4 AWG ampacity by temperature column
| Conductor | 60°C | 75°C | 90°C | 240.4(D) |
|---|---|---|---|---|
| Copper | 70 A70 A breaker | 85 A90 A breaker | 95 A100 A breaker | no cap |
| Aluminum | 55 A60 A breaker | 65 A70 A breaker | 75 A80 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 4 gauge wire
- 4 AWG copper: 70 A on a 60°C column and 90 A on a 75°C column.
- 4 AWG aluminum: 60 A on a 60°C column and 70 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: 70 A on the 60°C column and 90 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 75°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 60 A at 60°C and 70 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 4 gauge wire is used for
- 70 A to 90 A feeders and small services
- Concrete-encased grounding electrode conductors
- Equipment grounding conductor on 300 A circuits
Four gauge copper carries 70 A at 60°C, which is itself a standard rating, and 85 A at 75°C, which is not, so 240.4(B) permits the next standard rating of 90 A on the higher column. The higher figure is why 4 AWG is the usual feeder for a 90 A panel or a small service. Aluminum at this size carries 55 A, 65 A, and 75 A, which lands on a 60 A or a 70 A device.
It is the largest grounding electrode conductor 250.66(B) requires for a concrete-encased electrode, and Table 250.66 calls for 4 AWG copper where the service conductors are 2/0 or 3/0 copper, which is a typical 200 A residential service. Table 250.122 makes 4 AWG copper the equipment grounding conductor for a 300 A circuit.
The 60°C and 75°C answers straddle two standard ratings, so the same conductor is a 70 A circuit or a 90 A circuit depending on nothing but the termination markings, which 110.14(C) is what settles. At this size the terminals are lugs in a panel rather than devices in a box. Voltage drop is the usual reason to go one size up on a long feeder, and 250.122(B) then raises the equipment ground with the phase conductors.
Grounding with 4 AWG
As an equipment grounding conductor, 4 AWG is sized from the overcurrent device rather than from the phase conductors. Table 250.122 names 4 AWG copper on the 300 A row and 4 AWG aluminum on the 200 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, 4 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 2/0 or 3/0 AWG, and aluminum where the service runs 2/0 or 3/0 AWG. It is the largest grounding electrode conductor 250.66(B) requires for a concrete-encased electrode.
Voltage drop and run limits
Chapter 9 Table 8 lists 4 AWG copper at 0.308 ohms per 1000 ft at 75°C and aluminum at 0.508 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 |
|---|---|---|---|
| 4 AWG copper at 70 A | 80 ft | 165 ft | 190 ft |
| 4 AWG aluminum at 55 A | 60 ft | 125 ft | 145 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 4 AWG sits
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 6 AWG copper | 55 A | 65 A | 75 A |
| 4 AWG copper | 70 A | 85 A | 95 A |
| 3 AWG copper | 85 A | 100 A | 115 A |
4 AWG questions
- How many amps can 4 gauge wire handle?
- 4 AWG copper handles 70 A at 60°C, 85 A at 75°C, and 95 A at 90°C. No 240.4(D) cap applies at this size, so the breaker follows the column: 70 A at 60°C and 90 A at 75°C. Aluminum at the same size handles 55 A, 65 A, and 75 A.
- What size breaker for 4 gauge wire?
- 70 A on the 60°C column and 90 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 75°C ampacity. It relaxes the device rather than the conductor, so a load at the full rating still needs the larger size. Aluminum takes 60 A at 60°C and 70 A at 75°C.
- What is 4 gauge wire used for?
- 70 A to 90 A feeders and small services; concrete-encased grounding electrode conductors; equipment grounding conductor on 300 A circuits. Four gauge copper carries 70 A at 60°C, which is itself a standard rating, and 85 A at 75°C, which is not, so 240.4(B) permits the next standard rating of 90 A on the higher column. The higher figure is why 4 AWG is the usual feeder for a 90 A panel or a small service. Aluminum at this size carries 55 A, 65 A, and 75 A, which lands on a 60 A or a 70 A device.
- What size ground wire with 4 gauge conductors?
- 4 AWG copper is the equipment grounding conductor for 300 A circuits in Table 250.122, and 4 AWG aluminum is the ground for 200 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 4 gauge a grounding electrode conductor?
- Yes, in one place. 4 AWG copper is the grounding electrode conductor where the service conductors are 2/0 or 3/0 AWG, and 4 AWG aluminum is the grounding electrode conductor where the service conductors are 2/0 or 3/0 AWG. It is the largest grounding electrode conductor 250.66(B) requires for a concrete-encased electrode.
- Is 4 AWG aluminum as good as copper?
- It carries less. In Table 310.16, 4 AWG aluminum is 55 A, 65 A, and 75 A against 70 A, 85 A, and 95 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.