CUPAL washer size guide: which size should I choose?

In short

The CUPAL bimetal washer is a copper-aluminium transition washer for bolted electrical joints — it sits between a copper cable lug and an aluminium busbar to eliminate galvanic corrosion. It is not a temperature washer and not a bimetallic thermostat strip: it does not work from thermal expansion but acts as an electrical transition element, seating like metal against like metal. Stocked sizes run from M6 to M20, all 2 mm thick.

M6–M20: 11 stocked sizes
2mm: Thickness
70/30: Al / Cu ratio
412Ft: From net price

Choosing a CUPAL washer looks straightforward at first glance, but picking the right size is critical to making a reliable, corrosion-free joint. This guide walks through every available size and unit price and helps you make the right choice.

What is a bimetal washer?

The CUPAL bimetal washer is a copper-aluminium transition washer for bolted electrical joints. It goes into the bolted joint between a copper cable lug and an aluminium busbar (or vice versa) and keeps the two dissimilar metals from touching directly — which eliminates galvanic corrosion.

An important distinction: this is not the temperature-sensing "bimetal" strip found in thermostats, thermal switches or circuit-breaker trips. That kind of bimetal works from the differing thermal expansion of two metals and bends when heated. The CUPAL bimetal washer, by contrast, is an electrical transition element: it does not move, and thermal expansion is not the point — the point is that its copper side seats against copper and its aluminium side against aluminium, so like metal meets like metal at every contact face. A second distinction matters just as much: toothed copper-alloy washers are also sold as transition washers, and there the transition is not carried by the material but made as the bolt is tightened and its profile pierces the aluminium oxide film — bimetallic or toothed transition washer shows which one a drawing means.

The two sides of the washer are not separate pieces: an EN AW-1050A aluminium carrier and a Cu-ETP copper layer, joined inseparably by cold roll-bonding and diffusion welding. The mass ratio is typically 70/30 (aluminium/copper). The critical Cu/Al interface therefore stays inside the composite, protected from moisture, where corrosion cannot start.

Correct installation: the copper side faces the copper cable lug, and the aluminium side faces the aluminium busbar. The electrochemistry behind galvanic corrosion is covered in a separate article: galvanic corrosion prevention.

Full size table with unit prices

The table below lists every standard CUPAL washer size and net list price. All 11 sizes are available ex stock; the gross price with VAT is the net figure × 1.27.

Bolt size	Inner dia.	Outer dia.	Thickness	Max. cable cross-section	From net price
M6	6.4 mm	16 mm	2 mm	120 mm²	437 Ft
M8	8.4 mm	18 mm	2 mm	120 mm²	412 Ft
M10	10.5 mm	23 mm	2 mm	240 mm²	677 Ft
M10 (large)	10.5 mm	26 mm	2 mm	300 mm²	759 Ft
M12	13 mm	26 mm	2 mm	240 mm²	801 Ft
M12 (large)	13 mm	30 mm	2 mm	400 mm²	1,131 Ft
M12 (extra)	13 mm	46 mm	2 mm	500 mm²	2,357 Ft
M16	17 mm	35 mm	2 mm	500 mm²	1,234 Ft
M16 (large)	17 mm	46 mm	2 mm	630 mm²	2,357 Ft
M20	21 mm	37 mm	2 mm	500 mm²	1,768 Ft
M20 (large)	21 mm	46 mm	2 mm	630 mm²	2,357 Ft

Important: Every CUPAL washer is 2 mm thick, which ensures both adequate conductivity and mechanical strength. The full list price with SKUs is on the CUPAL washer product page. If what you have is a distributor designation rather than a size — 880111, VCUP2, ICALCU10CS — the CUPAL part number cross-reference maps it onto our own sizes.

Material, standard and certificate

Alongside size, the reliability of the joint comes from the material grade and its certification:

Base material: EN AW-1050A aluminium (min. 99.5% Al) and Cu-ETP electrolytic copper — both grades optimised for electrical conduction.
Material certificate: every shipment ships with an EN 10204 2.2 manufacturer's certificate as standard; a 3.1 inspection certificate is available on request. The difference between the two is explained in a separate article: EN 10204 material certificate 2.2 vs 3.1.
Application context: the bimetal transition element helps meet the standards that govern joints between dissimilar metals (IEC 61238 for cable connectors, IEC 61439 for low-voltage switchgear) — on contact resistance, temperature rise and long-term stability.

How do I choose a size?

The right CUPAL washer size is determined by three factors:

1. Bolt size (M-size)

The most important parameter: the inner diameter of the CUPAL washer must match the bolt of the joint. An M10 bolted joint requires an M10 CUPAL washer.

2. Cable cross-section

The washer's outer diameter covers the contact face of the cable lug. For a larger cable cross-section, choose a washer with a larger outer diameter so that the entire contact face is covered.

3. Cable lug size

Check the size of the cable lug's contact face. Ideally, the outer diameter of the CUPAL washer should cover the full contact face of the cable lug.

Practical examples

Low-current application (e.g. lighting distribution board)

Typical bolt size: M8 or M10
Cable cross-section: 16–70 mm²
Recommended CUPAL washer: M8/18mm or M10/23mm

Medium-current application (e.g. main distribution in a switchgear cabinet)

Typical bolt size: M10 or M12
Cable cross-section: 70–240 mm²
Recommended CUPAL washer: M10/26mm or M12/30mm

High-current application (e.g. industrial main distribution, transformer connection)

Typical bolt size: M12 or M16
Cable cross-section: 240–630 mm²
Recommended CUPAL washer: M12/46mm, M16/46mm or M20/46mm

Installation tips

Clean the surfaces before installation — both the cable lug and the busbar.
Check the orientation: the copper side (copper-coloured) should face the copper cable lug, and the aluminium side (silvery) should face the aluminium busbar.
Apply the correct torque to the bolt as specified by the material-pair or equipment manufacturer.
Do not use lubricants on the contact faces — they can impair conductivity.
A spring washer may also be used alongside the CUPAL washer to maintain pressure.

Unsure about the size?

If you are unsure which size is right, get in touch! On working days we reply within a few hours and help you select the correct size.

Browse our products: CUPAL washers | Galvanic corrosion prevention | EN 10204 material certificate | Request a quote

Frequently asked questions about CUPAL washer sizes

The CUPAL bimetal washer is a copper-aluminium transition washer for bolted electrical joints. Placed between a copper cable lug and an aluminium busbar, it eliminates galvanic corrosion because like metal meets like metal at every contact face. The copper and aluminium layers form a single composite, joined inseparably by diffusion welding.

No. The CUPAL bimetal washer is an electrical transition element, not a temperature sensor. The bimetal strip used in thermostats and thermal switches works from the differing thermal expansion of two metals and bends when heated; the CUPAL washer, by contrast, does not move — it seats a copper side against copper and an aluminium side against aluminium to prevent galvanic corrosion. Same name, completely different function.

In 11 stocked sizes, from M6 to M20, with inner diameters from 6.4 mm to 21 mm and outer diameters of 16–46 mm, all 2 mm thick. Net list prices start from 412 Ft. Several bolt sizes also come in a larger outer-diameter version for larger cable cross-sections.

A CUPAL washer sits in the bolted joint between a copper part and an aluminium one. It keeps the two dissimilar metals from touching directly and so eliminates galvanic corrosion. Each face goes against its own metal — whether that is a copper lug on an aluminium busbar or an aluminium lug on a copper busbar.

The bolt size sets the washer's inner diameter, and the cable-lug cross-section sets the outer diameter. Use M6 for small lugs (up to 120 mm²) and low-current distribution boards. Use M12 for medium- and high-current joints (240–500 mm²), such as a switchgear main distribution or a transformer connection, where a larger bolt and a larger contact face are needed.

Three factors determine the size. First, the bolt size (M-size) must match the washer's inner diameter — an M10 joint needs an M10 washer. Second, the cable cross-section: choose a larger outer diameter for larger cross-sections. Third, the cable lug size: the washer's outer diameter should ideally cover the lug's full contact face.

Every standard CUPAL washer is 2 mm thick. This thickness ensures both adequate electrical conductivity and mechanical strength in the joint.

The copper side (copper-coloured) must face the copper cable lug, and the aluminium side (silvery) must face the aluminium busbar. Installing it the wrong way round defeats the purpose of the bimetal transition.

For 240–630 mm² cable cross-sections and M12–M16 bolts, choose a large outer-diameter washer (M12/46 mm, M16/46 mm or M20/46 mm) so the washer covers the full contact face of the cable lug.

Do not use lubricants on the contact faces — they can impair conductivity. A spring washer may be used alongside the CUPAL washer to maintain joint pressure. Tighten the bolt to the torque specified by the equipment manufacturer.