6 AWG vs 8 AWG
| 6 AWG | 8 AWG | |
|---|---|---|
| Diameter | 4.115 mm | 3.264 mm |
| Cross-section | 13.3 mm² | 8.37 mm² |
| Resistance (copper) | 1.3 Ω/km | 2.06 Ω/km |
| NEC 60 °C ampacity | 55 A | 40 A |
Which is thicker
6 AWG has 1.59× the copper of 8 AWG in cross-section, and carries current with 37% less resistance per metre. AWG numbers run backwards because the scale counts drawing passes: more passes, thinner wire, higher number.
Voltage lost over a run at 40 A
| One-way run | 6 AWG | 8 AWG |
|---|---|---|
| 10 m (33 ft) | 1.04 V (0.9%) | 1.65 V (1.4%) |
| 20 m (66 ft) | 2.07 V (1.7%) | 3.3 V (2.7%) |
| 30 m (98 ft) | 3.11 V (2.6%) | 4.95 V (4.1%) over 3% |
| 50 m (164 ft) | 5.18 V (4.3%) over 3% | 8.24 V (6.9%) over 3% |
Figures are for a 120 V supply and count both conductors, because the current returns along the neutral. 8 AWG stays inside 3% out to about 21 m; 6 AWG reaches 34 m. That gap is the usual reason to upsize a wire the ampacity table already allows.
What each is rated to carry
On the NEC 310.16 60 °C column, 6 AWG copper is rated 55 A and 8 AWG is rated 40 A. Those figures set the breaker, and they are not the whole story: insulation type, ambient temperature, how many conductors share a conduit and your local code all move them. Circuit design is an electrician's call, not a table's.
Frequently asked questions
What is the difference between 6 AWG and 8 AWG?
6 AWG is the thicker wire: 4.115 mm across against 3.264 mm, with 1.59× the cross-section. Thicker copper means lower resistance — 1.3 Ω/km against 2.06 Ω/km — so less voltage is lost along a run.
Can I use 6 AWG instead of 8 AWG?
Going thicker is electrically safe — 6 AWG has less resistance and runs cooler than 8 AWG at the same current. The practical limits are cost, stiffness and whether it fits the terminals. Going the other way, to thinner wire, is the direction that needs the ampacity table and an electrician.
How far can 8 AWG run at 40 A?
About 21 m (69 ft) before the drop reaches 3% of a 120 V supply. 6 AWG reaches 34 m (112 ft) at the same current. The 3% figure is the informational note in NEC 210.19, not a hard rule, but it is the number most designs work to.
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