Selecting the right cable for an underground service connection involves balancing four factors: current-carrying capacity, voltage drop compliance, DNSP approval, and cost. Get the balance wrong and you're either over-specifying (killing your margin) or under-specifying (failing compliance). Here's how to work through it systematically.

Copper vs Aluminium

The first decision is conductor material. Both copper (Cu) and aluminium (Al) are used in underground service connections, but they have very different characteristics:

Copper: Higher conductivity (lower resistance per metre), smaller cable diameter for the same current rating, easier to terminate, and more durable. But it's significantly more expensive — often 2–3× the price of equivalent aluminium cable.
Aluminium: Lower cost per metre, lighter weight, but requires larger cross-sections to carry the same current. Aluminium terminations need specific lugs and anti-oxidant compound to prevent corrosion at connection points.

For residential service connections, most DNSPs accept both materials. However, some specify a preference or mandate one over the other. Ausgrid, for instance, commonly specifies copper for consumer mains in certain configurations, while other DNSPs are material-agnostic as long as the cable meets current rating and voltage drop requirements.

Cost impact: On a typical 30m residential underground connection, switching from 16mm² Cu to 25mm² Al can save $150–$300 on cable alone — a meaningful difference when you're quoting competitively.

XLPE Insulation

Cross-linked polyethylene (XLPE) is the standard insulation type for underground service cables in Australia. It replaced older PVC-insulated cables for several reasons:

Higher operating temperature: XLPE is rated for 90°C continuous (vs 75°C for PVC), which means higher current-carrying capacity for the same cable size
Better moisture resistance: Critical for direct-buried and in-conduit applications where the cable is exposed to ground moisture
Longer service life: XLPE cables are rated for 30+ years underground with proper installation
Mechanical toughness: More resistant to abrasion during cable pulling through conduit

The most common cable designation you'll see on underground service jobs is X-HF (XLPE insulated, halogen-free sheath) or X-90 in various configurations. Multi-core cables (e.g., 2C+E or 4C+E) are typical for single-phase and three-phase services respectively.

Current Ratings and Common Sizes

AS/NZS 3008 provides current-carrying capacity tables for different cable sizes, materials, and installation methods. For underground cables installed in conduit (the most common method for service connections), typical ratings are:

16mm² Cu XLPE: ~85A (in conduit) — suits most 63A single-phase residential services
25mm² Cu XLPE: ~110A — common for 100A single-phase services or where voltage drop on longer runs requires upsizing
35mm² Cu XLPE: ~140A — used for three-phase connections or high-demand single-phase installations
25mm² Al XLPE: ~85A — aluminium equivalent to 16mm² Cu in terms of current rating
35mm² Al XLPE: ~105A — aluminium equivalent to 25mm² Cu
50mm² Al XLPE: ~135A — used for larger services or long cable runs where voltage drop drives the sizing

Note: These are indicative ratings. Actual current-carrying capacity depends on installation method, ambient temperature, soil thermal resistivity, grouping with other cables, and depth of burial. Always refer to AS/NZS 3008 tables for design calculations.

What Actually Drives Cable Selection

In practice, two factors dominate cable selection for underground services:

DNSP minimum specification: Most DNSPs publish a minimum cable size for each service rating. If the DNSP says 16mm² Cu minimum for a 63A service, that's your starting point — you can go larger but not smaller.
Voltage drop: On runs longer than about 25–30 metres, voltage drop often forces you to upsize beyond the minimum current-rated cable. A 16mm² Cu cable that comfortably carries 63A might fail the 5% voltage drop limit at 45 metres, requiring an upgrade to 25mm².

This is where accurate route measurement matters. A 5-metre measurement error on a borderline cable run can be the difference between a compliant 16mm² cable and a mandatory upgrade to 25mm² — a cost difference that directly affects your quote.

Bringing It Together

The cable sizing workflow for an underground connection should follow this sequence: identify the service rating, check the DNSP's minimum cable specification, measure the actual cable route distance, calculate voltage drop, and upsize if required. MAINS handles this automatically — when you draw a cable route on the map, it calculates the real distance, runs the voltage drop check against AS/NZS 3008, and recommends the smallest compliant cable size for both copper and aluminium options.