Connecting aluminium and copper — bimetallic corrosion and why cross-section doesn't scale 1-to-1

The cable-terminals guide and the contact-resistance measurement guide cover terminal connections and measuring transition resistance in general. This article covers a specific risk that neither addresses: what happens when an aluminium conductor is connected directly to a copper conductor or terminal — a situation that occurs increasingly often as aluminium cable is used for larger cross-sections (main supply cables, large-consumer connections) as a cheaper alternative to copper.

The problem: galvanic (bimetallic) corrosion

When two different metals are in contact in the presence of a conductive liquid (moisture, condensation), galvanic corrosion occurs — also called bimetallic corrosion — driven by the difference in electrode potential between the two metals. Aluminium and copper form one of the least favourable combinations in this series: on direct contact, the aluminium gradually corrodes away, so that over time the connection:

Loosens — the corrosion product (aluminium oxide) occupies more volume than the original metal, reducing clamping force.
Develops a higher transition resistance — aluminium oxide is itself a poor conductor, unlike the aluminium core underneath it.
Starts generating heat — a higher transition resistance at the same current means more power loss (P = I²R) at exactly that point, which further accelerates corrosion: a self-reinforcing process that eventually leads to a burnt terminal or a loose connection.

Note: this process develops gradually, often over months to years. An aluminium-copper connection that measures a perfectly fine transition resistance at handover may still have deteriorated by the time of a re-inspection years later — this is exactly why periodic contact-resistance measurement (see the contact-resistance measurement guide) is especially relevant for this type of connection.

Solutions: how an aluminium-copper connection is made safe

Contact paste (for example an "alu-plus"-type paste) — applied inside the terminal before the aluminium conductor is clamped, this paste blocks fresh oxidation at the contact point and keeps the connection conductive.
Specially certified aluminium or bimetallic terminals — terminals specifically designed and certified for aluminium (or for an aluminium-copper transition), with a geometry and clamping force matched to the creep behaviour of aluminium (aluminium gradually deforms under sustained pressure, "creeps", which causes a copper-designed terminal to lose clamping force over time).
Bimetallic connectors — a factory-made transition between an aluminium and a copper part, where the actual aluminium-copper interface is produced under controlled, corrosion-resistant conditions (often via explosion welding or a comparable process), rather than in an ordinary terminal in the field.

Why cross-section isn't simply "converted"

Aluminium has lower conductivity than copper (roughly 61% of copper's conductivity at equal cross-section). This means that an aluminium conductor at the same cross-section has a noticeably lower permissible current than a copper conductor — as a rough rule of thumb, an aluminium conductor needs roughly 1.5 to 1.6 times the cross-section of a copper conductor to reach the same current-carrying capacity. This is part of why wiring standards typically only permit aluminium in fixed installations from a practical minimum cross-section upward (for example, 16 mm² under the UK's BS 7671, Table 52.3): below that, the lower ampacity and the creep behaviour (see below) no longer outweigh aluminium's cost advantage. An aluminium cable therefore needs to be sized larger than the copper cable it replaces — never by simply ordering the same cross-section in aluminium on the assumption that it performs "the same".

Practical relevance

When assessing or connecting an installation with aluminium main supply cables (common with large-consumer connections or older commercial installations), always check whether connection points where aluminium transitions to copper (for example at the meter cabinet, a main distribution board, or a connection to an existing copper site network) are made using contact paste, a certified bimetallic terminal, or a bimetallic connector — never an ordinary copper terminal without any additional measure. This applies both to new installations and when assessing an existing installation during an NEN 3140 inspection.

Common mistakes

Clamping aluminium directly into an ordinary copper terminal without contact paste or aluminium certification — this eventually leads to galvanic corrosion and a heat-generating connection.
Keeping the same cross-section when replacing copper with aluminium — because of aluminium's lower conductivity, the cross-section must be increased to retain the same current-carrying capacity.
Measuring a good transition resistance once and assuming it stays that way — bimetallic corrosion is a gradual process; periodic re-measurement is needed, especially for this type of connection.
Overlooking aluminium creep — a terminal designed and torqued for a copper conductor does not retain its clamping force on an aluminium conductor, even without any corrosion occurring.