Aluminium cables — cross-section equivalence relative to copper and the 1.6× rule of thumb

The guide on aluminium/copper connections and bimetallic corrosion covers the galvanic corrosion risk at a terminal connection between an aluminium and a copper conductor. This article covers a different, prior aspect of aluminium cabling: how much larger an aluminium conductor needs to be than a copper conductor to safely carry the same current, and why.

The physical cause: resistivity

Aluminium has a higher electrical resistivity than copper: at equal cross-section and length, an aluminium conductor offers approximately 1.6× more resistance than a copper conductor. For the same current, that produces significantly more heat generation (I²R losses) in an aluminium conductor of equal cross-section. To achieve the same temperature rise (and thus the same current-carrying capacity) at the same current, an aluminium conductor therefore needs a larger cross-section than its copper counterpart — approximately in the same order of magnitude as that 1.6× resistance ratio, although the exact practical ratio per cross-section deviates somewhat due to the non-linear ampacity tables.

The practical rule of thumb and IEC 60364-5-52 table values

In practice, the rule of thumb often applied is that an aluminium conductor needs approximately 1.6× the cross-section of the equivalent copper conductor for the same current-carrying capacity. Because cables are only available in standardised cross-section steps, this typically translates in practice to one to two cross-section steps up relative to the copper reference. An example based on the IEC 60364-5-52 table values for XLPE-insulated cable under reference installation method C: a 50 mm² copper cable has a current-carrying capacity of approximately 179 A; the nearest aluminium equivalent for that lies around 70-95 mm², depending on whether rounding up or down to the nearest standard cross-section.

Note: this article gives the rule of thumb and the underlying principle; for an actual design the real ampacity table of IEC 60364-5-52 (or the manufacturer specification) must always be consulted for the specific combination of insulation material, installation method and environmental correction factors — see also the guide on reference methods — rather than relying solely on the 1.6× rule of thumb.

Why choose aluminium despite the larger cross-section

Despite the larger required cross-section, aluminium cabling remains attractive for certain applications:

Weight: aluminium, even at the larger cross-section needed for equal current-carrying capacity, has a significantly lower weight per supplied metre than copper — relevant for long high-voltage or medium-voltage runs, overhead lines, and applications where the weight of the cable support structure is a limiting factor.
Material cost: at comparable market prices per kilogram, aluminium is often still cheaper per ampere carried over a given length than copper due to its lower density, despite the larger required cross-section — this price advantage varies with the raw material market and must be reassessed per project.
Field of application: aluminium is more commonly applied at larger cross-sections (main supply cables, MV cables, overhead lines) where the weight advantage matters most, and less commonly for small final-circuit cross-sections where the connection technique (terminal tightness, material creep under pressure) requires relatively more attention.

Practical relevance

When replacing or upgrading an existing copper supply with an aluminium cable — or conversely, when assessing an existing aluminium supply — the cross-section equivalence must always be checked against the actual ampacity tables for the applied insulation and installation method, not by simply keeping "the same cross-section": an aluminium cable with the same cross-section as the copper cable it replaces has a significantly lower current-carrying capacity and, at unchanged load, can lead to overheating.

Common mistakes

Applying an aluminium cable with the same cross-section as copper without enlarging the cross-section — this leads to a significantly lower actual current-carrying capacity than the original copper cable.
Treating the 1.6× rule of thumb as an exact, universal value instead of an approximation — the actual ampacity tables of IEC 60364-5-52 deviate somewhat per cross-section and installation method from a simple linear factor.
Adjusting only the cross-section, but not the connection technique — aluminium requires different terminals, tightening torques and (with terminal connections to copper) corrosion prevention, see the guide on bimetallic corrosion.
Deciding solely on weight or material price without weighing in the larger physical cross-section (and thus larger bending radius and space requirement in cable trays/conduits) in the design.