Solar DC cable sizing and design for Perth homes: what installers check

The DC cable between your solar panels and inverter is one of the most permanent parts of your installation — it's installed in conduit, routed through the roof cavity, and not easily replaced. Getting the cable sizing right at installation matters for long-term safety and system efficiency.

This guide explains what qualified Perth installers assess when sizing DC cable, what the relevant standards require, and what to check on a quote.

What DC cable sizing involves

DC cable sizing for solar has two primary goals:

1. Limiting resistive losses — undersized cable has higher resistance, which converts some of your solar generation into heat rather than electricity
2. Preventing fire risk — undersized cable operating near current limits generates heat, which in a roof cavity creates fire hazard

The main factors:

• Maximum current the string will carry (determined by panel short-circuit current × number of parallel strings)
• Length of the DC cable run (longer run = more resistance = more heat = need for larger cable)
• Ambient temperature (cable rating is derated at high temperatures)
• Installation method (conduit in open air vs enclosed in roof cavity — the latter is hotter)

Cable sizing and Perth's temperature

AS/NZS 5033:2021 requires that cable current ratings be derated for the installation environment. In Perth, this matters significantly:

Roof cavity temperature: A typical Perth roof cavity on a hot summer day reaches 50–70°C ambient. Cable rated at 40°C ambient (a common laboratory baseline) must be derated. A 6mm² DC cable rated for 40A in open air may be derated to 28–32A in a sealed roof cavity during Perth summer.

Conduit in direct sun: Conduit running exposed on a north-facing roof in Perth can reach 80°C+ surface temperature in summer. Cable inside this conduit is derated significantly — this is why properly installed systems route conduit under the roofline or in shaded locations where possible.

What this means for you: Your installer should be using AS/NZS 5033 derating tables for the actual installation environment — not just cable catalogue ratings under laboratory conditions. This is one reason why a slightly larger cable (6mm² rather than 4mm²) is often specified on longer runs in Perth, even where the base current calculation would allow the smaller cable.

Common cable sizes in Perth residential systems

| String current (typical) | Typical cable size (short run, < 10m) | Typical cable size (long run, > 20m) | |---|---|---| | Up to 10A (small system) | 4mm² | 6mm² | | 10–15A (typical 6.6–10kW) | 6mm² | 6mm² or 10mm² | | 15–20A (larger system with parallel strings) | 6mm² | 10mm² |

These are indicative — the correct size depends on the specific panels, string configuration, cable run length, and installation environment for your property.

DC cable standards and markings

AS/NZS 5033:2021 requires that DC cable used for solar PV be specifically rated for that application:

What to look for:

• Cable marked TUV 2PfG1169 or EN50618 H1Z2Z2-K — these are the European and Australian-accepted DC cable standards for solar PV (UV-resistant sheath, double insulated, rated for outdoor continuous use)
• Standard house wiring cable is NOT appropriate for rooftop DC runs — it degrades in UV and heat exposure much faster

A well-documented installation should include the cable specification in the installation documentation provided at handover.

Conduit requirements

AS/NZS 5033:2021 and the AS/NZS 3000 wiring rules require that DC cabling be protected:

Roof cavity runs: Cable routed through roof cavities must be in conduit or be protected against mechanical damage. The conduit keeps cable away from roofing nails, maintains cable separation from AC wiring (required by the standards), and limits fire spread.

External conduit: Where DC cable runs down an external wall or along a roof surface, UV-rated conduit (or cable trunking with UV-resistant rating) is required. Grey PVC electrical conduit yellows and becomes brittle in Perth UV within 5–7 years — specify UV-stabilised or black HDPE conduit for external runs.

Roof penetrations: Each penetration through the roof surface must be properly sealed and flashed to prevent water ingress. A well-installed system has flashing kits at each roof penetration — not just silicone sealant.

What to check on a quote or post-installation

On the quote:

• Does the specification list the cable size (mm²) explicitly?
• Is there a mention of conduit material and routing?
• For longer cable runs (> 15m), has the installer accounted for voltage drop?

Post-installation (at handover):

• Ask for the as-installed documentation showing cable routing and sizes
• Check that conduit at external runs is UV-rated (should be black HDPE or UV-stabilised grey)
• Confirm roof penetrations are properly flashed
• Check that DC cable does not run alongside AC cable without separation (required by AS/NZS 3000)

Voltage drop

For long DC cable runs (> 20m), voltage drop becomes a consideration in addition to current rating. A 2% voltage drop limit is commonly used for DC PV cabling — this limits the resistive loss to 2% of the string voltage.

Example: A 400V DC string with 20m of cable at 10A current. Using 6mm² cable:

• Resistance per metre: ~3.3 mΩ/m for 6mm² copper
• Total resistance (20m × 2 = 40m of conductor): 0.132Ω
• Voltage drop: 10A × 0.132Ω = 1.32V = 0.33% of 400V

In this example, 6mm² is adequate. If the run were 80m (uncommon in residential), the voltage drop would become 5.3% — too high, and a larger cable size would be needed.

Perth residential systems typically have DC runs of 5–25m. Runs beyond 25m should prompt your installer to document the voltage drop calculation.

What happens with undersized or poorly installed DC cable

• Resistive losses: Constant power loss proportional to current² × resistance. A 1% voltage drop means 1% of DC power lost as heat every hour the system operates.
• Heat damage over time: Cable operating near its derated limit degrades its insulation over time — the insulation becomes brittle, cracks, and creates ground fault or arc fault potential.
• DC arc faults: DC arc faults are more dangerous than AC faults because DC arcs don't self-extinguish at current zero crossings. A damaged DC cable in a roof cavity that arcs is a serious fire risk.

Modern inverters (Sungrow SH, Fronius GEN24) include DC arc fault detection (AFDD) as a built-in feature on some models — this monitors the DC string current waveform for arc signatures and trips if one is detected.

DC cable sizing is one of the less visible but important elements of a solar installation. The right cable size for Perth's roof temperatures differs from what you'd specify in Melbourne's milder climate. A legitimate CEC-accredited installer will size to AS/NZS 5033 requirements using Perth-specific derating — if a quote doesn't list cable sizes, ask.