25 mm2 to AWG: 3 AWG, and Why It Is Hard to Buy

25 mm² converts to 3 AWG, and this is one of the few sizes where the nearest gauge and the safe substitute are the same answer.

3 AWG is 26.7 mm² — 7% larger than 25 mm², and it clears the resistance limit. On paper the conversion is clean. The complication is commercial, not technical.

The resistance check

Metric conductors are verified by DC resistance per kilometre at 20 °C. For 25 mm² the limit is 0.727 Ω/km for solid and stranded, 0.780 Ω/km for flexible.

4 AWG is 16% smaller than 25 mm² and misses the limit. 3 AWG has 7% more copper and clears it comfortably. So the arithmetic answer and the engineering answer agree, which is unusual on this site.

The problem with 3 AWG

Odd-numbered gauges exist in the standard. They are not stocked.

Building wire, THHN, welding cable and armoured cable are almost universally supplied in even gauges — 14, 12, 10, 8, 6, 4, 2 — and then the 0-series. 3 AWG appears in the ampacity tables and in the wire gauge standard, and then you try to buy a drum of it.

So the practical question is which even gauge to use:

2 AWG is the safe commercial substitute. It overshoots by a third, which is more copper than the design called for, but it meets the requirement and it is available everywhere.

4 AWG is a downgrade. It carries 85 A at 75 °C against the 96 A of 25 mm² clipped direct, and it does not satisfy the conductor resistance limit. Use it only if the ampacity and volt-drop calculation for the actual installation supports it, and record that you checked.

If you can source 3 AWG, take it — it is the correct answer and it avoids a 34% overshoot. Just do not assume it is on the shelf.

Ampacity

25 mm² copper, three loaded conductors clipped direct at 30 °C:

The gap between 4 AWG and 25 mm² is around 11 A at the 75 °C rating. On a feeder that is not a rounding difference.

All of these are before derating. In a 40 °C enclosure with three circuits bunched, the 25 mm² XLPE figure of 119 A becomes:

119 A × 0.91 (40 °C) × 0.70 (3 circuits) = 75.8 A

Conductor diameter

A solid conductor of 25 mm² works out to 5.642 mm across. Solid copper at this size and above is intended for particular cable types such as mineral insulated rather than general use, so in practice you are buying stranded.

Solid 3 AWG is 5.82 mm and solid 4 AWG is 5.19 mm, for reference.

The spread from 5.7 mm to 7.8 mm is 37%, and at this size that is two lug sizes. Compression lugs are ordered ahead of the cable, so getting the construction wrong means hardware that does not fit when the drum arrives. See conductor classes 1, 2, 5 and 6.

Where 25 mm² is used

Sub-main and distribution feeders, larger motor circuits, generator tails, and main earthing conductors on medium services. It is the size where cable stops being hand-formed and starts being pulled and crimped.

FAQ

What is 25 mm² in AWG? 3 AWG. At 26.7 mm² it is 7% larger than 25 mm², and it meets the 25 mm² conductor resistance limit.

Can I use 4 AWG instead of 25 mm²? 4 AWG has 16% less copper and misses the 25 mm² resistance limit by 13%. It carries about 85 A against 96 A. It is a downgrade — only if the circuit calculation supports it.

Why is 3 AWG hard to find? Odd-numbered gauges are defined in the wire gauge standard but are rarely stocked. Building wire and cable are supplied in even gauges and the 0-series. 3 AWG is a real size that most distributors do not carry.

What should I use if 3 AWG is unavailable? 2 AWG. It has 34% more copper than 25 mm², which is more than needed, but it meets the requirement and it is available. See 2 AWG.

What is 25 mm² in mm diameter? A solid conductor would be 5.642 mm across. Real 25 mm² cable is stranded and measures up to 6.6 mm, or 7.8 mm for flexible constructions.

What is 25 sqmm in AWG? Same question — sqmm and mm² are the same unit. 3 AWG, or 2 AWG as the commercially available substitute.

Reference

Work the other direction from 4 AWG or 2 AWG, or use the converter.