8 AWG ampacity by temperature column
| Conductor | 60°C | 75°C | 90°C | 240.4(D) |
|---|---|---|---|---|
| Copper | 40 A40 A breaker | 50 A50 A breaker | 55 A60 A breaker | no cap |
| Aluminum | 35 A35 A breaker | 40 A40 A breaker | 45 A45 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 8 gauge wire
- 8 AWG copper: 40 A on a 60°C column and 50 A on a 75°C column.
- 8 AWG aluminum: 35 A on a 60°C column and 40 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: 40 A on the 60°C column and 50 A on the 75°C column. Aluminum at this size takes 35 A at 60°C and 40 A at 75°C. 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 8 gauge wire is used for
- 40 A range and subpanel circuits in NM-B cable
- 50 A circuits in conduit with 75°C terminations
- Grounding electrode conductor on small services
Eight gauge is the first size the small-conductor rule leaves alone, so its breaker follows the column ampacity directly: 40 A in the 60°C column, which is what 8/3 NM-B provides, and 50 A in the 75°C column, which is what the same conductor carries in conduit. The gap between those two answers is a termination question rather than a conductor question, and it is why one size is sold as a 40 A cable and as a 50 A conductor.
It is also the smallest grounding electrode conductor Table 250.66 ever calls for: where the service conductors are 2 AWG copper or 1/0 aluminum or smaller, the grounding electrode conductor is 8 AWG copper or 6 AWG aluminum. 250.66(A) then caps the sole connection to a rod, pipe, or plate at 6 AWG copper, which is why ground rods stop needing larger conductors long before the service does.
Aluminum at this size carries one standard step less than copper in every column, so an aluminum 8 AWG replaces copper 8 AWG only where the circuit is 40 A or below, and the terminations have to be listed for it. Box fill is 3.00 cubic inches per counted conductor under Table 314.16(B). Above this size the conductors get hard to bend inside a device box, and terminations move to lugs in a panel or a disconnect.
Grounding with 8 AWG
As an equipment grounding conductor, 8 AWG is sized from the overcurrent device rather than from the phase conductors. Table 250.122 names 8 AWG copper on the 100 A row and 8 AWG aluminum on the 60 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, 8 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 AWG or smaller. 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 8 AWG copper at 0.778 ohms per 1000 ft at 75°C and aluminum at 1.28 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 |
|---|---|---|---|
| 8 AWG copper at 40 A | 55 ft | 115 ft | 130 ft |
| 8 AWG aluminum at 35 A | 40 ft | 80 ft | 90 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.
Box fill with 8 AWG
Table 314.16(B) allows 3.00 cubic inches per counted conductor at this size. Every conductor that originates outside a box and terminates or is spliced inside it counts once, each device yoke counts as two of the largest conductor in the box, and all the equipment grounding conductors together count as one. The total is compared against the box volume from Table 314.16(A). Table 314.16(B) stops at 6 AWG, so a larger conductor is sized by the pull and bend rules in 314.28 instead.
Where 8 AWG sits
| Conductor | 60°C | 75°C | 90°C |
|---|---|---|---|
| 10 AWG copper | 30 A | 35 A | 40 A |
| 8 AWG copper | 40 A | 50 A | 55 A |
| 6 AWG copper | 55 A | 65 A | 75 A |
8 AWG questions
- How many amps can 8 gauge wire handle?
- 8 AWG copper handles 40 A at 60°C, 50 A at 75°C, and 55 A at 90°C. No 240.4(D) cap applies at this size, so the breaker follows the column: 40 A at 60°C and 50 A at 75°C. Aluminum at the same size handles 35 A, 40 A, and 45 A.
- What size breaker for 8 gauge wire?
- 40 A on the 60°C column and 50 A on the 75°C column. Aluminum takes 35 A at 60°C and 40 A at 75°C.
- What is 8 gauge wire used for?
- 40 A range and subpanel circuits in NM-B cable; 50 A circuits in conduit with 75°C terminations; grounding electrode conductor on small services. Eight gauge is the first size the small-conductor rule leaves alone, so its breaker follows the column ampacity directly: 40 A in the 60°C column, which is what 8/3 NM-B provides, and 50 A in the 75°C column, which is what the same conductor carries in conduit. The gap between those two answers is a termination question rather than a conductor question, and it is why one size is sold as a 40 A cable and as a 50 A conductor.
- What size ground wire with 8 gauge conductors?
- 8 AWG copper is the equipment grounding conductor for 100 A circuits in Table 250.122, and 8 AWG aluminum is the ground for 60 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 8 gauge a grounding electrode conductor?
- Yes, in one place. 8 AWG copper is the grounding electrode conductor where the service conductors are 2 AWG or smaller.
- Is 8 AWG aluminum as good as copper?
- It carries less. In Table 310.16, 8 AWG aluminum is 35 A, 40 A, and 45 A against 40 A, 50 A, and 55 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.