So you’re interested to learn about the sexy topic of solar fuses and safety! You’ve come to the right place. In this post we are talking about WHEN and HOW to fuse a solar system array.
By the time you’re finished with this post, you will have a concrete understanding of:
- Which type of solar arrays require fuses,
- How to calculate the correct size fuse, and
- Where in the solar array to install the fuse.
“If you’re ready, let’s go!”
Confused where to start with DIY solar? Check out our comprehensive camper van solar system guide to start from the very beginning.
Why Are Solar Fuses Important?
Solar fuses are important because they prevent fires that would occur if a solar array experiences a ‘short circuit’ situation.
What Is A Short Circuit?
A short circuit happens when one of the solar panels in an array 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 backwards towards the faulty panel.
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).
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 system. This is usually accomplished with a thin metal conductor that is designed to melt at a certain amperage and stop the flow of current through the fuse.
Fuses are useful because they halt an electrical problem before it has a chance to create a far more severe problem, like a fire.
Likewise, 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.
What Do Solar Fuses Look Like?
For solar ‘photovoltaic’ systems, the most common type are these MC4 in-line fuses.
MC4 fuses come in several different amp ratings (usually from 10-30amps), but the size of the exterior housing is pretty much identical.
The only difference is the thickness of the delicate conductor inside the housing (the part that is designed to melt in overcurrent situations).
These fuses are disposable. Meaning once there is an overcurrent event, the fuse will melt (as it’s designed to do) and it will need to be replaced. Luckily, fuses aren’t very expensive and usually cost between $10-15 each.
But what these fuses protect (i.e your home or camper van) is priceless.
When Do You Need A Solar Fuse?
Below, we will go over important solar terms to know and present the 4 most common solar array situations and decide whether or not each array requires a fuse.
Important Terms To Know
It’s important to know these two technical terms.
- Short Circuit Current – This is the current a solar panel produces in the event one of the other panels becomes faulty.
- Maximum Series Fuse Rating – This is the maximum current (amps) that a single solar panel (and it’s output cables) can accept before it overheats and becomes a fire hazard.
You can find both these two terms on the safety information pamphlet of your solar panels.
For an incredibly clear explanation on these two important terms, watch this video from Jayteenetics Tech. (Make sure to turn on English subtitles, it’s worth it).
Situation 1: A Single Solar Panel Array
This is easy. If your only solar panel short circuits, that panel won’t be at risk of a fire since there are no other sources of current for that panel to absorb.
Verdict: No Fuse Necessary
Situation 2: A Solar Panel Array Connected In Series
In a solar panel array connected in series, each individual panel accepts the current (amperage) given to it from the prior panel and outputs the same amount of amps.
Only the voltage is increased.
Therefore, if one of the PV panels experiences a short circuit, there is no risk of overheating since the short circuit current from the prior panel in the series is not enough to push past the shorted panel’s maximum series fuse rating.
Verdict: No Fuse Necessary
Situation 3: Two Solar Panels Connected In Parallel
With two solar panels connected in parallel, if one of the panels experiences a short circuit, the short circuit current from the only other functioning panel will flow into the faulty panel.
But because there is only one functional panel sending its short circuit current into the faulty panel, there is not enough current to push past the shorted panel’s maximum series fuse rating.
But for peace of mind, you should double check: Make sure that the listed short circuit current is LESS THAN the listed maximum series fuse rating for your solar panels.
Verdict: No Fuse Necessary
Situation 4: Three (or more) Panels Connected In Parallel
With more than two solar panels connected in parallel, this is where things get interesting.
If one of the panels experiences a short, it will now absorb ALL the short circuit current from the other functioning panels. The total current headed to the faulty solar panel is the sum of each panel’s individual ‘short circuit current’.
In most cases, accepting current from more than one panel will almost always BE GREATER than that panel’s ‘maximum series fuse rating’.
In this situation, a shorted panel is a fire risk.
Verdict: Fuse Required
Furthermore, an in-line fuse will need to be installed FOR EVERY panel in the parallel array. So if you have three panels connected in parallel, you will need three in-line fuses.
Keep reading to learn where in the solar array to install these in-line fuses.
Still Confused If You Need A Solar Fuse?
Check out this useful video from ‘Off-Grid Garage’. His explanation on how electricity flows during a short circuit incident is clear and understandable.
What Size Fuse Is Required For Your Solar Array
If it is determined that your solar array needs a fuse, calculating the fuse size is easy.
Fuse size = The solar panel’s maximum series fuse rating
In the above example, you can see that for this particular solar panel, the ‘maximum series fuse rating’ is 10 amps. Therefore, a 10 amp-rated solar fuse is required.
If, for some reason, your solar panel manufacturer doesn’t state the panel’s maximum series fuse rating, you will need to email the manufacturer to request it.
Find Your Solar Fuse
Solar Fuse Size
Where To Buy?
Building a camper van? Download our free e-Books with intuitive electrical, solar, and plumbing diagrams.
Where To Install The Solar In-Line Fuse
Assuming you have more than two solar panels connected in parallel, you will need to install an in-line fuse just BEFORE the current of each panel is combined with the rest of the array.
In most cases, this will be right on the MC4 branch connector that accepts the positive (red wire) current.
Now if a short circuit were to happen in one of the panels, 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.
Our Solar Array - Did We Install Fuses?
On our camper van, we connected two 180-watt solar panels in parallel.
Solar Panel Specifications:
In our case, because we have only two panels connected in parallel, we DID NOT fuse our solar array. This is because when one of our two panels experiences a short circuit:
- The remaining short circuit current (11.75 amps) does not exceed the maximum series fuse rating (20 amps) of the faulty panel.
- The remaining short circuit current (11.75 amps) does not exceed the maximum amperage rating of the source wires (10AWG wire = 30 amps rating)
Interested In The MOST Efficient (9BB) Solar Panels?
“9BB” solar panels are becoming the next big thing in the solar panel industry. Read our 9BB solar panel post to learn why these panels are more efficient AND have a longer usable life than traditional solar panels.
Final Thoughts: Fusing a Solar Array Is For Your Safety
Fusing a solar array, if it’s required, is a critical step to keep you and your camper van safe from potential fires. We hope this article was able to simplify the learning process for you.
If you’re interested in other ways you can keep your solar system safe from fires, check out our solar wire sizing guide. It’s informative and easy to understand.
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.
The Official Text
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 of 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
This post will teach you how to calculate both the maximum potential current and the maximum current safely allowed in a solar panel array so that you can determine IF your solar array needs to have fuses installed.
(For those curious about the official, overly wordy text, head over to Appendix 1 at the end of this post)