IEC 60228:2023 Explained: Conductor Classes and Limits

IEC 60228 is the reason metric cable comes in 1.5, 2.5, 4, 6, 10 mm² and not whatever number your conversion produced. It defines the nominal cross-sectional areas, the four conductor classes, and the resistance limit each size has to meet.

Edition 4.0 was published in December 2023. It is short — 48 pages including the French text — and almost all of its weight sits in four tables.

What the standard actually does

It specifies nominal cross-sectional areas from 0.5 mm² to 3500 mm², along with the number and size of wires, and the maximum resistance for each size. It covers solid, stranded and Milliken conductors in copper, aluminium and aluminium alloy for fixed installations, plus flexible copper conductors.

It does not cover conductors for telecommunications. That is why AWG sizes below 20 have no metric equivalent — 24 AWG belongs to the data cabling standards, not here.

One line in the scope explains a lot of confusion in the field: the standard relates to the conductor in the finished cable, not to the conductor as made or supplied for inclusion into a cable. What gets tested is what comes off the drum.

Nominal cross-section is a label, not a measurement

This is the single most useful thing to understand about the standard.

The defined term is nominal cross-sectional area — a value that identifies a particular size of conductor but is not subject to direct measurement. You cannot put calipers on a conductor and read 4 mm².

What you can measure is DC resistance per kilometre at 20 °C, and every nominal size carries a maximum. That is the requirement. The area is the name; the resistance is the test.

Everything else on this site follows from that. It is why 12 AWG is not 4 mm² even though people write it that way — 12 AWG runs 5.21 Ω/km against a 4.61 Ω/km limit, so it fails the only check that exists.

The four classes

There is no class 3 or 4 — both were withdrawn in earlier editions.

Classes 5 and 6 may also be used in fixed installations. The reverse is not permitted, and the reason is mechanical rather than electrical: a solid or coarsely stranded conductor work-hardens where it flexes and eventually cracks.

Class 6 is defined precisely as flexible conductors made with smaller diameter wires than class 5 for the same nominal cross-section. Same area, same resistance limit, same overall diameter — only the strand fineness differs. That is covered in conductor classes 1, 2, 5 and 6.

Class 2 has four constructions, not one

The stranded class is broader than most people assume:

Circular non-compacted — plain concentric stranding, minimum wire count per Table 4.

Circular compacted — drawn through a die to squeeze out the air. Minimum 1.5 mm² for copper, 10 mm² for aluminium. Smaller diameter for the same area.

Shaped — compacted into a sector profile so cores pack efficiently in a round cable. Minimum 25 mm².

Milliken — new as a formal clause in edition 4. An assembly of shaped conductors lightly insulated from each other, minimum 800 mm², built from four, five or six equal segments. The wire count is not specified, and the central element may be empty, a solid conductor, wires, or a plastic filler.

For compacted and shaped conductors, the ratio between the diameters of any two wires in the same conductor must not exceed 2 — except where pre-shaped wires are used.

The resistance tables

Four tables carry the requirements:

Tables 1 and 2 are tensile strength limits for solid and stranded aluminium. Note the renumbering: in the 2004 edition, the conductor tables were 1, 2, 3 and 4.

Metal-coated conductors get a slightly higher limit than plain, because the coating displaces a little copper. At 0.5 mm² it is 36.7 against 36.0 Ω/km.

Solid aluminium alloy conductors take the Table 3 value multiplied by 1.162, unless the manufacturer and purchaser agree otherwise.

The one row that behaves oddly

Flexible conductors are normally allowed more resistance than solid ones, because stranding lengthens the copper path.

At 50 mm² the limits invert. Class 1 and 2 are held to 0.387 Ω/km; class 5 is held to 0.386. It is the only size in the standard where flexible sits below solid, the gap is one part in four hundred, and it survived unchanged into edition 4.

It changes nothing practical. But it means you cannot use the solid figure as a conservative proxy for flexible at that one size. More on it in 50 mm² to AWG.

Minimum wire counts

Table 4 sets a minimum number of wires, not a fixed count:

Above 1600 mm² the count is unspecified — that is Milliken territory. Compacted conductors are allowed fewer wires than non-compacted, since the wires deform to fill the space.

Those numbers are not arbitrary. They are what you get packing circles in complete concentric layers: 1, then +6, then +12, +18, +24, +30. See copper wire strands.

The annexes

Annex A (normative) — how to measure resistance. Condition the cable, measure DC resistance on a complete length or a sample of at least 1 m, record the temperature, and correct with the Table A.1 factors. Calculate resistance per kilometre from the length of the complete cable, not from the individual cores.

Annex B (informative) — exact temperature correction formulae. For annealed copper, the factor is 254.5 / (234.5 + t). For aluminium, 248 / (228 + t).

Annex C (informative) — dimensional limits for circular conductors. This is the one people use most, and it is worth knowing it is informative guidance, not a requirement. It exists so cable and connector manufacturers stay dimensionally compatible. Binding diameters live in the product standard for the cable type.

What changed in edition 4

The standard lists three significant changes:

Milliken conductors got a formal description. Edition 3 mentioned segments only in a table footnote; edition 4 gives them a definition, a clause and a minimum size.

The range extended above 2500 mm². Class 2 now runs to 3500 mm², adding 1800, 3000, 3200 and 3500. Of the new sizes, 1800, 3000 and 3500 are marked non-preferred.

The 2500 mm² aluminium resistance was corrected, from 0.0127 to 0.0119 Ω/km. The foreword notes that legacy designs built on the old value may be maintained, and which value applies is by agreement between supplier and user.

Two further changes are visible in the tables but not in that list. 1400 mm² is no longer non-preferred — in edition 3 it carried the marker alongside 1800. And minimum wire counts now extend to 1600 mm², where edition 3 stopped specifying them at 1200.

Everything else is unchanged. Class 1 copper across all twenty sizes, class 2 copper, class 5 and 6 strand diameters, aluminium from 10 to 1200 mm², and all twenty-four rows of Annex C Table C.1 are identical to the 2004 edition.

Where AWG fits in

It does not, formally. IEC product standards do not specify cables with AWG or kcmil conductors.

The foreword carries a reference table of AWG and kcmil equivalents because Canada uses those sizes under sub-national regulation, but it is explicitly for reference. Two of its values differ slightly from the ASTM figures through rounding — 4 AWG appears as 21.2 mm² where ASTM gives 21.1, and 4/0 as 107 where ASTM gives 107.2.

That is the gap this site exists to bridge. See why AWG doesn't match metric sizes.

FAQ

What is IEC 60228?

The international standard for conductors of insulated cables. It defines the nominal cross-sectional areas from 0.5 to 3500 mm², the four conductor classes, the minimum wire counts and the maximum resistance for each size.

What is the current edition?

Edition 4.0, published December 2023. It replaces edition 3.0 from 2004.

What are the IEC 60228 conductor classes?

Class 1 solid and class 2 stranded for fixed installations; class 5 and class 6 flexible for cords and flexible cables. Class 6 uses finer wires than class 5 at the same nominal size.

Does IEC 60228 cover AWG sizes?

No. It specifies metric sizes only. The AWG and kcmil equivalents printed in the foreword are for reference and are not a basis for ordering.

What is the smallest size in IEC 60228?

0.5 mm². Finer conductors fall under the telecommunications and data cabling standards instead.

Is Annex C mandatory?

No. Annex C is informative — guidance for dimensional compatibility between conductors and connectors. Binding diameters are set in the product standard for the specific cable type.

How is compliance checked?

Construction is checked on the completed cable by inspection and measurement where practicable. Resistance is measured per Annex A and corrected to 20 °C using the Table A.1 factors.

Reference

Convert any AWG size against these limits with the AWG to mm² converter.