Why does aluminium cable need a larger cross-sectional area than copper for the same current rating?

Aluminium has a resistivity of approximately 2.82 × 10⁻⁸ Ω·m compared to 1.72 × 10⁻⁸ Ω·m for annealed copper — aluminium is about 1.64 times more resistive. This means that for the same cross-sectional area, an aluminium conductor will carry less current than a copper conductor at the same temperature rise. To achieve the same current-carrying capacity, the aluminium conductor must be larger in cross-sectional area. As a practical rule, an aluminium conductor rated for the same current as a copper conductor will be approximately 1.6 times the cross-sectional area. For example, 25mm² aluminium is broadly equivalent to 16mm² copper for current-carrying capacity under similar installation conditions. Check the specific values in BS 7671 Appendix 4 Tables 4D and 4E for your installation method.

What is anti-oxidant compound and why is it needed for aluminium terminations?

Aluminium rapidly forms an aluminium oxide (Al₂O₃) layer on its surface when exposed to air. This oxide layer has very high electrical resistance and will cause the connection to overheat if not removed before termination. Anti-oxidant compound (also called jointing compound or inhibitor paste) is a petroleum-based paste containing abrasive particles. When applied to the stripped aluminium conductor end, the abrasive particles penetrate and break through the oxide layer, and the petroleum base seals the connection from air to prevent re-oxidation. The correct procedure is: strip the conductor, immediately apply anti-oxidant compound, work the compound into the conductor strands, insert into the terminal, and tighten to the manufacturer torque specification. The compound must remain in the connection — do not wipe it off.

Can aluminium cables be used for domestic socket outlet circuits?

Aluminium cable is not suitable for domestic socket outlet circuits in standard wiring accessories. The reasons are: (1) Galvanic corrosion — direct contact between aluminium and copper terminals in a damp environment causes corrosion at the interface. (2) Creep — aluminium is a softer metal than copper and will deform under the compression of a screw terminal over time, leading to a loose connection and overheating. (3) Coefficient of thermal expansion — aluminium expands and contracts more than copper with temperature cycling, further loosening connections. (4) All standard UK wiring accessories (socket outlets, switches, ceiling roses) are designed for copper conductors. Aluminium cable is appropriate for large fixed conductors — service heads, meter tails, submain cables, and large industrial cabling — where connections are infrequent and properly managed with bi-metallic clamps and anti-oxidant compound.

What are the current ratings of aluminium cables under BS 7671?

BS 7671 Appendix 4 Tables 4E1A to 4E5A give current ratings for aluminium conductors in different installation configurations. For example, under Table 4E1A (single-phase, twin insulated cable, Method C — clipped direct): 16mm² aluminium is rated 73A, 25mm² is rated 96A, 35mm² is rated 119A, 50mm² is rated 142A. Compare these to Table 4D1A for copper: 16mm² copper is rated 87A, 25mm² is rated 114A — confirming that aluminium requires a larger CSA for the same rating. For the precise rating applicable to your installation, always apply the appropriate derating factors from Table 4C1 (grouping) and Table 4B (ambient temperature) to the base current rating.

Is aluminium cable used for solar PV and EV charging installations?

Aluminium cable may be used for the main supply cable from the consumer unit to an EV charger or solar PV inverter if the cable run is long and the cross-sectional area is large enough to justify it (typically 16mm² and above). However, the termination at the EV charger or inverter must be checked — many manufacturers specify copper-only terminations and will not warrant the equipment if aluminium is connected without a bi-metallic terminal or copper tail. For solar PV DC cabling between panels and the inverter, copper is standard — aluminium DC conductors are not used in residential PV installations. For commercial and utility-scale PV, aluminium armoured cable is used for AC connections between inverters and the grid connection.

What torque should be applied when terminating aluminium cables?

The termination torque for aluminium cables must follow the terminal manufacturer specifications exactly. Aluminium is softer than copper and more susceptible to cold flow (creep) under compression — over-tightening can deform the conductor and cause a loose connection after the compression relaxes. Under-tightening leaves an inadequate contact area that will overheat under load. Always use a calibrated torque screwdriver or wrench and record the torque value. For large aluminium conductors (25mm² and above) in compression-type terminals, re-tightening after the first thermal cycle may be specified by the manufacturer. This is particularly important in industrial switchgear and distribution boards where aluminium busbars or cables are used.

What are the weight and cost advantages of aluminium over copper?

Aluminium is approximately one-third the density of copper and has historically been around half the cost per kilogram, though prices fluctuate with commodity markets. For large cable sizes (35mm² and above) used in distribution networks, the weight saving of aluminium over copper can be significant — relevant for overhead lines, cable trays with weight limits, and long cable routes where cable pulling tension is a factor. In the UK distribution network, aluminium conductors are standard for overhead lines and underground distribution cables. For electricians, the cost and weight advantages of aluminium become meaningful at 25mm² and above — for smaller sizes, the termination care required and the need for larger CSA generally make copper more practical.

CABLE SELECTION GUIDE

Copper vs Aluminium Cable: Current Ratings, Termination and When to Use Each

Aluminium requires 1.6× the cross-sectional area of copper for the same current rating — and must be terminated with anti-oxidant compound. This guide covers BS 7671 Appendix 4 current ratings, correct termination, and when aluminium cable is appropriate in UK electrical installations.

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11 min readUpdated 2026-06-10Andrew Moore, Founder of Elec-Mate

Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.

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Key Takeaways

1Aluminium has approximately 61% of the electrical conductivity of copper. An aluminium conductor must be around 1.6 times the cross-sectional area of a copper conductor to carry the same current — for example, 25mm² aluminium has a similar current rating to 16mm² copper.
2BS 7671 Appendix 4 Tables 4D1A–4D5A cover copper conductors; Tables 4E1A–4E5A cover aluminium conductors. Current ratings are given for each installation method and conductor size.
3Aluminium conductors must never be terminated in standard copper terminals without anti-oxidant compound. The aluminium oxide layer that forms on the conductor surface must be removed and the connection sealed with anti-oxidant compound to prevent re-oxidation. BS 7671 Reg 526.1 requires the connection method to account for conductor material — terminals must be rated for aluminium.
4Aluminium PEN conductors (e.g. in meter tails) must be at least 16mm² CSA (Reg 543.4.201) and are prohibited in any part of the installation supplied through an RCD — including PME/TN-C-S systems where an RCD is inserted upstream.
5Aluminium cable is most commonly used in the UK for service heads, meter tails, distribution cables, and large industrial/commercial installations where the weight saving and lower cost justify the termination care required.
6Aluminium is not suitable for wiring accessories, standard socket outlets, or domestic circuits where connections are frequently disturbed. The galvanic corrosion risk between aluminium and copper terminals makes aluminium unsuitable for small-conductor domestic applications.

01 · Cable Selection Guide

Copper vs Aluminium Cable: A Technical Comparison for UK Electricians

Copper and aluminium are the two conductor materials used in electrical installations. Copper dominates domestic and most commercial wiring in the UK. Aluminium is used extensively in the distribution network and in large commercial and industrial installations where its weight and cost advantages become significant.

The choice between copper and aluminium affects cable sizing (aluminium requires a larger cross-sectional area for the same current rating), termination method (aluminium requires anti-oxidant compound and care with torque), and the suitability of the cable for the application. Both conductor types are covered in BS 7671:2018+A4:2026 — copper in Appendix 4 Tables 4D, aluminium in Tables 4E.

This guide explains the current ratings for both materials, the correct termination method for aluminium, and the applications where each material is appropriate.

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02 · Cable Selection Guide

Current Ratings: BS 7671 Appendix 4

BS 7671 Appendix 4 provides current-carrying capacity tables for cables in various installation configurations. Copper conductors are covered in Tables 4D1A to 4D5A; aluminium conductors in Tables 4E1A to 4E5A. The tables give current ratings in amperes for different conductor sizes and installation methods.

Comparative Current Ratings (Method C — Clipped Direct, Single Phase)

Conductor Size	Copper Rating (A)	Aluminium Rating (A)
16mm²	87A	73A
25mm²	114A	96A
35mm²	141A	119A
50mm²	168A	142A

Source: BS 7671:2018+A4:2026 Appendix 4, Tables 4D1A (copper, PVC twin-and-earth) and 4E1A (aluminium equivalent), 30 °C ambient, no grouping. Always verify the exact values from the relevant table for your cable type (e.g. Table 4D5A for XLPE/90 °C conductors gives higher ratings). Apply derating factors from Table 4C1 (grouping) and Table 4B (ambient temperature) before selecting a conductor size.

Use the Elec-Mate cable sizing calculator to apply the correct derating factors from BS 7671 Appendix 4 (grouping factor from Table 4C1, ambient temperature factor from Table 4B) to determine the derated current capacity for your specific installation conditions.

03 · Cable Selection Guide

Termination Requirements for Aluminium Conductors

Correct termination of aluminium conductors is critical. The aluminium oxide layer that forms on the conductor surface within seconds of stripping must be removed and the connection sealed to prevent re-oxidation. Failure to do this correctly causes a high-resistance joint that will overheat under load, potentially causing a fire.

BS 7671 Reg 526.1 requires that every connection shall provide durable electrical continuity and adequate mechanical strength, and that the means of connection shall take account of the conductor material (Reg 526.1(a)), conductor class and wire shape (Reg 526.1(b)), and cross-sectional area (Reg 526.1(c)). This means a terminal or lug that is not rated for aluminium cannot be used — even with anti-oxidant compound — unless the manufacturer has specifically approved and tested it for use with aluminium conductors.

Aluminium Termination — Critical Requirements

• Never terminate aluminium in standard copper-only terminals without anti-oxidant compound
• Strip the conductor and apply anti-oxidant compound immediately — do not allow the stripped end to sit in air
• Work the compound into the conductor strands with a brush or by working the conductor into the paste
• Use terminals rated for aluminium conductors — bi-metallic terminals where copper to aluminium connection is required
• Tighten to the manufacturer-specified torque using a calibrated torque tool
• Re-tighten after the first thermal cycle if specified by the terminal manufacturer

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04 · Cable Selection Guide

When Aluminium Cable is Appropriate

Aluminium cable is appropriate in the following UK applications:

Service heads and meter tails: DNO service heads in the UK commonly use aluminium conductors for the final connection to the meter. The tails from the service head to the consumer unit may be aluminium in older installations. Always use anti-oxidant compound and bi-metallic clamps when connecting aluminium tails to copper terminals in the consumer unit or isolator. Note: Reg 543.4.201 permits an aluminium conductor to serve as a PEN conductor only where its CSA is at least 16mm² and the part of the installation concerned is not supplied through an RCD — aluminium PEN conductors are prohibited downstream of any RCD (including PME/TN-C-S arrangements where an RCD is fitted between the origin and the point of use).
Distribution cables (submains): Large-CSA aluminium armoured cables are commonly used for submain cables between distribution boards and sub-distribution boards in commercial and industrial premises, particularly for runs of 25mm² and above where the weight and cost savings are significant.
Overhead lines: Aluminium conductors (with or without a steel core for mechanical strength — ACSR, Aluminium Conductor Steel Reinforced) are the standard for overhead distribution lines in the UK due to their weight and cost advantages over copper for long spans.

Not Appropriate For:

• Domestic socket outlet circuits and wiring accessories
• Small cross-sections (below 16mm²) in most applications — Reg 524.2.3(c) sets 25mm² aluminium as the minimum for a reduced neutral conductor in a polyphase circuit; general practice is 16mm² minimum for aluminium line conductors in fixed wiring
• Connections that will be frequently disturbed
• Flexible cables and cords

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05 · Cable Selection Guide

Jointing and Connections for Aluminium Cables

Aluminium-to-aluminium and aluminium-to-copper connections require specific jointing methods to prevent galvanic corrosion and ensure long-term reliability:

Compression joints: The standard jointing method for aluminium cables. A compression sleeve is crimped onto the stripped conductor ends using a calibrated compression tool. Anti-oxidant compound is applied before crimping. Compression joints are reliable and tamper-evident.
Bi-metallic terminals: Where aluminium cable must be connected to copper busbars or copper terminal blocks, bi-metallic (aluminium/copper) terminals prevent galvanic corrosion at the interface. The aluminium part of the terminal accepts the aluminium conductor; the copper part makes the connection to the copper equipment.
Mechanical connectors: Bolted-type mechanical connectors with serrated washers are used for larger conductors in distribution switchgear. The serrations cut through the oxide layer on contact. Anti-oxidant compound and correct bolt torque are essential.
Insulating barriers (inspection and older installations): Where aluminium conductors are in proximity to copper in an existing installation and a bi-metallic terminal cannot be used, BS 7671:2018+A4:2026 recognises the use of an insulating barrier — such as sleeving or a non-conductive washer — to prevent direct contact between the dissimilar metals and avoid electrolytic action. This approach is relevant when inspecting older systems or making repairs where conductor replacement is not practicable.

06 · Cable Selection Guide

Practical Considerations: Copper vs Aluminium on Site

Copper Advantages

• Higher conductivity — smaller CSA for same current
• Simpler termination — no anti-oxidant compound required
• Compatible with all standard wiring accessories
• More flexible — easier to route and terminate
• Standard for all domestic and most commercial wiring

Aluminium Advantages

• Lower cost per metre (especially large CSA)
• Approximately one-third the weight of copper
• Standard for distribution networks and submains
• Suitable for fixed large-CSA installations
• Lower pulling tension for long cable routes

07 · Cable Selection Guide

For Electricians: Cable Selection in Practice

For the vast majority of domestic and commercial electrical work, copper cable is the correct choice — it is simpler to install, easier to terminate, and compatible with all standard wiring accessories. Aluminium cable is the correct choice for large submain cables, distribution network connections, and applications where weight and cost at large CSA are significant factors.

When you encounter existing aluminium wiring (particularly in properties built in the 1960s and 1970s where solid aluminium domestic wiring was briefly used), inspect the connections carefully. Loose or corroded aluminium connections at wiring accessories are a Code C2 or C1 defect depending on severity.

This guide is written for qualified electricians working to BS 7671:2018+A4:2026 and reviewed against the current edition of the standard. Last reviewed: May 2026.

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Frequently Asked Questions: Copper vs Aluminium Cable

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Copper vs Aluminium Cable: Current Ratings, Termination and When to Use Each

Key Takeaways

Copper vs Aluminium Cable: A Technical Comparison for UK Electricians

Get the BS 7671 A4:2026 Cheat Sheet — free

Current Ratings: BS 7671 Appendix 4

Termination Requirements for Aluminium Conductors

Record cable type and termination details on your

When Aluminium Cable is Appropriate

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Jointing and Connections for Aluminium Cables

Practical Considerations: Copper vs Aluminium on Site

Copper Advantages

Aluminium Advantages

For Electricians: Cable Selection in Practice

Size cables correctly with BS 7671 Appendix 4

Frequently Asked Questions: Copper vs Aluminium Cable

Why does aluminium cable need a larger cross-sectional area than copper for the same current rating?

What is anti-oxidant compound and why is it needed for aluminium terminations?

Can aluminium cables be used for domestic socket outlet circuits?

What are the current ratings of aluminium cables under BS 7671?

Is aluminium cable used for solar PV and EV charging installations?

What torque should be applied when terminating aluminium cables?

What are the weight and cost advantages of aluminium over copper?

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