copper | 85 A | 100 A | 115 A |
| 2 AWG copper | 95 A | 115 A | 130 A |
3 AWG questions
- How many amps can 3 gauge wire handle?
- 3 AWG copper handles 85 A at 60°C, 100 A at 75°C, and 115 A at 90°C. No 240.4(D) cap applies at this size, so the breaker follows the column: 90 A at 60°C and 100 A at 75°C. Aluminum at the same size handles 65 A, 75 A, and 85 A.
- What size breaker for 3 gauge wire?
- 90 A on the 60°C column and 100 A on the 75°C column. Where an ampacity falls between standard ratings, 240.4(B) permits the next size up, so 90 A protects the 85 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 70 A at 60°C and 80 A at 75°C.
- What is 3 gauge wire used for?
- 100 A feeders built on 75°C terminations; single-conductor pulls in raceway; equipment grounding conductor on 400 A circuits. Three gauge carries 85 A at 60°C and 100 A at 75°C, which makes it the smallest conductor that carries a 100 A feeder with nothing left over. That match cuts both ways: there is no room above it for ambient correction or for the adjustment that applies to more than three current-carrying conductors, so any derating moves the answer to 2 AWG or 1/0.
- What size ground wire with 3 gauge conductors?
- 3 AWG copper is the equipment grounding conductor for 400 A circuits in Table 250.122. 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 AWG aluminum as good as copper?
- It carries less. In Table 310.16, 3 AWG aluminum is 65 A, 75 A, and 85 A against 85 A, 100 A, and 115 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.