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AS/NZS 3008.1.1-2017
Commercial Solar — Where the Electrical Engineering Gets Serious
A 30 kW commercial rooftop system running at full output pushes approximately 130 A through the AC main cable at 230 V single-phase — or 43 A per phase on a three-phase 400 V system. At this scale, cable selection and installation method become significant engineering decisions, not just compliance checkbox items.
Cable Sizing Process for Commercial AC Runs
Determine design current: IB = Inverter rated AC current × 1.25 (thermal safety margin per AS 5033)
Select installation method: Determine how the cable will be installed (in conduit underground, on cable tray in ceiling, in free air) — this determines the base capacity table to use in AS 3008
Apply derating factors: Multiply by grouping factor (if multiple cables), ambient temperature factor, and soil thermal resistivity factor (for underground cables)
Check voltage drop: Calculate Vdrop for the AC cable run; must remain within 5% total from the inverter to the main switchboard
Check short-circuit withstand: For cables >50 m, confirm the cable cross-section is adequate to withstand the maximum prospective fault current for the disconnection time of the upstream protection device
Cable Management for Commercial Installations
Method | Best Use | Key AS 3000 Requirements |
|---|---|---|
Perforated cable tray | Large flat roofs, plant rooms | Cables must be secured every 300 mm (horizontal), fill ≤40% of tray width for derating |
Conduit (PVC heavy duty) | Vertical runs, weatherproof areas | Max 40% fill, expansion loops every 6 m in sun-exposed locations |
Armoured cable (SWA) | Underground DC links, outdoor runs without conduit | Minimum burial depth 0.5 m general, 1.0 m under driveways; marker tape required |
Cable ladder | Heavy commercial, multiple large cables | Cables must be tied; spacing maintained for derating per AS 3008 |
Harmonic Considerations in Large Inverter Installations
Multiple inverters connected to the same AC bus can produce harmonic currents that add (rather than cancel) in the neutral conductor, requiring the neutral to be sized at 150–200% of the phase conductor for installations with significant harmonic load. Confirm with the inverter manufacturer's harmonic data before sizing the neutral in a commercial multi-inverter array.
Engr. Jason Morales — Founder, SolarEnergyPH
