HOW TO FUSE YOUR SOLAR PV SYSTEM

2021-04-01

When connecting Slocable 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. Fuses and circuit breakers are used to protect the wiring from getting excessively hot and also protect all devices connected in the system from bursting into flames or getting harmed if a short circuit happens. A good example is a 12V lead acid battery. If a short develops in your AC/DC inverter for example, a fuse between it and the battery will prevent a possible explosion of the battery and it will cut the circuit fast enough to prevent the wires from bursting into flames or getting hazardously hot. For this situation, the battery, wires, and AC/DC inverter will be securely disabled by the fuse. They are not necessary for the system to run properly, but we always suggest using fuses or circuit breakers for safety purposes. There are three different locations that we suggest 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.

To determine the fuse size required between the charge controller and battery bank you simply match the amperage rating on the charge controller.

The second fuse between your solar panels and charge controller is somewhat different to figure out. The size of this fuse is dependent on how many solar panels you have and how they are connected (series, parallel, or series/parallel). If the panels are connected in series, the voltage of each panel is added but the amperage stays the same. For instance, if you have four 100W panels connected in series, each producing 20 volts and 5 amps, the total output would be 80 volts and 5 amps. We then take the total amperage and multiply it by a safety factor of 25% (5A x 1.25) giving us the fuse rating of 6.25A or 10A if we round up. If you have a parallel connection, where the amperage of the panels is added up however the voltage stays the same, you would have to add up the amperage of each panel and then we add a 25% industry rule to figure out the fuse size. For instance, if you had four 100W panels hooked in a parallel connection, each panel produces about 5 Amps, so we would use this equation (4 * 5 * 1.25) = 28.75 Amps, so in this instance we would recommend a 30 Amp fuse.

Commercial solar panels with more than 50 watts have 10 gauge wires and can handle currents up to 30 amps. If these panels are connected in series, the current will not increase, so the string does not need to be fused. When you connect the panels in parallel, this is not the case, because when connected in parallel, the system currents add up. For example, if you have 4 panels, each of which can provide up to 15A of current, a short circuit in one panel will cause all 60 A of current to flow to the short-circuited panel. This will cause the wires leading to the panel to far exceed 30 amps, which may cause the pair of wires to catch fire. If it is a parallel panel, each panel needs a 30 amp fuse. If your panel is less than 50 watts and you only use 12 gauge wire, you need a 20 amp fuse.

The last fuse that we suggest in the system would be if you are using an inverter. Wiring and fusing from the battery to the AC/DC inverter is critical because this is where the maximum current can flow. This fuse would be between your inverter and the battery bank. The fuse size is normally stated in the manual and most inverters is likely to have built-in fuses/circuit breakers on the input and output (AC) sides of the device. 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 suggest a 150A fuse.

This is a brief introduction and summary for fusing your system. There are other aspects such as cable size/length and fuse/breaker types that are important. You can send an email for more information about solar products! If your take your time and use the right combination of rated parts, then the system should work well and you’ll sleep better knowing you engineered it to be safe and reliable.