Solar Panel Fuse Options: Protecting Your System from Overloads

Selecting a properly sized fuse for a photovoltaic (PV) system is critical to the safe, reliable, and long-term operation of this renewable energy source. Unlike typical power distribution and applications with controllers, fuses in PV systems will be subject to special conditions: prolonged exposure to the environment produces abnormal ambient temperatures, which in turn affects fuse performance, wire, and size selection.

In addition, unlike conventional circuits that typically have continuous load output, PV modules that are required to produce continuous current will require additional considerations when sizing fuses. With these conditions in mind, the fuses in the kit must be sized before the system is installed.

What is a solar panel fuse？

A solar panel fuse is a crucial component in solar energy systems. Let’s delve into its significance and role:

Solar panel fuses are designed to protect individual panels and their cables. They play a critical role in safeguarding the system from fault currents, such as DC breakers preventing short circuits. When a fault occurs, the fuse melts and breaks the circuit, preventing the flow of current and ensuring safety for the entire system.
However, when there is a controller in your system, the fuse will be useless, the controller and inverter will protect your components and safety.

The U.S. National Electrical Code (NEC) specifies requirements to protect PV systems from excessive currents. Fuses are used to protect cables and PV modules from line, line ground, and mismatch faults. Its sole purpose is to prevent fire and to safely open a faulted circuit in the event of an overcurrent event. However, in some cases, a blown fuse is not required.

• Individual in series (no fuses to blow)
• Two wires in parallel (no fused fuse required)
• Three or more wires in parallel (fused fuse required)

Typically, fuses can be added between different components in a complete solar power system, such as from the solar panel array to the charge controller, from the controller to the battery bank, and from the battery bank to the inverter. The requirements for fusing fuses may differ for each part of the installation, and the exact ratings depend on the amperage of those installations and wires.

To determine the appropriate fuse size, follow these steps:

Step #1: Find the maximum short circuit current (Isc) of the panels (usually indicated on the panel’s sticker at the back).

Step #2: Use the following formula: Fuse size = 1.56 × Isc.

(This value represents the minimum fuse rating needed for your solar panel operation)

Fuse sizes for common power solar panels

To determine the appropriate fuse size for different solar panel wattages, it’s essential to consider the operating current of the solar panels and apply a safety margin to accommodate for any possible current spikes or surges. Here’s a table that summarizes the suitable fuse sizes for 100W, 200W, 300W, and 400W solar panels, typically installed in a 12V system:

How to calculate:

Calculate the Operating Current: Divide the solar panel’s wattage by the system’s voltage. For example, a 100W panel in a 12V system generates approximately 8.33 amps.

Select the Fuse Size: Choose a fuse that is slightly higher than the calculated operating current to prevent nuisance blowing from slight overages yet still low enough to protect the circuit effectively.

Things to note：

Safety Margin: It’s typical to use a fuse size that can handle at least 125% of the panel’s maximum current to account for variable conditions, such as high irradiance days or unexpected surges.

DC Fuses: Always ensure that the fuses used are rated for DC applications, as DC behaves differently from AC current in fault conditions.

Considerations

• Incorrect fuse sizing can lead to fuses blowing too easily or not blowing at all during overcurrent or short-circuit situations.
• Environmental factors (such as prolonged exposure to direct sunlight) can impact fuse performance.
• Seek guidance from a licensed electrician or your solar panel installer for your specific setup if you have doubts about the correct fuse size.

Solar panel fuses

Solar panels over 50 watts have 10 wire gauges capable of handling up to 30 amps of current. When you have more than 3 panels connected in parallel, each panel can draw up to 15 amps, and if one panel shorts out, it directs all 40-60 amps of current to that shorted panel. This will cause the wires going to that panel to be well over 30 amps, creating a fire hazard.

If panels are connected in parallel, each panel requires a 30 amp fuse. If your panel is less than 50 watts and uses only 12 gauge wire, a 20 amp fuse is required.

In parallel systems

In parallel systems, the use of fuse boxes makes the installation of fuses easier and more standardized. Our first task is to determine the size of the fuses required for each panel, this is known as “combination” fuses.

• For example, if we consider a 12-volt 250 watt solar panel, we need to look at its short-circuit current (ISC), which is rated at 11.32 amps.
• According to the NEC, if the load is continuous, a factor of 25% needs to be added for the fuse calculator, thus increasing the current to 14.15 amps per panel.
• Considering that there may be more than one panel in a parallel installation, the amp fuse size calculator combined current could theoretically be as high as 56.6 amps.

In this example, our wire set from the convergence box to the charge controller utilizes an 8 AWG wire set (minimum), which is sufficient to handle 60 amps of current. Therefore, we should use a 60 amp fuse or circuit breaker to protect that wire set, which is also consistent with the maximum capacity of the selected charge controller.

From charge controller to battery

With Pulse Width Modulation (PWN) charge controllers, the worst-case scenario for in and out of the controller is that the current is the same, so the fuse and wire sizes can be matched.

For example, we recommend a 60 amp fused fuse for a 60 amp PWM charge controller, placing it between the unit and the battery bank.

Battery fuse/circuit breaker to inverter

Wiring and fusing from the battery to the inverter is critical as this is where the most current is drawn. Similar to the charge controller, the recommended wires and fuses should be obtained from the inverter manual.

We have prepared a fuse holder on its positive cable which is capable of holding 50 amps of current. A typical 600 watt 12V pure symbol wave inverter draws up to 50 amps continuously, in which case a 55-60A cable with at least a 6 AWG wire is required.

Frequently asked questions about solar panel fuses