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AS/NZS 3008.1.1-2017 AS/NZS 3008.1.2-2017
Why Cable Selection Matters So Much in Solar
In a conventional household circuit, the worst that happens with an undersized cable is a tripped breaker. In a solar DC circuit, there are no breakers protecting individual string cables from sustained overload — and the fault currents can be substantial. An undersized DC cable that operates at 90–100% of its rated current for hours every sunny day will have its insulation life reduced from decades to years.
AS/NZS 3008 provides the scientific basis for cable selection: current-carrying capacity, voltage drop calculation, and grouping/derating factors for cables installed in various environments.
AS/NZS 3008.1.1-2017 — Australian Conditions
Part 1.1 covers Australia specifically. It differs from Part 1.2 (New Zealand) primarily in the reference ambient temperature: Australia uses 40°C ambient as the design baseline for most locations, with higher temperatures applicable in Queensland and NT.
The Four-Step Cable Selection Process
Determine design current (IB) — For a DC string: Isc × 1.25 × 1.25 (temperature correction × safety factor per AS 5033). For AC inverter output: inverter rated output current × 1.25
Apply derating factors — grouping (multiple cables together), ambient temperature, and installation method (Table 22 through Table 35 of AS 3008)
Select cable cross-section — from the capacity tables to meet or exceed the derated design current
Check voltage drop — must not exceed AS/NZS 3000 limits (typically 5% end-to-end for solar; some networks specify tighter limits on AC)
DC Solar Cable — Australian Requirements
Parameter | Requirement | Reason |
|---|---|---|
Voltage rating | 1000V DC (or 1500V DC for commercial) | String VOC can reach 600V DC at low temperature |
UV resistance | UV-stabilised outer sheath (black) | 20+ years of direct sun exposure on roof |
Temperature rating | 90°C conductor (XLPE insulation minimum) | Cable surface temperature on dark roof can exceed 70°C ambient |
Cable type | H1Z2Z2-K (TUV 2 Pfg 1169 solar cable) | Double insulation — required for accessible DC string runs |
Connector compatibility | MC4 or H4 — matched pairs only | Mixed connector brands are prohibited under AS 5033 |
Key Derating Factors for Roof-Mounted DC Cables
Installation Method | Typical Derating | Notes |
|---|---|---|
On roof surface (under panels), single cable | 0.7 – 0.8 | Surface temperature can be 30°C above ambient |
In conduit on roof surface | 0.6 – 0.7 | Grouping in conduit significantly reduces capacity |
In cable tray, touching | 0.75 – 0.85 | Per Table 22 grouping factors |
In roof cavity (moderate temperature) | 0.80 – 0.90 | Better than exposed; still above standard ambient |
Single cable in free air | 1.0 (no derating) | Baseline condition of AS 3008 tables |
Voltage Drop — DC Strings
Voltage drop in DC string cables reduces energy yield — a 3% voltage drop in the string means 3% less energy to the inverter throughout the life of the system. The calculation for a DC string:
Vdrop = (2 × L × Imp × ρ) / A
Where: L = one-way cable length (m) • Imp = string operating current (A)
ρ = resistivity of copper at 90°C = 0.0225 Ωmm²/m • A = conductor cross-section (mm²)
The factor of 2 accounts for the return conductor (positive + negative). AS/NZS 3000 recommends keeping voltage drop below 5% across the entire installation. For solar, best practice is to keep DC string cable voltage drop below 1–2% to maximise yield.
Minimum Cable Sizes — Practical Guide
Circuit | Minimum Size | Recommended Size |
|---|---|---|
DC string cable (standard residential) | 4 mm² | 6 mm² |
DC main cable (multi-string to inverter) | 6 mm² | 10–16 mm² |
AC inverter to switchboard (<5 kW) | 2.5 mm² | 4 mm² |
AC inverter to switchboard (5–10 kW) | 4 mm² | 6 mm² |
Three-phase AC (<15 kW) | 4 mm² per phase | 6 mm² per phase |
Battery interconnect (<200 Ah) | 35 mm² | 50–70 mm² |
Engr. Jason Morales — Founder, SolarEnergyPH
