Aluminium vs Copper Cable Sizing in Australia: AS/NZS 3008 Guide
11Sep
Ahmed Tayeh
Cable Sizing
Aluminium vs Copper Cable Sizing in Australia: AS/NZS 3008 Guide
Aluminium or copper? For most Australian electricians working on residential and light commercial installations, the answer is simple: always copper.
For electrical engineers specifying large feeders, industrial sub-mains, solar farm collection circuits, or MV cables on infrastructure projects, the answer is more nuanced.
Understanding aluminium vs copper cable sizing requires understanding:
The fundamental material difference between the two conductors
What termination requirements govern where aluminium can and cannot be used
Where the economics clearly favour one over the other
This guide covers all of it, with the specific numbers, table references, and worked comparison that make the decision straightforward.
What Is the Fundamental Difference Between the Two?
C15.2.webp179.57 KB Every sizing difference between aluminium and copper traces back to one fundamental material property: electrical conductivity.
Copper is a highly conductive metal with a conductivity of around 100% IACS (International Annealed Copper Standard).
Aluminium is a less expensive alternative with a conductivity of around 61% IACS.
Aluminium's electrical resistivity is 2.826 × 10⁻⁸ Ω·m compared to copper's 1.724 × 10⁻⁸ Ω·m. This gives aluminium approximately 61% of copper's conductivity by volume.
The practical consequence: a larger aluminium cross-section is needed to carry the same current as a smaller copper conductor. Typically, aluminium needs to be approximately 1.6× the copper cross-section to carry the same current.
This is the 1.6× rule. It is a useful starting approximation, but the precise sizing difference depends on the installation method, the ambient temperature, and the correction factors applied. The only reliable answer for a specific installation is to look up both conductor types in the correct AS/NZS 3008 table and compare directly. C15.3.webp167.88 KB
When Does Aluminum Win?
A 300 mm² aluminium cable (equivalent to 185 mm² copper in current rating) weighs approximately 50% less than the copper cable.
Aluminium is 61% the conductivity but only 30% the density and 25% the price. When you account for the larger cross-section needed, aluminium still costs 40% of the equivalent copper cable for the same current rating. The economic case for aluminium strengthens with cable size. At smaller sizes, the difference is not compelling:
At 25 mm² Cu vs 35 mm² Al: The size difference is small and the cost saving modest. Copper's installation simplicity and flexibility advantage outweighs the aluminium saving.
At 95 mm² Cu vs 150 mm² Al: On a 200 m cable run, the material cost difference can exceed AUD $15,000 per circuit. At this scale, aluminium is typically the specification that requires justification for copper substitution, not the other way around.
At 185 mm² Cu vs 300 mm² Al: The aluminium cable costs approximately 40% of the copper equivalent and weighs 50% less. For building risers, site distribution, and solar farm MV collection circuits, the cost advantage is decisive.
Aluminium is cost-effective for larger feeders and longer runs. Below 16 mm², the size penalty, termination complexity, and reduced flexibility make copper the practical default.
How Does AS/NZS 3008.1.1:2025 Handle Both Conductors?
AS/NZS 3008.1.1:2025 provides separate current rating columns for copper and aluminium conductors within the same installation method tables.
Current ratings come from the AS/NZS 3008.1.1:2025 Section 3 tables (Tables 3.9 to 3.20), and voltage-drop impedances from Tables 4.1 to 4.10, with three conductor options:
Copper (Cu)
Copper (Flexible)
Aluminium (Al)
For aluminium conductors, AS/NZS 3008 has restrictions on minimum size and insulation type:
Minimum Conductor Size: 16 mm² for solid aluminium; 25 mm² for stranded aluminium conductors used for fixed wiring
Insulation Requirement: Aluminium conductors must use XLPE (X-90) insulation for most applications above the minimum size. Aluminium conductors need special terminations to avoid creep failure.
The reason for the XLPE requirement is aluminium's thermal expansion behaviour. Aluminium expands and contracts more than copper with temperature cycling. PVC-insulated aluminium conductors can develop micro-gaps at termination points over time that increases contact resistance and creates a fire risk.
XLPE's harder insulation and improved chemical compatibility with aluminium termination hardware significantly reduces this risk.
How to Size an Aluminum Cable vs Copper?
The sizing process is identical for both conductors but the values at each step differ.
Step 1: Design Current (Ib)
The design current formula is the same regardless of conductor material:
Ib = P ÷ (V × PF) Three-phase: Ib = P ÷ (√3 × VL-L × PF)
This is where the conductors diverge. Using AS/NZS 3008 for cables clipped direct, 40°C ambient (no correction factor):
Copper — 95 mm² Cu/XLPE, clipped direct: approximately 260 A ≥ 246 A ✓
Aluminium — 150 mm² Al/XLPE, clipped direct: approximately 260 A ≥ 246 A ✓
Aluminium will generally require a larger size since 50 mm² aluminium tabulates 92 A versus 50 mm² copper at around 120 A in the same installation method. The size ratio at this current level:
150 mm² Al vs 95 mm² Cu — approximately 1.58×, consistent with the 1.6× rule
Step 3: Voltage Drop Comparison
This is where aluminium's higher resistivity has a direct performance impact. Aluminium cables have a higher resistance than copper cables, which can result in a larger voltage drop of around 2.5% per 100 m (based on a current of 100 A and a conductor temperature of 75°C).
For the 150 kW feeder, 80 m run, using the power factor method:
Both pass the AS/NZS 3000 5% limit comfortably on this 80 m run. On longer runs, the voltage drop difference between 95 mm² Cu and 150 mm² Al becomes more significant and must be checked for both conductors individually.
Step 4: Short-Circuit Withstand
The adiabatic equation (S = (I × √t) ÷ K) uses a different K constant for aluminium and copper:
Copper (PVC): K = 115
Copper (XLPE): K = 143
Aluminium (XLPE): K = 94
The lower K constant for aluminium means a larger minimum conductor size is required to withstand the same fault current and clearing time.
On circuits with high prospective fault currents, this check may require upsizing the aluminium conductor beyond what current-carrying capacity and voltage drop require.
What Is the Critical Constraint on Aluminum Use?
Aluminium's economic advantages are entirely contingent on correct termination. Poor aluminium terminations are one of the most common causes of electrical fires in industrial and commercial installations.
When connecting copper and aluminium cables, it's important to use bimetallic lugs to prevent corrosion. Copper cables are more durable and resistant to corrosion, fire, and heat.
The termination requirements for aluminium conductors are:
Bimetallic Lugs for Copper-to-Aluminium Connections. Where an aluminium conductor terminates at copper busbars or copper terminal blocks, bimetallic lugs must be used. Standard copper compression lugs must not be used on aluminium conductors: the electrolytic reaction between copper and aluminium in the presence of moisture causes progressive corrosion at the joint, increasing resistance and creating a fire risk.
Anti-Oxidant Compound. Aluminium forms an oxide layer rapidly when exposed to air. This oxide layer is non-conductive and increases contact resistance. Anti-oxidant compound must be applied to the conductor strands before compression termination to displace the oxide layer and prevent re-oxidation.
Aluminium-Rated Terminal Blocks. Switchboard terminal blocks specified for copper conductors may not have adequate contact pressure for aluminium. Aluminium-rated terminal blocks with appropriate spring pressure maintain contact despite aluminium's thermal expansion cycling.
Torque-Specified Fasteners. Aluminium termination bolts must be torqued to the manufacturer's specified value, both during installation and at the first maintenance inspection. Under-torqued aluminium connections develop high resistance over time; over-torqued connections damage the conductor strands.
Minimum Size Restrictions. AS/NZS 3000 limits aluminium conductors to 16 mm² solid or 25 mm² stranded for fixed wiring applications. Small aluminium conductors cannot achieve reliable terminations with standard hardware. This is why copper is mandatory below the minimum threshold.
Do I need a larger aluminium cable than copper to carry the same current in Australia?
Yes. Aluminium's electrical conductivity is 61% of copper's by volume, which means an aluminium conductor must have a larger cross-sectional area to carry the same current. Typically, aluminium needs to be approximately 1.6× the copper cross-section.
Is aluminium cable cheaper than copper for Australian electrical installations?
At 2026 commodity prices, aluminium cable delivers 40–55% material cost savings compared to copper for equivalent current-carrying capacity in sizes above 50 mm². Below 16 mm², the size penalty, termination complexity, and reduced flexibility make copper the practical default.
Cable Sizing
