Solar Fusing Requirements

In Australia, the fusing requirements for solar panel installations are governed by the AS/NZS 5033:2021 standard, "Installation and safety requirements for photovoltaic (PV) arrays," which works in conjunction with AS/NZS 3000 (the Wiring Rules). This standard is critical for ensuring the safety, efficiency, and compliance of your solar system.

Key considerations for fusing include:

• Overcurrent Protection: Fuses (or circuit breakers) are mandatory to protect the solar PV system from overcurrent events, which can lead to overheating, equipment damage, or fire.
• Parallel Strings: If you have three or more strings of solar panels connected in parallel, each string must have its own overcurrent protection device (fuse) on the positive conductor. This prevents "circulating currents" if one panel or string faults, protecting the entire array. While not strictly mandatory for one or two parallel strings, it's generally good practice to fuse them as well.
• Fuse Sizing: The fuse rating must be carefully calculated. A common rule of thumb specified in AS/NZS 5033:2021 is that the fuse rating should be around 1.5 times the short-circuit current (Isc) of the string, and less than or equal to the maximum series fuse rating specified by the panel manufacturer.
• Voltage and Type: Fuses must have a voltage rating higher than your maximum system voltage and be specifically designed for DC PV applications (often marked as "PV fuses").
• Location and Accessibility: Fuses should be located as close as practical to where the strings connect in parallel, often in a string combiner box or at the inverter. Disconnection points (like MC4 connectors) should be readily accessible and labelled.

Choosing the right fuse size for your solar panel array is critical for safety and optimal performance. An undersized fuse will blow too frequently, causing system downtime, while an oversized fuse won't offer adequate protection against overcurrents, potentially damaging equipment or even causing a fire.

To correctly size your solar panel fuse, you'll primarily use the Short-Circuit Current (Isc) of your solar panels and a safety factor. Here's a quick guide:

1. Find your Panel's Isc: This value is usually listed on the back of your solar panel or in its datasheet. It's the maximum current the panel can produce under ideal conditions.
2. Calculate Total Isc: If you have multiple panels wired in parallel, sum the Isc of all panels. If wired in series, the Isc remains the same as a single panel.
3. Apply a Safety Factor: Electrical codes (like the National Electrical Code - NEC in the US, or AS/NZS 5033 in Australia) typically require a safety factor of 125-156% (or 1.25 - 1.56). This accounts for unusual conditions like increased solar irradiance.
   • Fuse Size = (Total Isc) x 1.25 - 1.56
4. Choose the Next Standard Fuse Size: Once you've calculated the required current, select the next commercially available fuse size that is equal to or greater than your calculated value. For example, if your calculation yields 18A, you might choose a 20A fuse.

Important Considerations:

• Voltage: Ensure the fuse's voltage rating is higher than your maximum system voltage.
• Fuse Type: Use fuses specifically designed for DC circuits, often labeled as "PV fuses" or "UL listed for PV."
• Consult a Professional: If you're unsure or dealing with a complex system, always consult a qualified solar installer or electrician. Incorrect fuse sizing can have serious consequences.

Always ensure your solar installation, including its fusing, is carried out by a qualified and accredited solar installer in accordance with the latest Australian Standards to guarantee safety and compliance.