How To Properly Fuse A Solar System

Solar panels have recently become much more common as people all over the world begin to shift toward a greener and more sustainable way of life. Going off-the-grid requires some people to be able to do their own housework, and in circumstances like this, knowing how to correctly wire the solar panels is essential. 

In most cases, a solar system fuse is required between a solar panel and its charge controller because fuses and circuit breakers protect the wiring from overheating. This also avoids any appliances from catching fire or being damaged in the event of a short circuit. However, if the solar panels are wired in series, a fuse is rarely required.

What Is A Solar Fuse?

Solar fuse is a kind of fuse especially meant for solar power systems, serve as a critical line of defense against electrical faults in your solar system. They are designed to protect the solar equipment against overheating, overloading, or short circuits that might occur. When an electrical fault occurs, the fuse acts as a weak link in the circuit, melting and breaking the connection to prevent the flow of excessive current. The solar fuse can be chosen based on several factors such as physical size and shape, amperage rating, breaking capacity, and many more.

Without proper use of fuses, a short circuit or overload can cause wires to overheat, insulation to melt, and even start a fire. Fuses help mitigate these risks by quickly interrupting the flow of electricity before damage can occur. They also protect your solar panels from reverse current, which can occur when a panel becomes shaded or faulty, potentially causing it to draw current from other panels in the string.

When to Fuse Solar Panels

The NEC provides guidelines for solar panel fusing based on the short circuit current (Isc) of the panels. The code requires that the fuse rating be at least 156% of the panel’s Isc. For example, if a solar panel has an Isc of 10A, the minimum fuse rating would be 15.6A, rounded up to the next standard fuse size of 20A.

Series wiring: Situations You Do Not Need To Fuse

When solar panels are wired in series, the current flowing through each panel is the same. In this configuration, a single fuse at the end of the string is usually sufficient to protect the entire array. The solar array does not need to be fused if the short circuit current of the solar array is less than the maximum series fuse rating of the solar panel. Because of the following, there is no added protection or benefit by fusing this kind of array:

Three 200W panels are connected in sequence. Each solar panel has a maximum series fuse rating of 15A and a maximum series short circuit current of 10.2A, 9.8A, and 9.8A, respectively.

Given that the maximum series fuse rating is 15A, we can assume that the internal wires, diodes, connections, and other parts of the real solar panel can withstand a maximum current of 15A.

Since the array’s short circuit current is 10.2A, it is reasonable to state that if a short circuit or other malfunction were to occur within one of the solar panels, the panel would be equipped to manage the situation because the short circuit current cannot surpass the panel’s maximum fuse rating.

It will ultimately never permit the fuse to blow. Make the fuse endure bothersome blows on a constant basis when it is performing normally. Due to this, a fuse is not necessary for this situation, according to code.

Parallel wiring: when fusing is necessary

However, when solar panels are connected in parallel, each string carries its own current. In this case, fuses are required for each parallel string to prevent a short circuit in one string from affecting the others. The NEC requires that each parallel-connected string be individually fused, with the fuse rating not exceeding the maximum series fuse rating of the panels.

WHERE TO FUSE?

There are three different locations where the fuse can be installed in the solar panels. The first is between the battery bank and the charge controller. Secondly, it can be placed between the charge controller and the solar panels. Lastly, the fuse can also be present between the inverter and the battery bank.

A fuse or breaker installed between the charge controller and the battery bank can protect against short circuits and overloads. This fuse should be rated to handle the maximum current output of the charge controller.

The fuse between the charge controller and solar panel functions efficiently to protect against the overheating of wires while at the same time protecting the appliance against any damage.

A fuse or breaker installed between the battery bank and the inverter can also protect against short circuits and overloads. The fuse should be rated to handle the maximum current draw of the inverter.

What Type/Size Of Fuse Do We Need?

For photovoltaic (PV) systems to operate safely, dependably, and over the long term, the fuses must be properly sized. In contrast to customary electrical power distribution and control applications, solar system fuses are susceptible to special circumstances. Long-term exposure to environmental factors can lead to aberrant ambient temperatures, which in turn impacts conductor choices, sizing, and fuse performance. 

Additionally, PV modules produce continuous currents, unlike traditional circuits, which are typically sized based on continuous loads, necessitating additional considerations for sizing fuses. Given these circumstances, a special technique for sizing solar fuses in PV systems is required.

Solar panel fusing 

The commercial solar panels with over 50 watts utilize 10 gauge cables that can withstand the current flows of up to 30 amps. In such instances, fusing is not a necessity if the panels are interlinked in the series. However, the situation changes significantly when panels are interconnected in parallel. For instance, if there are four panels, each rated for 15 amps, a short in one of them could cause all 60 amps to be directed at the shorted panel. This will result in the wires leading to that panel receiving significantly more current than 30 amps, potentially causing that wire pair to catch fire. Therefore, in parallel connections, it is crucial to install fuses on each panel to prevent excessive current flow that can lead to overheating and fire hazards.

Fusing for a parallel/combiner box

A combiner box is used in a parallel system to contain the fuses and breakers for each panel as well as one or more “combined” fuses that connect to the charge controller or grid tie inverter. We must first calculate the worst-case current that will flow based on our unique panels before sizing this “combined” fuse/breaker. It’s important to note that the total amperage of all the fuses in the combiner box should not exceed the maximum input rating of the charge controller or the ampacity of the wires leading from the combiner box.

Charger to Battery Fuse or Breaker

Because the worst-case amps flowing to and from a Pulse Width Modulated (PWN) charge controller is the same, the fuse and wire size can match. Contrarily, MPPT charge controllers have the capacity to both reduce voltage and increase the current flowing between the controller and the battery bank, necessitating a recalculation of the exact size of the wire and fuse or consulting the charge controller manual. For their Solar Boost 50 (amp) charge controller, for instance, Blue Sky advises installing a 60-amp fuse/breaker between the device and the battery bank. Once more, pick a wire with the correct rating.

Fusing Between the Battery Bank and Inverter

The last fuse or breaker between the battery bank and inverter should be sized based on the inverter’s continuous power rating and the maximum surge current it can handle. Consult the inverter manufacturer’s specifications to determine the appropriate fuse or breaker size. As a general rule, the fuse should be rated at 1.25 to 1.5 times the inverter’s continuous current rating. For example, if your inverter has a continuous power rating of 2000W at 24 volts, the continuous current would be 2000W ÷ 24V = 83.3A. In this case, you’d choose a fuse or breaker with a rating between 105A and 125A.

Tools Required 

• Angle finder
• Torpedo level
• Fish tape
• Chalk line
• Cordless drill (14.4V or greater), multiple batteries
• Unibit and multiple drill bits (wood, metal, masonry)
• Hole saw
• Hole punch
• Torque wrench with deep sockets
• Nut drivers 
• Wire strippers
• Crimpers
• Needle-nose pliers
• Lineman’s pliers
• Slip-joint pliers
• Small cable cutters
• Large cable cutters
• AC/DC multimeter
• Hacksaw
• Tape measure
• Blanket, cardboard, or black plastic 
• Heavy-duty extension cords
• Caulking gun
• Fuse Pullers

Differences Between Fuses and Breakers

Fuses and breakers both serve to protect your solar system from electrical faults, but they work in slightly different ways. Fuses are one-time-use devices that melt and break the circuit when they encounter an overcurrent situation. Once a fuse blows, it must be replaced.

Circuit breakers, on the other hand, can be reset after they trip. They use a mechanical switch to break the circuit when an overcurrent is detected. Once the fault is cleared, the breaker can be reset, restoring power to the circuit.

In solar systems, fuses are more commonly used for smaller currents, such as in solar panel strings or between the charge controller and battery. Breakers are often used for higher-current applications, such as between the battery bank and inverter.

Selecting the Appropriate Fuse Type

When selecting fuses for your solar system, it’s important to choose the correct type and size. The most common types of fuses used in solar applications are:

• ANL (blade) fuses: These fuses are commonly used in battery and inverter circuits and are available in sizes from 20A to 750A.
• Midget (ATC) fuses: These small, blade-style fuses are often used in solar panel strings and combiner boxes, with sizes ranging from 1A to 30A.
• Cartridge fuses: These cylindrical fuses are available in a wide range of sizes and are often used in higher-voltage applications.

When selecting a fuse size, always refer to the NEC guidelines and the specifications of your solar components. Fuses should be rated to handle at least 156% of the expected current, rounded up to the next standard fuse size.

Common Mistakes to Avoid When Fusing a Solar System

• Undersizing or oversizing fuses: Using a fuse that is too small can cause it to blow prematurely, while using a fuse that is too large may not provide adequate protection. Always size fuses according to NEC guidelines and component specifications.
• Neglecting to fuse parallel-connected panels: Each parallel-connected string must be individually fused to prevent a short circuit in one string from affecting the others. Failing to fuse parallel strings can lead to damage and fire hazards.
• Improper fuse placement or installation: Fuses should be installed as close to the power source as possible, and always on the positive wire. Improper placement or installation can reduce the effectiveness of the fuse and potentially cause damage to your system.

Proper Fuse Installation Techniques

When installing fuses in your solar system, follow these best practices:

• Use the correct fuse holder or breaker box for the fuse type and size.
• Install fuses on the positive wire, as close to the power source as possible.
• Use proper wire sizes and connectors, ensuring a tight and secure connection.
• Label each fuse or breaker clearly, indicating its purpose and rating.
• Keep fuses and breakers clean and free from corrosion, replacing them as necessary.

Maintenance and Troubleshooting

Regular maintenance and inspection of your solar fuses and breakers can help prevent problems and ensure the longevity of your system. Periodically:

• Visually inspect fuses and breakers for signs of corrosion, damage, or overheating.
• Check connections for tightness and cleanliness.
• Test the continuity of fuses using a multimeter to ensure they are still functional.
• Replace any blown fuses or tripped breakers, investigating the cause of the fault before restoring power.

If you experience repeated fuse failures or breaker trips, it may indicate a more serious problem with your solar system. Consult a professional solar installer or electrician to diagnose and resolve the issue.

Conclusion

Any solar panel system that includes a fuse between the panels and the charge controller is highly advised since it will shield electrical appliances and gadgets from power surges and keep the wires from overheating or catching fire as a result of an overcurrent. Even though a smaller PV system with panels connected in series may not require a fuse, it is always advisable to be safe than sorry. A fast-blow fuse is the safest option for the system. Refer to Beny for better solutions for solar power systems.