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 to just 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) in order 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 below simplified wiring diagram 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.
If you’re ready, let’s get to it.
From the solar panels, to charge controller, to the batteries, and everything in-between, this eBook has you covered. 12+ pages of detailed diagrams, product recommendations, and links to additional resources.
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 experience 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 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
Single Panel = No Fuse
Panels In Series (Single String) = No Fuse
Two Panels In Parallel (Two Strings) = No Fuse
Three Or More Panels In Parallel = FUSE!
Read our series vs. parallel post to learn about the pros and cons of the different ways to wire solar panels together.
Where To Install Solar Fuses?
If your solar array will require solar fuses, you must install them just before the positive (red) branch connector. Each solar panel will require a 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 is equal to 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 the panel were to experience more than this rated current, the panels & wire will overheat and a fire may start.
During a short circuit event in an array with 3 (or more) panels wired in parallel, it is often the case that the resulting current which will flow into the shorted (faulty) panel WILL be greater than that panels max series fuse rating. Therefore, a solar fuse must be installed to stop the flow of this potentially damaging current.
Solar Fuse Recommendations
If you now know what size solar fuse you need for your solar system, you can refer to our table below to pick up your fuses.
Solar Fuse Size (Amp Rating) | Where To Buy? |
10A | |
15A | |
20A | |
30A |
Part 2:
Solar Disconnect
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 fuses from Part 1, which aren’t needed in every situation, your solar system will 100% require a disconnect breaker/switch to be compliant with National Electric Code standards.
Correct Solar Wire Size
Using the correct size solar wires is just as important as installing a solar disconnect. Check out our solar wire size guide to learn how to calculate the right size wires for your system.
What Size Solar Disconnect Do You Need?
To correctly size the amp-rating of your solar disconnect OCPD, follow the below four-step process.
- Identify short circuit current – Isc(A): This info is found on your panel’s info sticker.
- Calculate max current of array: If panels are in series, max current is equal to single panel Isc(A). If panels are in parallel, sum the Isc(A) of each panel.
- Multiply max current by 1.56: This is the NEC safety margin.
- Round up to the nearest solar disconnect amp rating
Example: Two panels are wired in parallel. The short circuit current 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.
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 for when you are conducting routine maintenance or troubleshooting problems on your system.
If you’re ready to pick up a disconnect breaker, we provide a link to the very same device we use in our camper van.
A DC Solar Breaker is used to protect the solar wires between the panels and the charge controller. It also provides an easy way to disconnect the panels during regular electrical maintenance.
Part 3:
DC Circuit Breaker (Or Fuse)
The last part in fusing a solar system involves installing an in-line fuse (or breaker) 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 (or Fuse) Do You Need?
Calculating the correct circuit breaker size involves a simple two-step process:
- Identify amp-rating of charge controller*
- Multiply this amp-rating by 1.25: This is the NEC safety margin.
- Round up to the next circuit breaker size
* Note: If you don’t yet have a charge controller, 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.
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!
If you’re ready to pick up a DC circuit breaker, we provide a link to the very same device we use in our camper van.
This circuit breaker protects the solar wire from overheating and is installed between the solar charge controller and the positive bus bar.
Summary
Solar Panel Fusing Review
And that’s it for fusing a solar panel system. In summary, you will need to install:
- Solar fuses before positive branch connector (only for 3+ panels connected in parallel)
- Solar disconnect before charge controller
- DC breaker after charge controller
Review the below solar fuse wiring diagram 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.
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Summary
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 and you and your solar system safe is worth your time and effort.
And 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, send us an email to the address listed in our footer.
Happy building!
Additional Reading
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 it’s way towards the faulty panel instead of towards 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) to catch on fire.
(Refer to our not-so-subtle graphic below).
Additional Reading
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
- 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 over current 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:
- The conductors have sufficient ampacity for the maximum current circuit.
- The currents from all sources do not exceed the maximum overcurrent protective device rating specified for the PV module or electronic power converter
- 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 or 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
Additional Reading
Appendix C: How Solar 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 has a chance to 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 backwards 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 towards 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 on how electricity flows during a short circuit incident is clear and understandable.
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