Connecting Copper to Aluminium: Which Part for Which Joint

Connecting copper to aluminium is not one job but five, and each takes a different part: a crimped bimetal lug where an aluminium cable lands on a copper terminal, a butt-splice where an aluminium conductor meets a copper one in line, a bimetal washer or a cut plate where a copper lug bolts onto an aluminium busbar, the same washer turned over where an aluminium lug bolts onto a copper busbar — and, for solid domestic wiring below 10 mm², nothing we make at all. This page sends you to the right part in the right size, or tells you plainly that we are not your supplier.

Which joint are you actually making?

Two things decide the part: which side is aluminium, and how the two conductors physically meet. The cross-section decides whether we can help at all.

Solid aluminium house wiring, 1.5–4 mm²: we are not your supplier

If the aluminium in front of you is 1.5, 2.5 or 4 mm² solid conductor in a domestic installation — the original aluminium wiring in an older flat, a sealed meter tail you are not permitted to replace — then nothing in our range fits it. Our smallest bimetal cable lug is 10 mm², and butt-splices start at 16 mm² on the aluminium side. There is no smaller version and no exception to ask us about.

At that size the correct answers are the ordinary ones: a clamp terminal approved for aluminium conductors, used with an oxide-inhibiting contact paste, or replacing the aluminium run with copper. Both are sound engineering there, and neither is a product we supply. Take that advice from the installer who can see the circuit.

Aluminium conductor onto a copper terminal: bimetal cable lug

The commonest industrial version of the problem: an aluminium cable terminating on a copper busbar, breaker or terminal block. A bimetal lug puts the transition inside the part — the aluminium barrel is crimped onto the cable, the copper palm bolts to the copper terminal, and no dissimilar pair is ever exposed.

The tubular eye-lug covers 10–625 mm² with bolt holes from M8 to M20, palm dimensions to DIN 46329, in 52 stocked SKUs. The cheapest is ICALCU108 at 10 mm²/M8, HUF 1,981 net. A worked example from the middle of the range: a 240 mm² aluminium cable landing on an M16 stud is ICALCU24016, HUF 7,194 net. The SKU reads as cross-section then bolt size, so the part number is a lookup once you know those two numbers.

Where the device expects a copper pin rather than a bolt-through hole — common on isolators and some breakers — the pin lug covers aluminium 16–300 mm² with pin diameters Ø6 to Ø18 mm, 11 SKUs, from ICALCU16B6V at HUF 4,926 net to ICALCU300B18V at HUF 16,939 net.

Lug and splice prices on this page are net list, effective 20 July 2026; gross is net × 1.27. The cable-lug product page carries the current figures.

There is a second route to the same joint. Where an aluminium cable lands on a copper terminal, it is often faster and cheaper to crimp the plain aluminium lug your wholesaler already stocks and have us cut a copper-aluminium washer to that lug's palm than to source a bimetal lug in your exact geometry. That is the cut-to-size route, and it is covered further down this page.

One thing to check before you order: a bimetal lug is only the answer while the cable end is still open. If the cable is already terminated and only the bolted interface is dissimilar, the washer is the answer, whichever way round the metals sit — a plain copper lug on an aluminium bar, or a plain aluminium lug on a copper bar. Both bolted cases are covered below.

Aluminium conductor to copper conductor, in line: Al/Cu butt-splice

Sometimes there is no terminal at all: an aluminium conductor is joined end-to-end with a copper one, on an overhead-line repair or where an aluminium run is extended in copper. That is the Al/Cu butt-splice — a bimetal sleeve crimped onto the aluminium at one end and the copper at the other, transition sealed inside.

It is specified by the two conductor cross-sections and nothing else: aluminium 16–500 mm² against copper 10–400 mm², 46 stocked SKUs, from ICALCU1610V (Al 16 / Cu 10) at HUF 2,711 net up to ICALCU500400V (Al 500 / Cu 400) at HUF 43,588 net. There is no bolt size on a splice — if you are looking for one, you are in the wrong row of the table at the top of this page.

Copper lug bolted to an aluminium busbar: bimetal washer or cut plate

A copper cable lug bolted flat onto an aluminium busbar, an aluminium enclosure, an earthing bar or a mast fitting. With nothing between them, the aluminium corrodes under the palm. The washer solves it by one rule, which is worth stating before the sizes because it also settles the mirror case in the next section: each face of the washer meets its own metal. The copper face meets copper, the aluminium face meets aluminium, and the Cu-Al interface itself is sealed inside the disc where no electrolyte reaches it. In this joint that means the copper face against the copper lug palm and the aluminium face against the aluminium bar. CUPAL prescribes no torque of its own — tighten to the figure the bolt and the equipment call for.

A note on names. Drawings that call for a "transition washer" sometimes mean a profiled copper-alloy washer, whose raised pattern bites through the aluminium oxide film as the bolt is tightened — copper alloy throughout, the transition made at assembly. What follows is a roll-bonded bimetal, where the transition is built into the material. If your drawing does not say which, how to tell the two apart settles it in a few seconds.

The max. lug cross-section column is the largest lug palm that the outer diameter still covers fully. All 11 sizes are 2 mm thick, lead time is 2–10 working days from order to delivery, and there is no minimum order on washers. Net list prices are published on the product page — see net list prices for all 11 washer sizes, which also carries a cross-reference to the designations other distributors use. The size guide gives the bore diameter for each bolt size. If what you are holding is a distributor code rather than a size — 880111, Cupal M 10 Al-Cu, ICALCU10CS — the part number cross-reference maps it onto a stocked size. For a copper cable on an aluminium mast or antenna pole, where salt spray makes the same joint fail far faster, see copper cable to an aluminium mast.

Two numbers decide it: the bolt and the lug palm

The bolt sets the bore: an M10 joint takes an M10 washer. The width of the lug palm sets the outer diameter, because the palm has to seat fully on the washer face rather than overhang it. If the palm is wider than the largest stock washer for your bolt, step up the diameter or have a plate cut to the actual footprint — a washer that covers only the area around the bolt leaves the dissimilar contact exposed where it matters most. Every size is 2 mm thick, so going up in diameter does not change the bolt length. Surface preparation, orientation and inspection are set out step by step in the installation guide.

Aluminium lug bolted to a copper busbar: the same washer, turned over {#aluminium-lug-copper-busbar}

The mirror of the case above, and just as common: the lug is aluminium and the bar is copper. An aluminium cable lug on a copper busbar, on a copper terminal or on a copper earth bar.

The part does not change, only its orientation. Because each face of the washer meets its own metal, the stack here runs aluminium lug, then the washer's aluminium face, then its copper face, then the copper bar — two like-metal contacts, with the Cu-Al interface sealed inside the disc exactly as before. The washer is not a handed part — there is no left and right, no top and bottom — but it is orientated: the only thing you decide is which face goes against which conductor. The same 11 stocked sizes, M6 to M20, serve this joint.

Check the faces before you fit it: the reddish face is copper, the light, silvery face is aluminium. On an oxidised part, light sanding settles it — and if sanding reveals no copper face at all, what you are holding is not a bimetal washer. Note that the installation guide sets out the copper-lug-on-aluminium-bar orientation; here the same steps apply with the faces reversed.

If the aluminium lug is already crimped, you do not need a bimetal cable lug here — buying one is the expensive way to get this wrong. A bimetal lug puts the transition inside the termination, which only helps you while the cable end is still open, and yours is not. The only dissimilar contact left is the one under the bolt, and that is what the washer removes. The alternative is cutting off a good lug and re-crimping to a bimetal one, which starts at HUF 1,981 net (ICALCU108) and reaches HUF 7,194 by the 240 mm²/M16 size — to fix a joint the washer fixes without touching the termination.

We do not supply the aluminium lug itself. Our ICALCU lugs are bimetal only — an aluminium barrel with a copper palm or pin — so buy the plain aluminium lug wherever you normally buy lugs. What we supply is what goes between it and the bar.

Sizing is unchanged from the case above: the bolt sets the bore, and the aluminium lug's palm sets the outer diameter — measure the palm rather than inferring it from the cable cross-section. If it overhangs the largest washer for your bolt, have a plate cut to the actual footprint.

When no stock size fits: we cut it from your DXF

Stock sizes are faster and cheaper as long as your joint fits them. When they do not — a lug palm that matches no standard washer, a busbar transition with its own hole pattern, a terminal plate to a given contour — we cut the part from board.

Discs, strips, rectangles and arbitrary contours, in 0.5, 1.0, 1.5 and 2.0 mm, from a DXF, PDF or JPG drawing. Minimum is one 500 × 500 mm board of raw material; a 1.0 mm board is HUF 60,000 net and a 0.5 mm board HUF 37,500 net — board prices net list, effective 1 February 2025, gross is net × 1.27 — and the per-piece price is that board plus cutting labour, set by shape complexity and quantity. Quote within 24 hours, production in 5–10 working days, and the EN 10204 certificate covers cut parts exactly as it covers stock sizes.

This is the route behind the plain-aluminium-lug option above: a washer cut to the palm you already have, rather than a bimetal lug sourced to your geometry. Where the transition has to be flexible or wrapped, as in lightning protection, CUPAL strip is the same material in band form. More on cutting to size and the sheet it comes from.

Why paste and a stainless washer are not the answer

Copper and aluminium in contact with an electrolyte form a galvanic cell; on installed, passivated electrodes the open-circuit voltage of the pair typically settles around 0.5–1 V. Aluminium is the less noble metal, so it corrodes, and the corrosion product is an insulating oxide — contact resistance rises, the joint heats, the heat accelerates the corrosion, and the joint loosens. Contact paste keeps the electrolyte out for a while and a stainless washer merely moves the dissimilar pair rather than removing it; neither ends the couple, and both need re-inspection. The mechanism is covered properly in galvanic corrosion prevention.

A four-row summary is enough to choose; CUPAL vs the transition methods works each one through in full.

What the standards say — and what they do not

This is where specifications get written loosely. IEC 61238 for cable connectors, IEC 61439 for low-voltage switchgear and DIN 43671 for busbars set requirements on the installed joint — contact resistance, temperature rise, long-term stability. They are not certifications of the CUPAL material, and nobody can hand you a certificate saying a washer is "IEC 61439 approved". What a bimetal transition does is make those requirements achievable, and keep them achievable over the equipment's life.

The paperwork you can name in a bill of materials: an EN 10204 2.2 material certificate with every delivery as standard, 3.1 on request, both covering custom-cut parts — cutting does not affect the certification of the base material. The raw material comes from an ISO 9001:2015 certified supplier, and the grades carry Werkstoff numbers under DIN 17007 (aluminium 3.0255) and DIN 1787 (copper 2.0065). The EN 10204 article explains what separates a 2.2 from a 3.1.

The material data behind the washer, the cut plate and the strip is short: 70 % aluminium / 30 % copper by layer thickness, conductivity 41.9 m/(Ω·mm²), bond strength 70–110 N/cm. That ratio is worth checking against whatever else you are quoted, because most sellers publish no cladding ratio at all — which makes it impossible to specify against a drawing.

What to send us for a quote

• Cross-section and bolt size. Cable cross-section in mm² and the bolt or stud the joint lands on. That is enough to name the lug, the splice or the washer.
• The lug palm width, or a drawing. If the palm is wider than a stock washer, send a DXF, PDF or JPG — or a photograph with the bolt size, which is often enough to start.
• Quantity, and the certificate you need. 2.2 as standard, 3.1 if your specification calls for it.

Send those three and the quote comes back within 24 hours on working days. Ask for a quote, or size a cut part in the configurator.

Not sure which of the five joints you have? Send us the dimensions — we answer within minutes on working days, and the priced quote follows within 24.