When connecting a solar PV system, the most ideal approach to add assurance is by using MC4 fuses or solar circuit breakers. The correct use of fuses and circuit breakers is important to maintain safety.
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, when and where you should place fuses for optimal efficiency, and what size of solar panel fuse you need.
Role of a 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. (Source from Wikipedia)
So, the purpose of a fuse is mainly to prevent the wires from catching fire or getting dangerously hot, and either the wires, parts of the system, appliances, or even your camper vans catching on fire.
Do I Need A Fuse For My Solar System?
There is a lot of debate on the forums about the need for a fuse between the solar panel and the charge controller. Many solar users think the fuse is not needed. Well, actually of course you need! Every electrical circuit you should fuse just for the sake of safety. There are four reasons.
• The Charge Controller is Too Small – The primary reason to install a fuse or breaker is when the voltage from the solar panels is too much for the charge controller to handle.
• Wire is not in proper size – It is POSSIBLE to do some damage even with low voltage and low power panels if you select thin wires.
• Lightening is a Possibility – Even though there are grounds, a lightning strike to the panel could send an electricity spike to the solar controller and destroy it.
• Prevent a Fire – If the wiring, solar controller, or solar batteries get too hot, they can combust and start a fire, especially due to their aging.
When and How to fuse a solar system?
There are three different locations where we recommend installing fuses or breakers: first, between the charge controller and battery bank, second, between the charge controller and solar panels, and third would be between the battery bank and inverter. We consider the fuse size based on the specific location.
Between your solar panels and charge controller
We often wire our solar system in several ways. They are series, parallel, or a combination of both.
Calculating the fuse size is easy.
Fuse size = The solar panel’s Maximum series fuse rating (or Isc x 1.56)
You can find both these two terms on the safety information pamphlet of your solar panels.
If there is no maximum series fuse rating on the specification of the solar panel, you can calculate with the formula. Usually, a 100-watt solar panel needs a 10 amp fuse.
► Wiring solar panels in series
Most of the time you will be wiring your panels in series. This is because MPPT charge controllers have a high input voltage and it's cheaper to wire them in series. Each individual panel accepts the current (amps) given to it from the prior panel and outputs the same amount of amps. The array’s voltage is added together while the current (or amps) stays the same. In that case, you only need to have one fuse on the positive wire coming from the solar panels.
For example, if the short circuit current is 6.2 amps, and the maximum series fuse rating is 10 amps, any fuse you add would need to be 10 amps. Or you can calculate the minimum fuse size. It will be 6.2 amps x 1.56 = 9.6 amps. In this case, you would use a 10 amp fuse. You can use a general inline mc4 fuse.
► Wiring solar panels in parallel
if you use a PWM charge controller, you will most likely wire in parallel. The amperage of the panels is added up but the voltage stays the same. You need to fuse every parallel panel. As you can see in the following image, every parallel panel is fused.
Just like the previous example, we will use a 10 amp fuse for F1 to F3.
We now need to calculate the fuse size for F4. We need to use a short circuit current and multiply it by three panels. Then we multiply it by the safety factor of 1.56 and we become 29 amps.
6.2A x 3 x 1.56 = 29A
We will use a 30 amp fuse for f4.
► Wiring solar panels in series and parallel
if you have a larger system you might want to combine series and parallel. Here the same rules apply: fuse every series string as indicated in the example.
This is an example of a 3s2p system. There are three panels in series and two parallel strings.
We need three fuses in total. One for each series string and one for the combined parallel array.
Fuse 1 and 2 will be 10 amps like our previous examples. Fuse 3 will be 6.2 amps times two parallel strings times 1.56 equals 19.3 amps. We will use a 20 amp fuse.
As you might already know, in series the voltage gets added up and in parallel, the currents get added up. That means we can use a small wire size in series, and we need to use a bigger wire size in parallel. If we want to calculate the wire sizes of the last system, we can do so as follows:
The wire from the solar panels to the charge controller needs to be calculated according to the short circuit current of the solar panel. This is 6.2 amps times 1.56 equals 9.6 amps. The combined voltage of the three solar panels is 52.5 volts.
If we calculate this for the voltage drop we can see that a 14 gauge or 2.5-millimeter square wire is good for 25 feet long. The other wire from the combiner box to the charge controller needs to be thicker because it combines the current from two series strings. So, 6.2 amps times 1.56 times two strings equals 19.3 amps. The voltage is the same at 52.5 volts.
Let's say the length is 25 feet, then we need a 12 gauge or 4-millimeter square wire.
Between the charge controller and battery bank
To determine the fuse size needed between the charge controller and battery bank, you need to match the fuse size according to the type of your charge controller.
If you are using a PWM charge controller, the highest number of amps flowing to and from the controller will be the same. Therefore, your fuse size and wire size can also be the same. That means, if you have a 20A PWM charge controller, you can use a 20A solar fuse.
However, if you are using an MPPT charge controller, you will need to consult the charge controller manual for the exact fuse size needed, as this type of controller can lower the voltage as well as increase the current flowing between the controller and the battery.
Between the battery bank and inverter
The last fuse that we recommend in the system would be the situation when you are using an inverter. This fuse would be between your inverter and the battery bank. The fuse size is usually stated in the manual and most inverters already have built-in fuses/breakers. The rule of thumb that we use here would be:
Continuous Watts / Battery Voltage times 1.25
For example, a typical 1000W 12V inverter draws up around 83 continuous amps and we would add the 25% safety factor which comes out to 105 Amps, so we would recommend a 150A fuse.
Conclusion
The fuse or circuit breaker will protect all the solar components from the solar panels and solar batteries to the inverter. The small price you pay to add a fuse or a breaker to your solar system can save you thousands of dollars should an energy spike occur.