4 Gauge Wire in mm: Diameter, mm2 & Ampacity

4 gauge wire measures 5.19 mm across. Its cross-section is 21.1 mm², or 41,740 circular mils.

Order 25 mm² — 18% more copper. The size below, 16 mm², is 24% less.

The two standards disagree on this one

Almost every AWG size converts to the same number in both reference systems. 4 AWG is one of the few where they do not.

The American standard gives the cross-section as 21.1 mm². The metric standard, in its own list of AWG equivalents, prints 21.2 mm².

The gap is 0.05% and has no practical consequence. It exists because the two documents round at different points in the calculation. It is worth knowing only so that you recognise both figures as the same conductor when you meet them on different drawings.

The same thing happens at 4/0, where one standard gives 107.2 mm² and the other 107.

Ampacity

4 AWG copper carries 70 A at 60 °C, 85 A at 75 °C, and 95 A at 90 °C, for not more than three current-carrying conductors at 30 °C ambient.

The metric equivalent at 25 mm², three loaded conductors clipped direct at 30 °C:

At this size the terminal rating usually decides which column you may use, not the conductor. Equipment terminals rated 75 °C mean you take the 75 °C figure even if the cable insulation is rated 90 °C. Check the rating stamped in the enclosure at both ends before reaching for the higher number.

Derating a service conductor

25 mm² XLPE clipped direct is 119 A on the table. A service run through a 40 °C roof space with two circuits together:

119 A  ×  0.91 (40 °C)  ×  0.80 (2 circuits)  =  86.6 A

Ambient is the harsher factor on service conductors, because they often run through the hottest part of a building. At 50 °C the XLPE factor is 0.82, and for PVC it falls to 0.71 — PVC loses nearly a third of its rating in a hot roof space where XLPE loses under a fifth. That difference is one of the better arguments for specifying XLPE on a run you know will get hot, and it is a bigger effect than most people expect.

4 AWG against the 25 mm² limit

The resistance limit for 25 mm² is 0.727 Ω/km. A 4 AWG conductor measures 0.82 Ω/km, missing by 13%. It clears 16 mm² easily, at 1.15 Ω/km. So the usual pattern holds — 4 AWG sits between two metric sizes and satisfies the requirements of neither the one above nor, usefully, anything else. See why AWG doesn't match metric sizes.

Note that the next gauge up, 2 AWG, is one of only two that does meet its metric limit. If you have any freedom over conductor size on a conversion, stepping to 2 AWG and 35 mm² gets you a genuinely clean pairing.

Conductor diameter

5.19 mm is solid wire. Solid copper above 25 mm² is intended for particular cable types rather than general use, so at this size you are in stranded territory. At 25 mm²:

Class 5 at 7.8 mm is 50% wider than the 5.19 mm solid figure. At service entrance sizes, compression lugs and glands are selected on diameter and ordered ahead of the cable, so getting this wrong means hardware arriving that does not fit. See conductor classes 1, 2, 5 and 6.

Where 4 AWG is used

Service entrance conductors, main feeders on residential and light commercial services, large sub-panels, and grounding electrode conductors where the service size requires it.

It is a distribution conductor. Nothing about its selection is casual — the ampacity, the voltage drop, the terminal ratings and the physical termination all need checking against the actual installation.

FAQ

How many mm is 4 gauge wire?

5.19 mm in diameter for solid wire, which is 204.3 mils. Stranded 4 AWG measures wider.

What is 4 AWG in mm²?

21.1 mm² in the American standard, or 21.2 mm² in the metric standard's own conversion list. The difference is rounding. The metric cable size to order is 25 mm².

How many amps can 4 gauge wire carry?

70 A at 60 °C, 85 A at 75 °C, 95 A at 90 °C, before derating. The terminal rating of the equipment at each end often limits which column you may use.

Is 4 AWG the same as 25 mm²?

No. 25 mm² has 18% more copper, and a 4 AWG conductor misses the 25 mm² resistance limit by 13%.

Can I use 16 mm² instead of 4 AWG?

It has 24% less copper and carries about 76 A against 85 A at the comparable rating. On a service conductor that is a significant reduction.

Is 4 gauge enough for a 100 amp service?

At 75 °C, 4 AWG copper is rated 85 A before derating, which is short of 100 A. Service conductor sizing also follows specific rules that differ from general ampacity, so check the requirements that govern your installation rather than the table alone.

Reference

Work the other direction from 25 mm² to AWG, or use the converter.