8 Gauge Wire in mm: Diameter, mm2 & Ampacity
8 gauge wire in mm is 3.26 mm across, with a cross-section of 8.37 mm2. Here is the metric size to order, the ampacity, and the 28% cost of rounding down.
8 gauge wire measures 3.26 mm across. Its cross-section is 8.37 mm², or 16,510 circular mils.
Order 10 mm² to replace it. That is 19% more copper. The size below, 6 mm², is 28% less — one of the steepest round-down penalties on the site, and this is the gauge where that decision costs the most.
The round-down trap
At branch-circuit sizes, taking the smaller metric size is a minor economy. At 8 AWG it is a feeder decision.
| Option | Cross-section | Against 8 AWG | Ampacity (PVC, method C) |
|---|---|---|---|
| 6 mm² | 6 mm² | −28% | 41 A |
| 8 AWG | 8.37 mm² | — | 50 A at 75 °C |
| 10 mm² | 10 mm² | +19% | 57 A |
6 mm² carries 41 A where the 8 AWG it replaces carries 50 A. That is not a rounding difference. On a sub-panel feed it is the difference between a circuit that works and one that nuisance-trips under load.
The temptation is real because 10 mm² is a noticeably bigger cable to pull and terminate. Take it anyway, unless the load calculation genuinely supports 6 mm².
Ampacity
8 AWG copper carries 40 A at 60 °C, 50 A at 75 °C, and 55 A at 90 °C, for not more than three current-carrying conductors at 30 °C ambient. Unlike 10 AWG and smaller, there is no overcurrent device cap overriding the table here — 8 AWG is the first gauge where the ampacity column is the answer.
The metric equivalent at 10 mm², three loaded conductors clipped direct at 30 °C:
| Insulation | 8 AWG | 10 mm² |
|---|---|---|
| PVC / 70–75 °C | 50 A | 57 A |
| XLPE / 90 °C | 55 A | 71 A |
The XLPE gap is worth noticing — 71 A against 55 A. Some of that is the extra copper, and some is that the metric method assumes a cable clipped to a wall with free air around it, which is a friendlier installation than a conductor bundled in a raceway. Do not read the difference as free capacity.
Derating a sub-panel feed
Feeders run through enclosures, and enclosures are hot.
Take 10 mm² PVC clipped direct, in a 40 °C plant room, with four circuits bunched in the same run:
57 A × 0.87 (40 °C) × 0.65 (4 circuits) = 32.2 AThe 57 A conductor is a 32 A conductor there. It would not feed a 40 A sub-panel.
Grouping is the bigger lever at feeder sizes, because feeders travel together. Two circuits is 0.80, four is 0.65, six is 0.57. If you can route them as a single layer on a perforated tray instead of bunched, the factors improve substantially — 0.82 for four circuits rather than 0.65. That routing decision is often worth more than going up a cable size.
8 AWG against the 10 mm² limit
The resistance limit for 10 mm² is 1.83 Ω/km. An 8 AWG conductor measures 2.06 Ω/km, missing by 13%.
So 8 AWG cannot be supplied against a 10 mm² specification. Going the other way — replacing an 8 AWG design with 10 mm² — is fine and gives margin. The pattern and its two exceptions are covered in why AWG doesn't match metric sizes.
Conductor diameter
3.26 mm is solid wire, and solid 8 AWG is uncommon — at this size cable is stranded. At 10 mm²:
| Construction | Max diameter | Min strands |
|---|---|---|
| Class 1, solid | 3.7 mm | 1 |
| Class 2, stranded | 4.2 mm | 7 |
| Class 5, flexible | 5.1 mm | — |
Class 5 at 5.1 mm is 56% wider than the 3.26 mm solid figure. Size glands and ferrules off the construction you are actually receiving — see conductor classes 1, 2, 5 and 6.
Where 8 AWG is used
Sub-panel feeders, 40 and 50 A circuits, ranges and cooktops, larger air conditioning units, EV charging circuits, and equipment grounding conductors on bigger services.
It sits at the boundary between branch circuit and distribution work, which is why the round-down question comes up here more than anywhere else.
FAQ
How many mm is 8 gauge wire?
3.26 mm in diameter for solid wire, which is 128.5 mils. Stranded 8 AWG measures wider, up to 5.1 mm for flexible constructions at the equivalent metric size.
What is 8 AWG in mm²?
8.37 mm². The metric cable size to order is 10 mm², which is 19% larger.
How many amps can 8 gauge wire carry?
40 A at 60 °C, 50 A at 75 °C, 55 A at 90 °C, before derating for ambient temperature and conductor grouping.
Can I use 6 mm² instead of 8 AWG?
It has 28% less copper and carries about 41 A against the 50 A of 8 AWG at 75 °C. On a feeder that is a real downgrade. Only if the load calculation supports it.
Is 8 AWG the same as 10 mm²?
No. 10 mm² has 19% more copper, and an 8 AWG conductor misses the 10 mm² resistance limit by 13%.
Is 8 gauge big enough for a 50 amp circuit?
At 75 °C insulation, 8 AWG copper is rated 50 A before derating. Whether it remains adequate depends on the ambient temperature, the number of conductors sharing the route, and the terminal temperature rating of the equipment at both ends.
Reference
| Property | 8 AWG |
|---|---|
| Diameter, solid | 3.26 mm |
| Diameter, solid | 128.5 mils / 0.1285 in |
| Cross-sectional area | 8.37 mm² |
| Area | 16,510 cmil |
| Resistance, solid copper at 20 °C | 2.06 Ω/km |
| Ampacity, copper 60/75/90 °C | 40 / 50 / 55 A |
| Metric size to order | 10 mm² (+19%) |
| Metric size below | 6 mm² (−28%) |
Work the other direction from 10 mm² to AWG, or use the converter.
Zakaria El Intissar
Electrical Engineer
Electrical engineer with 12+ years of experience in power systems. I create practical electrical calculators and standards-based technical guides that engineers can apply immediately in their daily work.
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