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!”
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Tables of Contents
What The National Electric Code Says
Making sense of the “Solar Photovoltaic Systems” section of the National Electric Code is a good place to start when learning about fusing a photovoltaic array.
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.
NEC 690.9 (A) (1-2) (2020 Edition) - The Official Text
(For those curious about the official, overly wordy 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.
Why Are Fuses Important In Solar Arrays?
In order to understand why fuses are important, it’s critical to know what happens when a single solar panel in an array experiences a short circuit.
What Happens During A Short Circuit In A Solar Panel?
It is rare, but sometimes solar panels can experience a short circuit and stop working. Shucks!
Intuitively, you might think that electricity simply stops flowing through the faulty panel. Unfortunately, it doesn’t quite work like that.
When a panel experiences a short circuit, it continues to allow outside current (generated from other solar panels in the array) to enter it’s wiring and diodes.
In some cases this is OK and safe.
But in other cases, depending on how the solar array is wired, this can be quite BAD.
Because electricity prefers to flow along a path of least resistance, a shorted PV panel (with now conveniently very low resistance) that is connected in parallel to other PV panels is now forced to absorb the total current produced by the other functioning panels.
A shorted PV panel is like a black hole. It just sucks in all the nearby current.
And if the amount of current that a faulty panel absorbs is greater than the maximum current it can safely handle, then that panel (and wiring) can overheat and catch on fire.
And this can, in turn, lead to your entire home (or camper van) to catch on fire.
The Role Of A Solar Fuse
An electric fuse is a device that is designed to stop the flow of electricity if an excessive amount of current is forced through the fuse. 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.
In a short circuit situation, the fuse belonging to the faulty panel will melt and be destroyed. But a fire will be prevented.
What Do Solar Fuses Look Like?
Electrical fuses come in all different shapes and sizes. But for solar PV systems, the most common type are these MC4 in-line fuses.
MC4 In-Line Fuse
MC4 fuses come in several different amp ratings (usually from 10-30amps), but the size of the exterior housing are 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 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 your camper van) is priceless.
Important Terms To Know Before Explaining Fuse Guidelines
Before we explain guidelines for fusing your solar PV array, it’s important to know two technical terms.
Short Circuit Current
This is the current a solar panel produces in the event a short circuit occurs in a solar array.
- In a parallel array, because the current is added together, the combined total short circuit current is the sum of the individual short circuit currents from the remaining functioning panels after one of the panels experiences a short circuit.
Maximum Series Fuse Rating
This is the maximum current (amperage) that a single solar panel (and it’s output cables) can accept before it overheats and becomes a fire hazard due to overcurrent.
You can find both these two terms on the safety information pamphlet of your solar panels.
Guidelines For Fusing Your Camper Van Solar Array
So, the most important question.
“Do you need to add a fuse to your solar array?
And the answer is… “It depends!”
The biggest factor is whether the solar array is connected in series or parallel.
Below, we will go over the 4 most common solar array situations installed on RVs and camper vans and decide whether or not each array requires a fuse.
Situation 1: A Single Solar Panel Array
This is very 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.
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, you will need a 10 amp-rated MC4 in-line fuse to keep this panel safe from fires.
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.
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 175-watt solar panels in parallel.
Solar Panel Specifications:
- Short Circuit Current: 11.75 amps
- Maximum Series Fuse Rating: 20 amps
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)
Fusing a Camper Solar Array – Final Thoughts
Learning about solar fuses and calculating whether a solar array requires a fuse is nobody’s idea of a fun time. Just reading the official language from the National Electric Code was enough for our eyelids to feel heavy.
But fusing your solar array, if it is 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 and that you came away having successfully identified if it was required and how to fuse your solar system.
If you have any specific questions or comments related to solar fuses, please let us know in the comments section below.
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