How To Fuse a Solar Panel Array (With Diagrams)

Fusing a solar panel system is arguably the LEAST interesting part of the solar installation process. It’s tedious, involves numbers, and requires calculations. We can understand if you’re getting antsy just to connect the panels to your batteries to start charging them ASAP.

But fusing solar panels is a critical step that keeps you – and your loved ones – safe. Without fuses (& circuit breakers) your solar wires are exposed to over-current events, which can lead to the wires overheating and potentially catching on fire. Let’s avoid this!

In fact, installing fuses and circuit breakers, when appropriate, is required by the National Electric Code (NEC) to be in compliance with their standards. For more on the formal NEC text, scroll down to Appendix B.

Good To Know: Fuses and circuit breakers are commonly called “over-current protection devices” or OCPD. As the name implies, these devices protect your wires from abnormal over-current events.

In a solar system, there are three sections to consider when deciding whether you need an OCPD and what size fuse or breaker you’ll need.

• Part 1: Solar Fuses
• Part 2: Solar Disconnect
• Part 3: DC Breaker

Refer to the wiring diagram below to locate these three sections/parts.

This post will talk about all three solar fusing sections and teach you how, if required, to identify the correct size fuse/breaker for your system.

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Part 1: Solar Fuses (MC4)

Solar fuses are in-line fuses that protect the solar panels and source wires (the wires connected to the panels) when one of the panels experiences a short circuit. (Learn more about short circuits in solar arrays by scrolling down to Appendix A)

These fuses come with MC4 connectors, meaning you can easily plug them in and integrate them into your solar system. Solar fuses look like this:

For more in-depth information on how a solar in-line fuse works to protect your solar system, skip down to Appendix C.

Does Your Solar System Require Solar Fuses?

Only SOME solar systems require MC4 solar fuses. Whether your system needs these fuses depends on how many solar panels are in your array AND how these panels are wired together (series or parallel).

Here’s a quick overview to help you determine whether or not you need a solar fuse.

• One panel only? No fuse
• Panels wired in series? No fuse
• Two panels in parallel? No fuse
• Three or more panels in parallel? Fuse required

One Panel Only? No Fuse

If you will have only one solar panel, you DO NOT need a solar in-line fuse.

Panels Wired in Series? No Fuse

If you have multiple solar panels wired in series, you DO NOT need to install in-line fuses.

Two Solar Panels Wired in Parallel? No Fuse

If you only have two solar panels wired in parallel, you DO NOT need any solar in-line fuses.

Three or More Solar Panels Wired in Parallel? You DO Need In-Line Fuses

If you have three or more solar panels wired in parallel, you MUST install in-line fuses.

Should you wire solar panels in series or parallel? Read our series vs. parallel post to learn about the pros and cons of each wiring method.

Where To Install Solar Fuses?

If your solar array requires solar fuses, install them just before the positive (red) branch connector. Each solar panel will require an MC4 solar fuse. So, if you have three panels, you will need three fuses.

Refer to the wiring diagram below to see where these solar fuses are installed.

What Size Solar Fuse Do You Need?

Most solar fuses come in four amp ratings: 10A, 15A, 20A, & 30A. The size of the fuse you need equals your panel’s “Maximum Series Fuse Rating.” You can find this info on the panel’s specs sheet, which you can locate either on the info sticker on the backside of the panel OR on the panel’s online information page, like on Amazon.

Below is an example showing a 10A maximum series fuse rating for a sample panel.

In this example, you would need a 10A solar fuse to protect this particular solar panel.

What Is A “Maximum Series Fuse Rating”? The Maximum Series Fuse Rating is the greatest amount of current that the solar panel and its source wires can safely handle. If a current greater than this “max series fuse rating” passes through the panel and wire, there is a risk of overheating and a fire.

If you already know the maximum series fuse rating of your solar panel(s), locate the correct in-line fuse you need from the table below.

Part 2: Solar ‘Disconnect’ Circuit Breaker

The wire that connects the solar panels to the solar charge controller must also be protected from over-current events. In most situations, this is done with a solar disconnect circuit breaker. Below is an example of what a solar disconnect looks like.

Unlike the solar in-line fuses from Part 1, which aren’t required in every situation, your solar system will 100% require a disconnect breaker/switch to comply with National Electric Code (NEC) standards.

Unsure what solar wire size you need? Check out our solar wire size guide to learn how to calculate the right wire size for your system.

What Size Solar Disconnect Do You Need?

To correctly size your solar disconnect’s amp rating, follow the four-step process below.

1. Identify short circuit current – Isc(A): This info is found on your panel’s info sticker.
2. Calculate the max current of your array:
   • If panels are wired in series, the max current equals a single panel’s Isc(A).
   • If panels are wired in parallel, sum each panel’s Isc(A) together.
3. Multiply max current by 1.56: This is the NEC safety margin.
4. Round up to the nearest solar disconnect amp rating

Example: Two panels are wired in parallel. The short circuit current – IscA – of one panel is 11.5A. Therefore, the max current of the array is 23A (11.5A + 11.5A). After multiplying 23A * 1.56, we get 35.88A. Finally, round up to a 40A-rated solar disconnect.

If you already know which size solar circuit breaker you need, select the breaker from the table below.

(Other solar disconnect sizes are available on the Amazon product page.)

Product Review: DC Solar Disconnect (CHTAIXI)

In our video, we unbox the CHTAIXI solar disconnect breaker, detail the important features, and show you how to connect this device to solar wires. Enjoy!

Another useful benefit of having a disconnect breaker is that you can manually disconnect and isolate the solar panels from the rest of the electrical system. This is useful when you are conducting routine maintenance or troubleshooting problems on your system.

Part 3: DC Circuit Breaker (or Fuse)

The last part of fusing a solar system involves installing an in-line circuit breaker (or fuse) between the solar charge controller and the bus bars.

Note: In our wiring diagrams, we connect the charge controller to the bus bar, which is then wired to the auxiliary battery (not pictured). But you can also wire the charge controller directly to the batteries.

In our example, we use a DC circuit breaker, but you can also install a fuse and achieve the same result.

What Size Circuit Breaker Do You Need?

Calculating the correct circuit breaker size involves a simple two-step process:

1. Identify amp-rating of charge controller*
2. Multiply this amp rating by 1.25: This is the NEC safety margin.
3. Round up to the next circuit breaker size

* Don’t have a solar charge controller yet? Read our post “How To Size A Solar Charge Controller.”

Example: If you have a 30A charge controller, multiply this amp rating by 1.25 to get 37.5A. Round up to get a 40A DC circuit breaker.

If you already know what size circuit breaker you need, select the model from the table below.

(Other circuit breaker amp sizes are available on the Amazon product page.)

Product Review: DC Circuit Breaker (T Tocas)

In our video, we unbox the T Tocas DC circuit breaker, detail the important features, and show you how to connect wires to this device. Enjoy!

Solar Panel Fusing Review

And that’s it for fusing a solar panel system. In summary, you will need to install:

1. Solar fuses before positive branch connector (only for 3+ panels connected in parallel)
2. Solar disconnect before charge controller
3. DC breaker after charge controller

Review the solar fuse wiring diagram below to see where each of the over-current protection devices is installed.

Enjoyed reading? Check out our ultimate van life solar guide to help with your installation project.

Conclusion: Fusing Solar Systems Is For Safety

We understand that the concepts behind correctly fusing solar arrays can be dull. That’s why we aim to make this post short, sweet, and straight to the point. But no matter how dry the content might be, it’s important to get your solar fusing right, and that’s because keeping you and your solar system safe is worth your time and effort.

If you follow our fusing guide and size your solar wires accurately, you should have a system that is compliant with NEC standards.

If you have any other questions regarding how to fuse a solar system, post a comment in the section below.

Happy building!

Appendix A: What Is a Short Circuit?

A short circuit in a solar panel happens when the solar panel becomes faulty and does not produce any more electricity from the sun.

If a solar array is wired in parallel, a single faulty solar panel can lead to a fire because all the electricity produced from the remaining functioning panels will force its way toward the faulty panel instead of toward the charge controller.

In the diagram below, we show what happens with the flow of electricity once one of the solar panels becomes faulty.

As you can see in the diagram above, instead of the electricity leaving the solar panels and heading towards the batteries, the electric current (amps) is now diverted towards the faulty solar panel.

So, in essence, a shorted PV panel is like a black hole. It just sucks in all the surrounding current.

And if the amount of current that a faulty panel absorbs is greater than the maximum current that it can safely handle, then that panel (and wiring) can overheat and catch on fire.

And this can lead to your entire home (or camper van) catching fire.

(Refer to our not-so-subtle graphic below).

Appendix B: What the National Electric Code Says

Below, we list the official text and the easier-to-digest ‘paraphrased’ version of what the National Electric Code says for fusing solar panel arrays.

690.9 Overcurrent Protection

1. Circuits and Equipment. PV system DC circuit and inverter output conductors and equipment shall be protected against overcurrent. Circuits sized in accordance with 980.8(A)(2) are required to be protected against overcurrent with overcurrent protective devices. Each circuit shall be protected from overcurrent in accordance with 690.9(A)(1), (A)(2), or (A)(3).

• Circuits Where Overcurrent Protection Not Required. Overcurrent protective devices shall not be required where both of the following conditions are met:

1. The conductors have sufficient ampacity for the maximum current circuit.
2. The currents from all sources do not exceed the maximum overcurrent protective device rating specified for the PV module or electronic power converter
3. Circuits Where Overcurrent Protection Is Required On One End. A circuit conductor connected at one end to a current-limited supply, where the conductor is rated for the maximum current circuit from that supply, and also connected to sources having an available maximum circuit current greater than the ampacity of the conductor, shall be protected from overcurrent at the point of connection to the higher current source.

The Paraphrased Version

Simply paraphrased, according to code NEC 690.9 (A) (1-2) (2020 Edition):

Fuses (aka “overcurrent protective devices”) are required if the maximum potential current (amps) flow is greater than the maximum amount of current that is safely allowed by both the electric wires and solar panels.

In other words,

A = Maximum potential current produced by solar array
B = Maximum current safely allowed in solar array

If A > B, then a fuse is required
If A < B, then a fuse is NOT required

Appendix C: How Solar In-Line Fuses Work

A fuse is a device that is designed to stop the flow of electricity if an excessive amount of current is forced through the wire. This is usually accomplished with a thin metal conductor that is designed to melt at a certain amperage and stop the flow of power through the fuse.

Fuses are useful because they halt an electrical issue before it can create a far more severe problem, like a fire.

A solar fuse installed in the right place helps to prevent faulty solar panels from overheating and catching fire because these fuses prevent an excessive amount of current from flowing backward into shorted PV panels.

If a short circuit were to happen in one of the panels with the proper solar fuses installed, the current from the other panels would flow toward the faulty panel and trip the fuse.

This cuts the flow of current before it can become a fire hazard.

Still Confused? Watch This Video

Check out this useful video from ‘Off-Grid Garage.’ His explanation of how electricity flows during a short circuit incident is clear and understandable.