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AS/NZS 3008.1.1-2017
Why Rooftop Cables Run Hotter Than You Think
AS/NZS 3008 cable current-carrying capacity tables are based on a 30°C ambient temperature (for some tables) or 40°C (for others). On an Australian rooftop on a summer day, the actual temperature of a cable lying on a metal roof surface can reach 70–80°C. A cable sized using unadjusted 40°C tables and installed on a 75°C roof surface is significantly undersized — its insulation life will be greatly reduced and it may operate near its thermal limit on hot clear days.
Temperature Correction Factors — AS 3008 Table 25
Actual Cable Surface Temperature | Correction Factor (90°C XLPE cable) | Correction Factor (75°C PVC cable) |
|---|---|---|
40°C (reference) | 1.00 | 1.00 |
50°C | 0.91 | 0.87 |
60°C | 0.82 | 0.71 |
70°C | 0.71 | 0.50 |
80°C | 0.58 | Not recommended |
For a DC string cable on a dark metal roof in Western Australia (cable surface temperature reaching 75°C), the correction factor is approximately 0.65 for XLPE cable. A standard 4 mm² solar cable rated at 38 A in free air is derated to approximately 25 A — meaning a string with ISC of 20 A requires the full correction factor applied before confirming adequacy.
Practical Solutions
Use 90°C-rated XLPE solar cable (H1Z2Z2-K) rather than 70°C PVC for all DC string runs — the higher temperature rating significantly improves derating performance
Route DC cables along the underside of the panel where they are shaded from direct sun and cooled by panel airflow
Avoid bundling DC cables in conduit on the roof surface — each additional cable in conduit reduces capacity by 10–20% through grouping derating
In extreme temperature locations (outback QLD, NT, WA), upsize DC cables by at least one size above the minimum calculated requirement
Engr. Jason Morales — Founder, SolarEnergyPH
