6 mm² converts to 9 AWG by cross-sectional area. As with the sizes either side of it, that gauge is not stocked and does not appear in the ampacity tables.
The real choice is 10 AWG or 8 AWG — and this is the one size where the undersized option comes genuinely close.
The closest near-miss on the site
For 6 mm² the resistance limit is 3.08 Ω/km for solid and stranded, 3.30 Ω/km for flexible.
| Gauge | Cross-section | Against 6 mm² | Resistance | Result |
|---|---|---|---|---|
| 10 AWG | 5.26 mm² | −12% | 3.28 Ω/km | fails by 6% |
| 9 AWG | 6.63 mm² | +11% | 2.60 Ω/km | passes |
| 8 AWG | 8.37 mm² | +40% | 2.06 Ω/km | passes |
A 6% miss is one of the narrowest anywhere. Most of the range sits at 12 or 13% — 14 AWG against 2.5 mm², 12 AWG against 4 mm², 6 AWG against 16 mm². Only 1 AWG, 3/0 and 4/0 come closer, at about 5% each.
It still fails. But if you are looking for the size where an AWG substitution is most nearly defensible on resistance alone, this is it.
The catch that undoes it
10 AWG is rated 30 A at 60 °C, 35 A at 75 °C and 40 A at 90 °C. Then the overcurrent protection rules cap the device at 30 A regardless of which column you used.
6 mm² clipped direct is rated 41 A for PVC and 52 A for XLPE.
So the two conductors are close on resistance and far apart on usable current. A 6 mm² circuit designed around 40 A cannot be served by a 10 AWG conductor sitting behind a 30 A breaker, no matter how near the resistance figures look.
That is the trap on this size. The number that suggests the substitution is fine is not the number that governs it.
8 AWG overshoots by 40%
The safe substitute is 8 AWG, and it is the largest overshoot among the common metric sizes — 8.37 mm² against 6 mm².
| Insulation | 6 mm² | 10 AWG | 8 AWG |
|---|---|---|---|
| PVC / 70–75 °C | 41 A | 35 A | 50 A |
| XLPE / 90 °C | 52 A | 40 A | 55 A |
8 AWG at 50 A comfortably covers what 6 mm² carries, with no overcurrent cap in the way. The cost is 40% more copper than the design specified.
There is no middle option. 9 AWG would be almost exactly right and you cannot buy it.
When 10 AWG works anyway
Most of the time, in practice — because most 6 mm² circuits are 32 A or less, and 10 AWG behind a 30 A breaker covers a large share of what 6 mm² actually gets used for.
Where it does not work:
- Circuits above 30 A. The overcurrent cap ends the discussion.
- Long runs. 3.28 Ω/km against 2.06 for 8 AWG is 59% more voltage drop per metre.
- Written specifications. The 6% resistance miss is still a miss.
Run the ampacity and volt-drop numbers for the actual route rather than reasoning from how close the resistance figures look.
Conductor diameter
A solid conductor of 6 mm² works out to 2.764 mm across. Both solid and stranded are common at this size.
| Construction | Max diameter | Min strands | Max strand dia. |
|---|---|---|---|
| Class 1, solid | 2.9 mm | 1 | — |
| Class 2, stranded | 3.3 mm | 7 | — |
| Class 5, flexible | 3.9 mm | — | 0.31 mm |
| Class 6, very flexible | 3.9 mm | — | 0.21 mm |
Solid 10 AWG is 2.59 mm and solid 8 AWG is 3.26 mm.
The spread from 2.9 mm solid to 3.9 mm flexible is 34%, which at this size is a ferrule size and often a gland size. See conductor classes 1, 2, 5 and 6.
Where 6 mm² is used
Cooker circuits, immersion heaters, shower supplies on smaller ratings, sub-circuits to outbuildings, and solar PV string cable. It sits above general-purpose 4 mm² and below the sub-main range.
On PV runs it is chosen for voltage drop rather than current, which is precisely the case where 10 AWG is the wrong substitution.
FAQ
What is 6 mm² in AWG?
9 AWG by cross-sectional area, at 6.63 mm². 9 AWG is not stocked and is absent from the ampacity tables, so the practical choice is 10 AWG or 8 AWG.
Can I use 10 AWG instead of 6 mm²?
10 AWG has 12% less copper and misses the 6 mm² resistance limit by 6% — the closest near-miss of any size. The bigger problem is the 30 A overcurrent cap on 10 AWG, against 41 A for 6 mm². Below 30 A it is usually fine; above that it is not an option.
Is 6 mm² the same as 8 AWG?
No. 8 AWG is 8.37 mm², 40% larger. It is the nearest stocked gauge that is not smaller than 6 mm².
Why can I not buy 9 AWG?
Odd-numbered gauges are defined in the wire gauge standard but distributors carry even gauges and the 0-series. 9 AWG is also missing from the standard ampacity tables. See how the AWG system works.
What is 6 mm² in mm diameter?
A solid conductor is 2.764 mm across. Stranded measures up to 3.3 mm, flexible up to 3.9 mm.
How many amps is 6 mm² cable?
41 A for PVC clipped direct, 52 A for XLPE, with three loaded conductors at 30 °C ambient. Both fall once corrected for the actual ambient temperature and the number of circuits sharing the route.
Reference
| Property | 6 mm² |
|---|---|
| Closest AWG by area | 9 AWG (6.63 mm², +11%) — not stocked |
| Commercial substitute | 8 AWG (8.37 mm², +40%) |
| Undersized option | 10 AWG (5.26 mm², −12%, fails by 6%) |
| Equivalent solid diameter | 2.764 mm |
| Max resistance, class 1 and 2 | 3.08 Ω/km |
| Max resistance, class 5 and 6 | 3.30 Ω/km |
| Max conductor diameter | 2.9 / 3.3 / 3.9 mm by class |
| Ampacity, PVC clipped direct | 41 A before derating |
Work the other direction from 10 AWG or 8 AWG, or use the converter.