Copper vs Aluminium Windings: Comparison for Motors and Transformers

A copper winding conducts better and is more compact, while an aluminium winding is lighter and much cheaper in material. Aluminium reaches around 60 percent of the conductivity of copper: conductor-grade aluminium 1350 achieves at least 35.5 MS/m (Aerumtec), Cu-ETP copper at least 58 MS/m (Aurubis, 2026 data sheet). For the same resistance, an aluminium winding therefore needs 1.63 times the cross-section. Because aluminium, at 2.70 g/cm³, has only a third of the density of copper at 8.9 g/cm³, it still weighs only half as much. Per kilogram, copper costs 4.7 times as much on the London Metal Exchange (cash, 23 October 2026), so the metal for a winding of equal resistance costs around ten times as much. That is why distribution transformers often use aluminium, while motors with tight space and large power transformers almost always use copper. This comparison works through the numbers for motors and transformers, explains why the terminations decide service life, and shows what the conductor material means for the slot liners, slot covers and layer insulation we make as insulation parts for motors and transformers.

Copper and aluminium as winding materials, side by side

The figures in forums and manufacturer blogs contradict each other: aluminium has 50 or 62 percent of copper's conductivity, it is a third lighter or 70 percent lighter. The reason is that the sources compare different things, sometimes equal mass, sometimes equal cross-section, sometimes the finished unit. For a winding, the comparison that counts is at equal resistance, because then the I²R losses are equal too. Material data come from the Aurubis, Aerumtec and Bikar data sheets, the temperature limits from Olivares-Galván and colleagues (IET Electric Power Applications, 2010).

One metre of copper conductor with a 10 mm² cross-section has a resistance of 1.72 mΩ and weighs 89 g. The same resistance in aluminium needs 16.3 mm², which gives 1.73 mΩ at 44 g. At LME prices of 23 October 2026, the copper metre contains US$1.28 of metal and the aluminium metre US$0.13. That is how a factor of 4.7 per kilogram becomes a factor of ten per winding. Copper prices swing widely, so work with the ratio and the date rather than the absolute price.

The finished unit does not become ten times more expensive. In a distribution transformer, the windings account for 22 ± 6 percent of material cost, with core, insulation, tank and fabricated parts making up the rest (Olivares-Galván). Conductivity values for other metals and alloys are listed in the article on electrical conductivity, thermal values in the article on thermal conductivity.

Aluminium has taken over the distribution transformer

The most thorough transformer study comes from Olivares-Galván, de León, Georgilakis and Escarela-Pérez. They optimised distribution transformers with copper and with aluminium windings for the lowest total owning cost, losses included. At November 2009 metal prices, aluminium was the cheaper choice below 190 kVA and copper from 190 kVA upwards, because the more compact copper winding needs less core, insulation, tank steel and oil. On average, the aluminium units in the study had a 15 percent longer winding, 20 percent more core mass and 45 percent more oil, while their windings were 25 percent lighter. Back then, copper cost about 3.5 times as much as aluminium per kilogram (US$6.47 against US$1.87); in October 2026 the factor is 4.7. Since then the calculation has shifted further in aluminium's favour.

Mandatory efficiency levels apply to both materials alike: in the US, a 500 kVA distribution transformer has to reach 99.35 percent, whichever conductor is used (Maddox). The manufacturer WEG reports that an aluminium transformer with equal losses has a 5 to 20 percent heavier core but almost the same total mass, and that eddy losses in aluminium are 38 percent lower for the same conductor volume. Most low and medium voltage dry-type transformers are wound with aluminium today (Elsco Transformers). Copper remains the better choice when

• the transformer is small: up to about 5 kVA, manufacturers wind with copper because thin aluminium wire is brittle and breaks during winding (Hammond Power Solutions),
• the rating is high: from about 1 to 3 MVA, copper is more practical to wind (Hammond), and almost all large power transformers have copper windings (Olivares-Galván),
• the dimensions are tight, because very compact designs require copper coils (Hammond),
• salt air or seawater is involved, because aluminium is more sensitive to chloride (Hammond recommends copper for marine units),
• high fault currents, vibration or frequent overloads are expected (Elsco).

Design, types and insulation system of the transformer are covered in the article on the electrical transformer.

Foil windings need layer insulation across the full strip width

In the low-voltage winding, the conductor is often a strip rather than a wire. The cast resin transformer maker SGB-SMIT almost always builds it as a strip winding, with exceptions below 250 kVA and above 3.6 kV. Below 0.2 mm the conductor is called foil, above that strip (Olivares-Galván). Hydro supplies aluminium transformer strip from 0.30 to 2.50 mm thick and 25 to 1,500 mm wide. Between turns lies a layer of insulation across the full strip width, traditionally a diamond-dotted paper of 150 to 300 µm with extra length at the edges (Alanod). Conventional cast resin transformers use a class F film for this (SGB-SMIT).

Hydro points to a weak spot: high, sharp burrs on the slit edge of the strip can pierce the insulating paper. Two things follow for the layer insulation. It has to come on a roll in exactly the strip width plus overhang, and its edge has to run as straight as the strip. In contract slitting we cut aramid paper, PET film and laminates to widths from 5 to 1,600 mm with a width tolerance of ±0.1 mm, on cores with inner diameters of 25, 55, 76 or 152 mm. Which materials suit the interlayer is covered in the article on layer insulation.

In electric motors the stator winding mostly stays copper

In an induction motor, aluminium sits mainly in the rotor. The squirrel cage is cast, and aluminium melts at 660 °C, copper only at 1,083 °C (Fassbinder, German Copper Institute, in de 20/2004). A copper rotor raises efficiency but is harder to cast. The enamelled-wire stator winding, by contrast, is almost always copper. Induction motors in continuous duty above 1 hp are wound with copper, and aluminium is an alternative for smaller motors that do not run continuously (Wikipedia, Magnet wire). Drain pumps for washing machines and dishwashers are one example: in a 2016 patent, Askoll describes an aluminium winding introduced because of the copper price, which turns out bulkier and has to shed more heat. Pumps and fan motors like these sit inside the appliances for which we supply insulation parts for household appliances.

The EU Ecodesign Regulation 2019/1781 has required class IE3 for three-phase motors from 0.75 to 1,000 kW since 1 July 2021 and class IE4 from 75 to 200 kW since 1 July 2023 (VEM). It leaves the conductor material open. For the same I²R losses, aluminium needs 63 percent more conductor cross-section, which means larger slots or a longer lamination stack.

Aluminium gets interesting where it is given more room in the slot than copper. Fraunhofer IFAM casts coils instead of winding them and reaches fill factors that wire never achieves:

• In a 330 kW DC machine from Lloyd Dynamowerke, cast interpole coils with a fill factor above 90 percent instead of around 50 percent replaced the copper coils. Resistance was no higher, temperature rise fell from 75 to 45 K and raw material cost per coil from 52 to 6 euros (IFAM 2013).
• In a 250 W pedelec motor, the fill factor rose from 32 to 60 percent, the motor became 10 percent lighter and delivered almost 20 percent more continuous power (IFAM 2019).
• For vehicle drives, the institute cites up to 50 percent less weight in the active part at almost the same efficiency. A copper interconnection ring offsets the extra space the aluminium coils need (IFAM, May 2019).

For hairpin traction motors, Petrelli and colleagues (University of Nottingham, ICEM 2022) studied aluminium in a 150 kW motor running at 18,000 rpm and consider it a promising candidate, mainly because of lower AC losses. The reason is skin effect: penetration depth grows with the square root of resistivity, so 3.3 mm in copper at 400 Hz becomes roughly 4.2 mm in aluminium. Production motors with a hairpin winding use copper flat wire today. The other winding types and their insulation are described in the article on the electric motor winding.

Terminations decide how long an aluminium winding lasts

The most common objection is that aluminium does not last as long. For the winding itself, that is not true. Copper and aluminium transformers use the same insulation systems and run at similar temperature rises, so their service life is similar (Hammond Power Solutions, AEMT). The bad reputation goes back to the 1960s, when fires broke out at the connections, never in the wire (Hammond). That is exactly where aluminium's four weaknesses lie:

• The oxide layer forms in air within nanoseconds (Hertweck and colleagues, METALL 5/2020) and is an excellent insulator (Olivares-Galván). A joint has to break through it and stay gas-tight afterwards (Hammond).
• Aluminium creeps. Under the pressure of a clamp it slowly flows away, contact force drops and contact resistance rises (Chipkin, METALL 5/2020).
• Aluminium expands about a third more than copper. Every thermal cycle loosens a bolted joint a little more (Chipkin).
• Copper and aluminium are 2.01 V apart in the electrochemical series. With moisture, the aluminium corrodes at the contact (METALL 5/2020).

WEG lists bolting, crimping and inert gas arc welding as proven methods. For more than 20 years, the cast resin transformer maker SGB-SMIT has joined lead bars and winding strip only by cold pressure welding at 400 kN, and motor makers use piercing crimps that break up the oxide under high pressure (ACHR News). The counter-position comes from rail engineering: in small, rarely maintained signalling transformers, resistance rises over the years because of aluminium oxide (Rail Engineer, 2019). Our view: aluminium is a good choice when the terminations are pressed or welded and the environment is dry. Where joints are bolted and nobody re-torques them, copper is the safe choice.

What the conductor material means for the insulation

The insulation does not care whether copper or aluminium carries the current. Thermal class and materials stay the same (Hammond). What changes are the dimensions and the boundary conditions, and with them the drawing of every insulation part:

1. The slot gets larger. At the same fill factor, aluminium needs 63 percent more slot area. A rectangular slot of 10 x 20 mm becomes 12.8 x 25.6 mm at the same proportions. The slot perimeter, and with it the blank of the slot liner, grows by 28 percent, and slot covers get wider. With 0.25 mm insulation, the share of insulation in the slot area drops from 7.5 to 5.9 percent.
2. The transformer coil gets thicker. In the 25 kVA example by Olivares-Galván, the high-voltage winding measures 55.7 mm radially instead of 38.0 mm, the low-voltage winding 25.2 mm instead of 14.2 mm. The winding is also 15 percent longer. That means more layers and more area of layer insulation.
3. The winding moves more. Aluminium expands a third more with every load change, and thermal expansion of the conductors can break the insulation (Olivares-Galván). At the slot exit and in the end winding, the insulation therefore needs a margin against abrasion.
4. Cut edges of strip and foil attack. Burrs on aluminium strip pierce thin paper (Hydro), so the interlayer needs thickness and puncture resistance with a margin.
5. The short-circuit limit is lower. In an oil-filled transformer of thermal class A, aluminium may reach 200 °C during a short circuit and copper 250 °C. Both limits exist to restrict ageing of the paper on the conductor (Olivares-Galván).

Our recommendation: when switching from copper to aluminium, take the insulation parts into the redesign and do not reuse an old drawing. Which materials belong in the slot is covered in the article on slot insulation, how the slot is closed in the article on slot liner covers. Papers and laminates by thermal class are compared in the article on insulating paper for electric motors, and the temperature limits in the article on insulation classes.

GOBA makes these parts from pressboard, NMN, DMD, aramid paper and NKN, and slot covers from all flat electrical insulating materials in every thermal class. We supply rolls, sheets, blanks, die-cut parts or formed parts, in thicknesses from 7 µm to 3 mm depending on the material, from a single piece and within two weeks at most when the material is in stock. For a quote we need the drawing, quantity and thickness, plus width, roll length, core and tolerances for rolls. Choose the right material from our insulation materials or ask our technical consulting team; we reply within 24 hours on working days.

Copper or aluminium: how to decide

The questions are listed in the order in which they narrow the choice most.

1. Is the installation space fixed? Then copper. Aluminium needs 63 percent more cross-section, and it has to go somewhere.
2. Do weight and metal cost matter more than volume, for instance in a distribution transformer? Then aluminium: the winding weighs half as much, and its metal costs a tenth for the same resistance.
3. How are the ends connected? Pressed or welded suits aluminium, bolted and unmaintained suits copper.
4. Is there salt air, moisture, vibration or high short-circuit force? Then copper.
5. A transformer up to about 5 kVA or a motor in continuous duty above 1 hp? Then copper. Thin aluminium wire breaks during winding, and a standard motor has no room for 63 percent more cross-section.

FAQ on copper and aluminium windings

How much more expensive is copper than aluminium?

On the London Metal Exchange on 23 October 2026, copper cash traded at US$14,415 per tonne and aluminium at US$3,043, a factor of 4.7. For a winding with equal resistance, aluminium needs only half the mass, so the metal value differs by a factor of about ten. Processing narrows the gap: for aluminium flat wire, an article in METALL 5/2020 puts the saving at a factor of three to four. Complete distribution transformers with copper windings cost two to three times as much, according to Maddox.

Does a motor with an aluminium winding have lower efficiency?

At the same wire cross-section, yes: resistance is 1.63 times higher and I²R losses rise by the same factor. Give the aluminium winding 63 percent more cross-section and the losses are equal, but the motor gets larger. The IE classes of EU Regulation 2019/1781 measure losses and allow both materials. Cast aluminium coils with a high fill factor can even beat copper wire, as the Fraunhofer IFAM tests show.

Does an aluminium winding last as long as a copper winding?

Yes, if the terminations are right. Both materials use the same insulation systems and reach similar temperature rises, so the insulation life is similar. Failures occur at joints that do not break the oxide permanently or that loosen through creep. Pressed and welded terminations avoid this, bolted clamps without maintenance do not.

Can an aluminium winding be replaced with copper during a rewind?

Yes, repair shops do this routinely. On paper, copper with around five eighths of the aluminium area is enough, but the association EASA does not consider this reduction advisable in every case. Coil dimensions should stay as close to the original as possible. Because the lamination stack stays, slot liners and slot covers usually keep their dimensions in a rewind.

Does an aluminium winding need different insulation?

The same materials and the same thermal class, but different dimensions. The slot gets larger for the same rating, the coil thicker, and the insulation has to cope with thermal expansion that is a third higher. In an oil-filled transformer of thermal class A, the short-circuit limit for aluminium is 200 °C instead of 250 °C, so that the paper on the conductor does not age faster.

Are aluminium windings used in electric cars?

Production traction motors use copper today, mostly as hairpins made of flat wire. Research is working on aluminium: Fraunhofer IFAM cites up to 50 percent less weight in the active part at almost the same efficiency, and a University of Nottingham study on a 150 kW motor sees aluminium hairpins as promising because of lower AC losses.