A copper aluminium bimetallic washer (often called a Cu-Al or cupal washer) is a flat transition piece with solid copper on one face and solid aluminium on the other, metallurgically bonded in between. Placed in a bolted joint with the copper side against the copper conductor and the aluminium side against the aluminium conductor, it removes the direct copper-to-aluminium contact that causes galvanic corrosion, oxide build-up and overheating. This guide explains how the washers work, how to choose the construction, size and layer ratio, and how to install them so the joint stays cool for decades.
Copper and aluminium are the two conductor metals of the power industry, and they meet constantly: aluminium cable on copper switchgear terminals, aluminium busbars connected to copper transformer bushings, copper earth bars bonded to aluminium structures, aluminium battery busbars on copper-plated cell terminals. Bolting the two metals directly together is one of the most common causes of hot joints in electrical installations.
A bimetallic washer solves this by moving the copper-to-aluminium interface inside the washer, where it is a sealed, metallurgical bond rather than a mechanical contact exposed to air and moisture. Each outer face then forms a like-to-like joint: copper against copper, aluminium against aluminium. Those like-metal contacts behave predictably, and the bonded interface inside carries the current with very low, stable resistance.
The washers are stamped, laser-cut or machined from copper-clad aluminium sheet or strip produced by roll bonding or explosive bonding. They are made in round, square, rectangular and custom shapes, with one or several holes, to suit everything from M6 control-panel terminals to M20 bolts on high-current busbars.
A direct copper-aluminium joint may test perfectly on the day of installation and still fail months or years later. Four mechanisms act together, and each makes the others worse.
Copper and aluminium sit far apart in the galvanic series; in seawater the potential difference is roughly 0.5 – 0.6 V. When moisture bridges the contact, the joint becomes a battery and the aluminium, as the anode, corrodes. Corrosion products are bulky and non-conductive, so contact area shrinks and resistance rises.
Aluminium forms a hard, insulating oxide film within seconds of exposure to air. Unless the film is broken during assembly and kept from reforming, current flows only through a few small metal-to-metal spots, which run hot.
Aluminium expands about 23 µm/m·°C and copper about 17 µm/m·°C, while steel bolts expand about 12 µm/m·°C. Each load cycle heats and cools the joint unevenly, so clamping force changes and the contact surfaces micro-slide (fretting).
Soft, high-purity aluminium slowly creeps under bolt pressure, especially when warm. As it flows, bolt tension drops, contact pressure falls and resistance increases.
The result is a feedback loop: higher resistance makes more heat, heat accelerates oxidation, creep and corrosion, and those raise resistance further. Thermal imaging surveys of distribution boards routinely find copper-aluminium joints running tens of degrees above neighbouring connections for exactly this reason. In the worst cases the joint fails open or starts a fire.
| Zone | What happens | Effect on the joint |
|---|---|---|
| Copper face | Copper layer contacts the copper conductor | Like-metal contact with no galvanic couple |
| Bonded Cu-Al interface | Metallurgical bond formed by roll or explosive bonding, sealed from air and moisture | Low, stable interface resistance that cannot corrode from outside |
| Aluminium face | Aluminium layer contacts the aluminium conductor | Like-metal contact; only the normal oxide film needs managing |
| Clamped area | Bolt load spreads across the full washer face | Larger effective contact area and more even pressure |
The galvanic couple has not disappeared; it has been moved to the bonded interface inside the washer, where electrolyte cannot reach it. The washer edge is the only place the two metals meet the atmosphere, which is why edge quality and, in damp locations, edge sealing matter.
A bimetallic washer is not an insulating washer. Its purpose is to conduct current with low resistance, so it must sit in the current path between the two conductors, not under the bolt head.
| Feature | Clad (bonded) bimetallic washer | Aluminium washer plated with copper on one side |
|---|---|---|
| Structure | Solid copper layer and solid aluminium layer bonded by rolling or explosion | Aluminium body with an electroplated copper coating on one face |
| Copper thickness | Typically 10 – 50 % of total thickness, i.e. tenths of a millimetre or more | A few micrometres to tens of micrometres |
| Interface | Metallurgical bond tested by bend, peel or shear | Plating adhesion; depends heavily on surface preparation |
| Resistance to damage | Copper layer survives scratching, abrasion and re-tightening | Thin layer can be scratched through, exposing aluminium to copper |
| Humid and outdoor service | Suitable with good edges and, if needed, sealing | Not recommended; corrosion can start at pores or scratches in the plating |
| Cost | Higher | Lower |
| Best suited to | Power distribution, transformers, switchgear, outdoor and long-life joints | Light-duty, dry indoor connections where cost is critical |
For any joint carrying significant current or exposed to humidity, specify a clad washer. A plated washer can look identical on delivery; ask for a cross-section photograph or a layer-thickness measurement on the certificate to confirm which construction you are buying.
| Layer | Common grades | Electrical conductivity (annealed, typical) | Why it is used |
|---|---|---|---|
| Copper | Cu-ETP: C11000 (ASTM B152), CW004A (EN 13599) | about 100 – 101 % IACS | The standard electrical copper; ductile and bonds well |
| Copper, oxygen-free | Cu-OF: C10200, CW008A | about 101 % IACS | Where hydrogen embrittlement or very high purity is a concern |
| Aluminium | 1060, 1050A, 1070 (EN AW-1060 / 1050A / 1070A) | about 61 – 62 % IACS | High-purity, highly conductive, soft enough to bond and stamp cleanly |
| Aluminium, electrical grade | 1350 (EC grade) | about 61 – 62 % IACS | Matches 1350 busbars and conductors |
| Aluminium, commercially pure | 1100 | about 57 – 59 % IACS | Slightly stronger and more formable; for some custom shapes |
Conductivity values are typical published figures for annealed material; the certificate gives measured values where specified.
The layer ratio is expressed as the percentage of total thickness that is copper. It controls cost, weight, how much copper is available for the contact face, and how much machining or cleaning the copper side can tolerate.
| Ratio (Cu / Al by thickness) | Copper layer in a 2 mm washer | Effective density | When to use it |
|---|---|---|---|
| 15 / 85 | 0.30 mm | 3.64 g/cm³ | Weight- and cost-sensitive joints in dry indoor service |
| 20 / 80 | 0.40 mm | 3.95 g/cm³ | The common standard for busbar and terminal transitions |
| 30 / 70 | 0.60 mm | 4.57 g/cm³ | Thicker copper face for rough mating surfaces, re-dressing or heavier clamping |
| 50 / 50 | 1.00 mm | 5.82 g/cm³ | Special designs where copper-side robustness matters more than weight |
Electrically, the washer is short and wide, so its own resistance is tiny compared with the two contact interfaces on either side. Changing the ratio therefore has little effect on joint resistance; the practical reasons to change it are mechanical robustness of the copper face, weight and cost. A 20/80 washer weighs well under half as much as a solid copper washer of the same size.
Central hole sized to the bolt, outer diameter sized to the terminal pad. The standard choice for single-bolt terminals on cable lugs, small busbars, switchgear and transformer studs.
Covers more of a square terminal or bar end than a round washer of the same width, giving more contact area for the same footprint.
Matches the width of the busbar, with one, two or four holes in standard busbar drilling patterns. Common on transformer low-voltage terminals and high-current links.
Notched, stepped, slotted or offset-hole shapes made to drawing for confined spaces, replacement parts or non-standard terminals.
As a rule, the washer should cover the whole overlapping contact area of the two conductors. A small round washer on a wide busbar joint only transfers current through the area it covers, leaving the rest of the overlap unused.
The table shows reference dimensions for round washers by bolt size, with calculated weights for a 20/80 clad washer. Outer diameters are larger than those of ordinary steel washers because the washer is a current-carrying element and needs contact area.
| Bolt | Hole d (mm) | Outer D (mm) | Thickness options (mm) | Weight at 2 mm (g) | Weight at 3 mm (g) |
|---|---|---|---|---|---|
| M6 | 6.5 | 14 | 1 · 2 · 3 | 1.0 | 1.4 |
| M8 | 8.5 | 18 | 1 · 2 · 3 | 1.6 | 2.3 |
| M10 | 11 | 22 | 1 · 2 · 3 | 2.3 | 3.4 |
| M12 | 13 | 28 | 1 · 2 · 3 | 3.8 | 5.7 |
| M14 | 15 | 30 | 1 · 2 · 3 | 4.2 | 6.3 |
| M16 | 17 | 35 | 1 · 2 · 3 | 5.8 | 8.7 |
| M20 | 21 | 40 | 1 · 2 · 3 | 7.2 | 10.8 |
Weights calculated from geometry and the weighted density of a 20 % Cu / 80 % Al clad structure (3.95 g/cm³). Other diameters, thicknesses and ratios are made to drawing.
Round washers in volume are normally stamped, which gives the best consistency once tooling is made. Square, rectangular and complex shapes, and short runs, can be laser-cut, waterjet-cut or machined, which avoids tooling cost but needs closer deburring. Laser cutting heats the edge, so the bond at the cut edge should be checked on first-off parts.
| Environment | Recommended construction | Additional protection |
|---|---|---|
| Dry indoor (switchboards, panels) | Clad washer; plated washer acceptable for light duty | Clean surfaces and correct torque are usually enough |
| Humid, condensing (substations, basements, pump rooms) | Clad washer only | Oxide-inhibiting joint compound on the aluminium face; consider edge sealing |
| Outdoor (transformers, poles, solar fields) | Clad washer with sound edges | Joint compound, sealing tape or heat-shrink boots over the joint; stainless or plated fasteners |
| Marine and salt spray | Clad washer, thicker copper layer | Full sealing of the joint and washer edge; regular inspection |
| Chemically aggressive (plants, electrolysis, fertiliser) | Clad washer, engineered for the site | Sealing compounds or enclosures selected for the specific chemicals |
Copper-plated aluminium washers should be avoided wherever condensation, rain or salt can reach the joint: once moisture finds a pore or scratch in the plating, the thin copper layer and aluminium form a galvanic cell under the plating, and the coating can lift.
| Method | How it works | Strengths | Limitations |
|---|---|---|---|
| Bimetallic washer | Clad Cu-Al washer between conductors | Simple retrofit; no change to conductors; sealed interface | Adds one more interface; washer must cover the contact area |
| Tin-plated aluminium busbar or lug | Tin layer on the aluminium contact face | Stable, solderable surface; widely used | Tin reduces but does not eliminate galvanic action; plating can wear |
| Bimetallic lug or connector | Friction-welded Cu palm and Al barrel | Ideal for terminating aluminium cable on copper terminals | Only for cable terminations; not for bar-to-bar joints |
| Copper-clad aluminium busbar | Copper layer bonded over an aluminium core | Copper contact surfaces with aluminium weight | Replaces the bar itself; higher cost than plain aluminium |
| Joint compound alone | Inhibitor paste excludes air and moisture | Cheap; useful supplement | Not a substitute for a transition in humid or outdoor service |
In practice these methods are combined. A transformer connection might use a copper-clad aluminium busbar on long runs, tin-plated aluminium where bars meet each other, and a bimetallic washer where a plain aluminium bar must meet an existing copper terminal.
A bimetallic washer placed under the nut or bolt head, outside the current path, does nothing to protect the copper-aluminium contact between the conductors. It must sit between the two conductor faces.
With copper against aluminium on both faces, the joint now has two galvanic contacts. Mark the copper face on drawings and check it at inspection.
A small round washer on a wide busbar overlap leaves most of the joint unused, so current crowds through a small area and the joint runs hot.
Electroplated copper looks the same as clad copper on delivery but can fail quickly once condensation reaches a scratch or pore.
Flat washers alone let bolt tension fall as aluminium creeps. Use conical spring washers or the maker's recommended hardware and re-check torque where procedures require it.
Non-conductive grease insulates the contact. Use only compounds designed for electrical aluminium joints, applied as a thin film.
Most of these mistakes are invisible once the joint is assembled, which is why thermal imaging after the first weeks of load is so valuable. A transition joint that runs at the same temperature as its neighbours is working; one that runs noticeably warmer should be opened, cleaned and remade before it degrades further.
The copper (reddish) side faces the copper conductor and the aluminium (silver) side faces the aluminium conductor. Installing it the other way round creates two copper-aluminium contacts instead of none.
No. A bimetallic washer has a solid copper layer metallurgically bonded to aluminium, typically tenths of a millimetre thick. A plated washer has a thin electroplated coating that can be scratched through and is unsuitable for humid or outdoor joints.
No. 20/80 is the most common standard, but 15/85, 30/70 and 50/50 are also made. The ratio affects weight, cost and how robust the copper face is far more than it affects joint resistance.
In dry indoor service, clean surfaces and correct torque are usually enough. In humid, outdoor or marine locations, apply an oxide-inhibiting compound on the aluminium face and seal the joint edges.
Only if it is flat, the copper face is intact and the edges show no separation. Because washers are inexpensive compared with the cost of a joint failure, most maintenance procedures replace them when a joint is remade.
Yes. A sample, a dimensioned photo or a drawing is enough to reproduce the shape; the copper layer thickness is confirmed by measuring a cross-section of the original where possible.
Match the hole to the bolt and make the washer large enough to cover the overlapping contact area of the two conductors. On wide busbar joints, use a rectangular transition plate drilled to the busbar pattern rather than small round washers.
Send the drawing or sample, bolt size, thickness, layer ratio and quantity. We will confirm the construction, inspection and delivered price.
