3/0 AWG in mm: Diameter, mm2 & the Metric Size

3/0 AWG measures 10.404 mm across. Its cross-section is 85.0 mm², or 167,800 circular mils.

Reading the 0-series

If the numbering looks odd, it is because the gauge series ran out of room.

AWG counts upward as conductors get thinner — 14, 16, 18 and so on. Going the other way it counts down to 1, and then stops. Larger conductors continue as 1/0, 2/0, 3/0, 4/0, spoken "one aught" through "four aught" and sometimes written 0, 00, 000, 0000.

So the order from thin to thick runs:

… 4 → 3 → 2 → 1 → 1/0 → 2/0 → 3/0 → 4/0 → 250 kcmil …

3/0 is the second largest gauge. Each step up the 0-series adds roughly 26% to the cross-section, the same ratio that governs the rest of the system. See how the AWG system works.

The metric options

Same pattern as 4/0: the round-up misses by about 5%, and the size below passes comfortably.

The difference is the gap. At 4/0 the two options are 11% and 12% either side — genuinely balanced. Here the drop to 70 mm² is 18%, which is a real reduction rather than a rounding decision.

So the choice is less comfortable:

95 mm² costs 12% more copper and covers everything. 70 mm² meets the resistance requirement but gives up 18% of the conductor, and carries 184 A against the 223 A of 95 mm².

For a straight substitution of an existing 3/0 design, 95 mm² is the safer call. 70 mm² only makes sense if the load calculation for the actual installation supports it.

Ampacity

3/0 copper carries 165 A at 60 °C, 200 A at 75 °C, and 225 A at 90 °C, for three current-carrying conductors at 30 °C ambient. Metric equivalents with three loaded conductors clipped direct at 30 °C: 95 mm² is 223 A for PVC, and 70 mm² is 184 A.

Note how close 3/0 at 200 A and 95 mm² at 223 A sit once you account for the 12% extra copper. The systems are describing similar conductors by different routes.

Derating applies to all of it. 95 mm² PVC in a 40 °C switchroom with three circuits bunched:

223 A  ×  0.87 (40 °C)  ×  0.70 (3 circuits)  =  136 A

Grouping is the harsher factor at this size, because feeders travel together and rarely alone.

Conductor diameter

10.404 mm is solid wire, which does not exist at this size in practice. The metric maximums at 95 mm²:

15.1 mm against 10.404 mm is a 45% spread. At 70 mm² the equivalent figures are 9.4, 11.0 and 13.1 mm.

Lugs and glands are selected on this number and ordered before the cable arrives. Use the manufacturer's overall conductor diameter for the specific construction rather than the solid figure. See conductor classes 1, 2, 5 and 6.

Where 3/0 is used

Service entrance conductors on larger residential and commercial services, sub-main feeders, generator and transformer connections, and large motor circuits. It sits between 2/0 and 4/0 in a range where the choice is often driven by what the terminals will accept rather than by the ampacity calculation alone.

FAQ

What is 3/0 AWG in mm?

10.404 mm in diameter as solid wire, which is 409.6 mils. Stranded 3/0 measures up to 12.9 mm, and flexible up to 15.1 mm at the equivalent metric size.

What is 3/0 AWG in mm²?

85.0 mm². The metric sizes either side are 70 mm² and 95 mm².

Is 3/0 the same as 95 mm²?

No. 95 mm² is 12% larger, and a 3/0 conductor misses the 95 mm² resistance limit by about 5%. It is the size to order when replacing a 3/0 design, not an equivalent.

Can I use 70 mm² instead of 3/0?

70 mm² is 18% less copper and carries 184 A against the 200 A of 3/0 at 75 °C. It does meet its own resistance limit, so it is not automatically wrong — but it is a downgrade. Check the load calculation.

What does 000 mean on a cable?

It is another way of writing 3/0. The 0-series is written either 1/0, 2/0, 3/0, 4/0 or 0, 00, 000, 0000.

How many amps is 3/0 copper?

165 A at 60 °C, 200 A at 75 °C, 225 A at 90 °C, before derating for ambient temperature and conductor grouping.

Reference

Convert any other size with the AWG to mm² converter.