4 mm² converts to 12 AWG by cross-sectional area. 12 AWG is 3.31 mm², which is 17% less copper.
This is the size where the two systems align worst, and there is no comfortable answer.
Table of Contents
Neither candidate fits well
| Gauge | Cross-section | Against 4 mm² | |
|---|---|---|---|
| Closest by area | 12 AWG | 3.31 mm² | 17% less |
| Safe substitute | 10 AWG | 5.26 mm² | 32% more |
12 AWG is meaningfully undersized. 10 AWG is meaningfully oversized. Nothing exists in between, because the AWG series steps 3.31, then 5.26, with no gauge at 4.
At most sizes the substitute overshoots by 20% or so and you accept it. Here it is 32%, which means a full size larger on the cable, the gland, the ferrule, and the conduit fill calculation.
The resistance test
For 4 mm² the limit is 4.61 Ω/km for solid and stranded conductors, 4.95 Ω/km for flexible.
| Gauge | Actual resistance | 4 mm² limit | Result |
|---|---|---|---|
| 12 AWG | 5.21 Ω/km | 4.61 | fails by 13% |
| 10 AWG | 3.28 Ω/km | 4.61 | passes |
12 AWG misses by 13%. If a specification calls for 4 mm² conductor resistance and 12 AWG is supplied against it, the test fails.
So the answer is 10 AWG, and the 32% overshoot is the cost.
When 12 AWG is defensible anyway
Worth being honest about this. If the circuit was designed around 4 mm² with margin — most are — then 12 AWG at 17% less copper may still satisfy the ampacity and voltage drop the installation actually needs.
What it will not do is pass a conductor resistance test against a 4 mm² specification. So the question is what you are up against:
- A specification or a factory acceptance test? 10 AWG. There is no argument to make.
- A working installation where you are matching an existing design? Run the ampacity and volt-drop numbers for the actual route. If 12 AWG clears them, it is a legitimate engineering decision, not a shortcut.
Document which one you did and why. That decision has a habit of resurfacing.
Conductor diameter
A solid conductor of 4 mm² works out to 2.257 mm across. Maximums by class:
| Construction | Max diameter | Min strands | Max strand dia. |
|---|---|---|---|
| Class 1, solid | 2.4 mm | 1 | — |
| Class 2, stranded | 2.7 mm | 7 | — |
| Class 5, flexible | 3.0 mm | — | 0.31 mm |
| Class 6, very flexible | 3.0 mm | — | 0.16 mm |
Solid 12 AWG is 2.05 mm and solid 10 AWG is 2.59 mm.
The spread here is worth attention if you are choosing hardware. Class 5 at 3.0 mm is 46% wider than solid 12 AWG at 2.05 mm. Terminal blocks, ferrules and glands all have to accommodate the construction you actually receive, not the one the conversion table implies. See conductor classes 1, 2, 5 and 6.
Note also the class 6 strand maximum of 0.16 mm against class 5 at 0.31 mm — nearly double the fineness. At 4 mm² that is a large number of very fine strands, and it is what makes the cable survive a drag chain.
Where 4 mm² is used
Higher-current socket circuits, cooker and water heater supplies, sub-main runs, motor circuits, and solar PV string wiring. It is the size above general-purpose 2.5 mm² and below the feeder range.
In PV work it is particularly common, because string cable runs long and voltage drop drives the sizing more than current does.
FAQ
What is 4 mm² in AWG? 12 AWG by cross-sectional area. 12 AWG is 3.31 mm², which is 17% smaller than 4 mm².
Should I use 12 AWG or 10 AWG for a 4 mm² design? 10 AWG if you are meeting a specification, because 12 AWG does not pass the 4 mm² conductor resistance limit. 12 AWG may be acceptable on a working installation if the ampacity and voltage drop calculation supports it.
Is 4 mm² the same as 12 AWG? No. 12 AWG is 3.31 mm², about 17% smaller, and it misses the 4 mm² resistance requirement by 13%.
What is 4 mm² in mm diameter? A solid conductor of 4 mm² is 2.257 mm across. Stranded measures up to 2.7 mm, flexible up to 3.0 mm.
Why is the jump from 12 AWG to 10 AWG so big? The AWG series is geometric — each step changes the area by about 26%. Two steps roughly double it. There is nothing between 3.31 and 5.26 mm² because the system was never designed to land on metric values. See why AWG doesn’t match metric sizes.
What is 4 sqmm in AWG? Same question — sqmm and mm² are the same unit. 12 AWG by area, 10 AWG as the safe substitute.
Reference
| Property | 4 mm² |
|---|---|
| Closest AWG by area | 12 AWG (3.31 mm², −17%) |
| Safe substitute | 10 AWG (5.26 mm², +32%) |
| Equivalent solid diameter | 2.257 mm |
| Max resistance, class 1 and 2 | 4.61 Ω/km |
| Max resistance, class 5 and 6 | 4.95 Ω/km |
| Max conductor diameter | 2.4 / 2.7 / 3.0 mm by class |
| Minimum strands, class 2 | 7 |