Do You Need a Fuse between a Solar Panel and Charge Controller?

For the most part, you do need a fuse between a solar panel and its charge controller as fuses and circuit breakers protect the wiring from getting too hot. This also prevents any appliances from catching fire or getting damaged should a short circuit occur. However, if the solar panels are wired in series, a fuse is often not necessary.

This article will take a closer look at what the role of the fuse is in the solar panel wiring system, how it relates to the charge controller, where you should place fuses for optimal efficiency, the difference between fuses and circuit breakers, and why fuses aren’t utilized for solar panels wired in series.

Role of the Fuse

A fuse is an electrical safety device used to protect an electrical circuit. The way it works is fairly simple — it contains a metal strip or wire that melts down when the current flowing through it becomes too much, essentially stopping the current’s flow. 

Basically, a fuse is a sacrificial device, which means that if it is employed to protect the circuit even once, it has to be either rewired or replaced, depending on the specific fuse type.

By protecting an electrical circuit from an overcurrent, a fuse also prevents risks like short circuits, mismatched loads, overloads, or even device failure or collapse. A circuit breaker can also be used for the same purpose, but it differs in base characteristics from a fuse (source). 

Role of the Charge Controller

In essence, a charge controller helps regulate the charge going directly into your battery storage from the solar panels. Its purpose is to prevent the battery from overcharging and to reverse the current flow during the night. The way it manages to do this is by utilizing a transistor to regulate the battery charge.

Should your battery reach its full charge capacity, the charge controller will stop it from charging further, and, on the other hand, if it starts to reach a critically low charge, it will then stop the discharging process. 

If you use a charge controller for your solar panel system, you are essentially maximizing your battery life and minimizing utility power use. In this way, a charge controller can significantly extend the longevity and efficiency of your entire solar panel system (source). 

Fusing Your Solar Panel System

As we’ve already discussed, the purpose of a fuse or circuit breaker is to protect the wires from getting overly hot and either the wires, parts of the system, or appliances catching on fire. They also protect devices from suffering damage should a short circuit occur. Now let’s look at the types of fuses and when and where to use them.

What Type of Fuse to Use

If you want to determine the size of the fuse to use between your solar panel and your charge controller, you will need to know how many panels you have, how they are connected, and the amps of each panel.

If your panels are connected in parallel, all you need to do is add up the panels’ amperage and then add 25% according to industry safety rules. This means that if you have 4100-watt panels of approximately 5 amps each, you’d have an equation that looked like this:

4 x 5 x 1.25 = 28.75 Amps

For this example, you would then simply round up to know what size fuse to get, which would be a 30-amp fuse.

Most commercial solar panels that are over 50 watts will have 10-gauge wires that can easily manage 30 amps or less of flow. If your solar panels are connected in parallel, the current will be additive. This means that if, for example, you have 4 panels, each capable of managing 15 amps, a short circuit in a single panel will draw all 60 amps towards it. 

If this happens, the wires that lead to this panel will, of course, far exceed the 30 amps it can manage, and this could result in a potential fire. For these panels, connected in parallel, it is important to have a 30-amp fuse for each one. 

If you have panels that are smaller than 50 watts, and if you are only using 12-gauge wires, a 20-amp fuse will be sufficient for each panel.

Where to Fuse Your Solar Panel System

There are several different locations where you can place the fuse in the solar panel system. The first is the one we’ve just discussed, where the fuse is positioned between the solar panel and the charge controller. 

This fuse type is usually used in a parallel system, and there would be a combiner box that holds the fuses to each panel, as well as one or more combined fuses that lead to the charge controller itself. It is of critical importance that the fuse is always placed on the plus (+) side between the charge controller and the panels. 

There are two different charge controllers — namely, a Pulse Width Modulation (PWM) charge controller and a Maximum Power Point Tracking (MPPT) charge controller. The main difference between the two is that with the PWM charge controller, the current drawn from the panel will be just more than the battery voltage. 

With an MPPT charge controller, the current drawn from the panel will be at its maximum voltage. 

The second type of fuse that is utilized in a solar panel system is one placed between the battery bank and the charge controller. With PWN (Pulse Width Modulated) charge controllers, the amps to and from the controller will be the same, so the fuse and the wire size can also be the same. 

For MPPT (Maximum Power Point Tracking) charge controllers, you will need to calculate the fuse size from the charge controller manual, as this type of charge controller can both increase the current flowing between the controller and the battery bank and lower the voltage. 

The final type of fuse you will need in a solar panel system will be between the battery and the inverter. This is critical, and you will definitely need this installed as this is where the maximum current flows (source). 

The inverter manual will have the fuse details, but it is more likely that the inverter will have a built-in fuse or breaker on both the input and output sides of the unit.

Why You Don’t Need a Fuse When Connected in Series

If your solar panels are connected in parallel, the current is additive, and, as we have already discussed, this is why a fuse will be necessary as the current can easily exceed the amps the wires can manage. 

On the other hand, if your solar panels are connected in series, the current is not additive and, therefore, fuses are typically not used. 

For example, if you had three panels with 30-amp wires and a 15-amp fuse between the charge controller and the panels, should there be a short circuit or a short to the ground, the 15-amp fuse wouldn’t trip as the maximum amps that could flow towards the short would only be 9.78 amps. 

Essentially, an active direct short simply wouldn’t be able to pull enough amps to trip the fuse. It is, therefore, not really feasible to try and protect this wire using a fuse because if your system was operating normally, there would be 9.33 amps flowing through the wire. To protect the wire, you’d then have to have a fuse that was between 9.33 and 9.78 amps. 

This is a very small margin, and you’d have to be able to work between higher than 9.33 for it to work optimally and less than 9.78 to offer protection from the short. 

Essentially, you’d be looking for a 9.5 amp fuse, which may not be readily available, and you run the risk of annoying circuit trips as the panels may produce more power than listed when they’re cooled, for example. 

This is the simple reason why many people choose not to install a fuse between the solar panel and charge controller should the panels be connected in series. 

Power Surges and Fuse Types

A fuse may be a necessity in the grander scheme of the solar panel system, and it is definitely recommended for it to be deemed safe by the authorities, especially if the panels are connected in parallel.

However, a fuse’s main purpose is to protect the system from a surge, and in a solar panel system, or photovoltaic (PV) system, a surge is a fairly uncommon occurrence in the majority of cases. Even so, a PV system is still an electrical system, and, as with any electrical system, it can result in a surge, especially if the panels are struck by lightning. 

Because a surge shouldn’t happen but can, a fuse is still necessary. The best type of fuse to use in the case of a PV system would be a fast-blow fuse. There are two different types of fuses: a slow-blow and a fast-blow fuse. 

Fast-blow fuses will blow the second that the flowing current reaches the fuse’s amp rating. This means that it is the safest type of fuse to use for solar panel systems. Slow-blow fuses, on the other hand, are designed to tolerate a greater number of startup surges and modest short-term overloads without blowing. 

You can see the difference between a fast-blow fuse and a slow-blow fuse, as a fast-blow fuse will usually use a thin wire, and slow-blow fuses will usually have a thicker, coiled wire (source).

Difference Between a Fuse and a Circuit Breaker

A fuse and a circuit breaker both serve a similar purpose, but they differ when you consider their base characteristics. The most important difference between a fuse and a circuit breaker is perhaps their lifespan. 

A fuse is a sacrificial device, which means that, should it be utilized to protect the system from an overcurrent, it will need to be replaced. Circuit breakers, on the other hand, work with a switch and can thus be reset should they have served their purpose. 

The way a fuse works is by melting a strip of metal inside of it when too much current flows through it, which essentially stops the circuit and prevents a power surge. Many people find this a hassle to replace and opt for circuit breakers instead. 

Circuit breakers are constructed with a metal strip or magnet on the interior and with an external switch. A circuit breaker has a clear top and bottom, which ensures that the electricity passes through them correctly. Should too much current flow through the circuit breaker, the metal strip will bend, which, in turn, flips the switch. 

If the switch flips, the circuit is interrupted. However, whereas a fuse will need to be replaced at this stage, the user can simply manually flip the circuit breaker’s switch back to its original position, and the process can repeat itself. 

A circuit breaker also makes it easy to interrupt the power to certain appliances or areas of the home, and that is why it is so often used.

Using a Circuit Breaker Between a Solar Panel and Charge Controller

Although a circuit breaker and fuse perform the same basic function, it is their different base characteristics that make a circuit breaker unsuitable for use between your solar panel and charge controller. The simple reason for this is that a circuit breaker is far more expensive than a fuse, and a solar panel system will be running on direct current (DC).

In a DC system, the minute that the circuit breaker switch flips, should a short occur, the possibility is higher for a DC arc. An arc, or arc flash, is what is known as an electrical breakdown of the resistance of air resulting in an electric arc. This sometimes occurs if there is sufficient voltage in an electrical system and a path to the ground. 

A DC circuit in a solar panel system can generate and sustain arcs of severe intensity, and this can cause fires or breakdowns. It is fairly common for a DC system to arc, and the circuit breaker’s role can make this type of arc detrimental for the whole system. 

If there is a circuit breaker present, the potential of a DC arc, as soon as the circuit breaker switch flips, will significantly deteriorate the lifespan of the entire system, but most likely the circuit breaker itself, or even damage it irreparably. 

It is substantially more expensive to replace a circuit breaker than a fuse, and for this reason alone, and the potential of hazardous DC arcs, a fuse is preferable for use between the panels of a solar panel system and the charge controller. 

Can You Install and Wire Your Own PV System?

Nowadays, more and more people are choosing to invest in green energy, and going off-grid has become fairly commonplace. This means that the demand for sustainable energy solutions, such as solar panels, has skyrocketed. 

It is possible to install solar panels by yourself and, overall, it can be a rewarding experience that can save you some money in the long run. It is much more cost-effective to cut contractors out of the equation and take on the job yourself if you are capable.  

The important thing to remember is that — since solar installation involves heavy lifting, electrical work, and potentially climbing onto high structures such as the roof of a building — safety should be prioritized at all times. 

It is best not to attempt the installation of a PV system all on your own, and if help can be called in, even amateur help, it will make the job just that much easier. 

The installation of a solar panel system is fairly simple, and it can be done by any relatively handy homeowner. When it comes to the fuse and wiring, It is always advised to call in the help of a professional should you not have experience in electrical wiring, as this is a potentially hazardous job and carries risks of electrocution, electric shock, or fire. 

Final Thoughts

It is highly recommended that any solar panel system incorporates a fuse between the panels and the charge controller, as this will protect appliances and devices from potential power surges and prevent the wires from getting too hot or catching fire due to an overcurrent.

Although a fuse may not be necessary if a smaller PV system has panels connected in series, it is always better to be safe rather than sorry, and when choosing the safest fuse for your system, it is wise to select a fast-blow fuse. 

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