DC Cable Sizing — AS/NZS 3008.1.1:2025 Complete Guide

The AS/NZS 3008.1.1:2025 edition introduces, for the first time, dedicated current rating tables and methodology for DC (direct current) cables operating up to 1500 V. Before this edition, engineers sizing DC cables in Australia and New Zealand had to rely on AC tables with manual corrections, IEC 60364-7-712, or manufacturer guidance — none of which carried the authority of the national standard.

This is a direct response to the explosive growth of DC electrical systems:

• Solar PV arrays: Utility-scale solar farms now commonly operate at 1500 V DC string voltage (up from the older 1000 V standard) to reduce balance-of-system costs.
• Battery energy storage systems (BESS): Commercial and utility-scale battery installations use DC cables from battery racks to inverters, often at voltages between 400 V and 1500 V DC.
• EV charging infrastructure: DC fast chargers operate at up to 500 V DC (with architectures up to 1500 V DC emerging).
• DC microgrids: Data centres and industrial facilities are increasingly adopting DC distribution to eliminate AC/DC conversion losses.

While the fundamental thermal principles are the same — a cable must carry the required current without exceeding its maximum conductor temperature — there are important differences between DC and AC cable sizing:

Because DC cables do not experience skin effect or proximity effect losses, the DC current ratings in the 2025 edition are marginally higher than the AC equivalent for larger conductor sizes. The approximate advantage for copper XLPE (X-90) single-core cables:

Conductor size    DC vs AC advantage (approximate)
  16 mm² Cu XLPE:  <1% higher than AC rating
  35 mm² Cu XLPE:  ~1% higher than AC rating
  95 mm² Cu XLPE:  ~2% higher than AC rating
 185 mm² Cu XLPE:  ~3% higher than AC rating
 300 mm² Cu XLPE:  ~4% higher than AC rating
 630 mm² Cu XLPE:  ~7% higher than AC rating

For aluminium conductors, the advantage is similar in percentage terms. This means that for large-scale solar farms using 300 mm² or 630 mm² aluminium DC cables, the 2025 DC tables may allow a smaller conductor than the AC-derived calculations previously used — a meaningful cost saving at scale.

The 2025 DC cable provisions cover these common installation scenarios:

Solar PV String Cables

String cables connect series-connected PV modules to string inverters or combiners. For 1500 V DC systems, the string cable typically carries 10–15 A at up to 1500 V DC. The 2025 edition provides current ratings for single-core DC cables in:

• Cable tray (above ground) — perforated and unperforated
• Clipped to structure (typical for rooftop solar)
• Buried direct in ground (ground-mount solar arrays)
• In conduit or duct (common for cable runs from array to inverter room)

Battery Energy Storage Systems (BESS)

DC cables from battery racks to inverters or DC busbars carry high currents (often 200–500 A per string) at voltages from 400 V to 1500 V DC. The 2025 edition includes derating considerations for proximity to battery thermal runaway zones, referencing AS/NZS 5139:2019.

EV Charging DC Fast Chargers

DC cables from the grid-side power conversion equipment to the charging connector supply currents up to 500 A at 200–1000 V DC (CCS2 standard). The 2025 provisions apply to the fixed wiring portion — the flexible charging cable is covered by the charger standard.

DC Microgrids

Emerging DC distribution systems in data centres and industrial facilities use 380 V or 750 V DC. The 2025 edition provides the first national standard basis for sizing these conductors.

The voltage drop formula for DC circuits is simpler than for AC because there is no reactive (inductive) component:

DC voltage drop formula:
  ΔV = 2 × I × R × L / 1000

Where:
  ΔV = voltage drop (V)
  I  = design current (A)
  R  = conductor resistance (Ω/km) at operating temperature
  L  = one-way cable length (m)
  2  = accounts for both positive and negative conductors

Note: The factor of 2 applies because current flows out on the positive
conductor and returns on the negative conductor. For a bipolar DC system
(+V, 0, -V), calculate each pole separately.

The voltage drop limit for DC circuits is not explicitly stated in AS/NZS 3008.1.1:2025 itself — it depends on the application standard. Typical limits:

DC cables use the same derating factor framework as AC cables. The 2025 edition derating tables (Tables 22–27) apply equally to DC circuits:

• Ambient temperature (Table 22): DC cables on solar arrays are often exposed to direct sunlight and high ambient temperatures. At 50°C (common on an Australian rooftop), the derating factor for X-90 insulation is approximately 0.87.
• Solar radiation: Cables exposed to direct sunlight require an additional derating per Clause 3.3.6. This is particularly relevant for solar PV string cables clipped to module frames.
• Grouping (Table 25): Multiple DC string cables in the same tray or conduit require grouping derating. Note the revised 2025 factors for unperforated trays.
• Soil thermal resistivity (Table 27): Ground-mount solar farms with buried DC cables in arid areas require significant derating — the new “very dry soil (3.0 K·m/W)” factor of 0.71 is critical for outback installations.

The combined derating for a typical ground-mount solar farm DC cable (50°C ambient, 6 strings in conduit, dry soil) can reduce the effective current rating to 50–60% of the base table value. This often drives cable sizes up by two or three commercial steps compared to a naïve reading of the base table.

While AS/NZS 3008.1.1:2025 primarily covers cable sizing (current rating and voltage drop), the protection of DC circuits has important implications for cable selection:

• Arc quenching: DC arcs are harder to extinguish than AC arcs because there is no natural current zero crossing. DC-rated circuit breakers and fuses must be used, and cable fault current withstand calculations should account for longer clearing times.
• Reverse current protection: In solar PV arrays, reverse current can flow through a faulted string. String fuse sizing must consider both forward and reverse current scenarios, which affects minimum cable size.
• Short-circuit withstand: The cable must withstand the prospective fault current for the time taken by the protective device to clear the fault. For DC systems with battery sources, fault currents can be very high (batteries have very low internal impedance). AS/NZS 3008 Table 52 provides the adiabatic short-circuit formula: I²t = k²S², which applies equally to DC and AC circuits.

ECalPro’s cable sizing calculator supports DC cable sizing to AS/NZS 3008.1.1:2025:

• DC/AC toggle: Select DC mode to use the 2025 DC-specific current rating tables. The voltage drop calculation automatically switches to the DC formula (no reactance component).
• Application presets: Choose from solar PV, battery storage, EV charging, or custom DC to pre-populate typical voltage and derating scenarios.
• Full derating chain: All derating factors (ambient temperature, grouping, solar radiation, soil resistivity) are applied with the 2025 values.
• Energy-aware voltage drop: For solar PV, the calculator shows the annualised energy loss from voltage drop to help optimise the cable size vs. cost trade-off.

→ Open the Cable Sizing Calculator (DC mode)

Free to use — no signup required for basic DC cable sizing calculations.

Technical content on this page is based on publicly available information about AS/NZS 3008.1.1:2025. ECalPro recommends purchasing the official standard from Standards Australia or Standards New Zealand for authoritative design guidance. This page does not reproduce copyrighted table values — use the standard for exact current ratings.