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Solar Fuse Sizing Guide for Solar Owners

Solar DC fuses on a fuse block
Solar DC fuses on a fuse block
Resource: https://forum.projectvanlife.com

When it comes to solar energy systems, one of the most important safety considerations is correctly sizing the fuses. This guide will help you understand why proper solar fuse sizing is so important for your PV system, and how to make sure you’re doing it correctly.

The Solar Fuse

A PV solar fuse is a critical component when it comes to protecting a solar system from electrical hazards. A properly sized fuse will help ensure that your system can safely handle any surges or spikes in electrical current, and prevent fires or damage to your equipment.

But while fuses for solar systems are important, they’re also one of the most misunderstood components. Many people do not know how to size them, which often results in electrical incidences such as wires that get too hot.

Why Solar Fuse Sizing Matters

To understand the need for proper solar fuse sizing, here’s how using the wrong size can lead to problems: Let’s say, for example, that you have a solar energy system with 5, 200-watt panels connected in parallel, and each panel produces a current of 4 amps.

Now, let’s assume that you choose a 10-amp fuse for each module in your system. This may seem to be fine since the maximum current output of your panels is only 4 amps. However, that wouldn’t be the correct way to size the fuses.

The reason has to do with the fact that your system can, without warning, experience surges that are much higher than the maximum output. For instance, a short circuit in one of the panels could cause the other panels to drive 16 amperes through the cables of the faulty panel and dangerously heat the circuit.

Solar Fuse Sizing Guide

So, how do you go about sizing solar fuse for your solar energy system? A lot depends on the specific solar fuse location. When fusing a solar system, there are generally 4 different locations where the fuses will be used:

  • In the solar panel circuits between one panel or string and another
  • In the DC combiner box for the cables leading to the charge controller or grid-tie inverter
  • Between the charge controller and battery
  • Between battery and inverter

Solar Panel Fuses

Manufacturers generally recommend a 30 Amp fuse for solar panel cable protection. That’s because most panels are installed with 10 gauge wires. These cables can safely carry 30A, which eliminates any possibility of fire due to overcurrents.

The fuses for solar panels are typically placed between each panel, or between strings of panels in a parallel configuration. This is to protect the panel cables from fault currents. The fuses are also normally installed at the combiner box.

Note that solar panel fusing does not apply to panels connected in series. In this configuration, the amperage does not exceed the ampacity of the wires. For that reason, fusing is not generally required.

Solar fuse combiner box/block
Solar fuse combiner box/block
Resource: https://www.thevanimals.com

Fuse Between Solar Panel and Charge Controller

This DC solar fuse (or fuses) is typically located in the combiner box. Its purpose is to protect the cables leading from the box to the charge controller.

For a grid-tied system, this would be the circuit between the fuse box and the inverter. Here are the sizing guidelines for the fuse between the solar panel and MPPT or charge controller.

  • For this solar fuse, sizing is done using the array short circuit current rating (ISC). This value can be found in the manufacturer’s specifications sheet.
  • Therefore, for a 5-panel system with an Isc of 5 amps, you would use a 10-amp fuse (5 x 1.56 = 7.8). If you have multiple strings of panels, then you would need to use a fuse that is rated for the sum of all the strings’ short circuit currents.
A 30 Amp solar fuse and its holder
A 30 Amp solar fuse and its holder
Resource: https://www.rv.net

Fuse Between Charge Controller and Battery

The next type of fuse is the fuse between the charge controller and battery cables. The size of this fuse will depend on the maximum current that can flow from the charge controller to the battery.

Here, you want to use a fuse that is rated 1.25 times the maximum current that can flow from the charge controller. Below is an example calculation when sizing a fuse between solar controller and battery:

  • If the charge controller is rated for a maximum current of 10 amps, then you would use a 12.5-amp fuse (10 x 1.25 = 12.5). It’s also important that you follow the recommended fusing as indicated in the solar charger manual.
  • That’s because solar charge controllers are different, and can be PWM or MPPT. They also sometimes may come equipped with overcurrent protection, and the fuse battery must be rated accordingly.
Solar fuse holder for battery fuse
Solar fuse holder for battery fuse
Resource: https://youtu.be/GRKcZEIWihg

Fuse Between Batteries and Inverter

The last type of solar DC fuse use is used between the battery and inverter. This fuse is there to protect the battery-inverter circuit from overcurrent. We recommend using your inverter’s manual to determine the fuse size in this situation as some inverters contain fuses while others don’t.

Generally, when sizing the fuse between battery inverter cables, the standard method is to use the inverter’s maximum current output and multiply it by 1.25. Using the formula, let’s size a fuse when using a 1,000-watt, 12-volt inverter.

  • To get the maximum current output of the inverter, we need to divide the inverter’s wattage by its voltage. Therefore, for a 1,000-watt, 12-volt inverter, we have 1,000 watts / 12 volts = 83.3 amps.
  • Next, multiply the amp rating by 1.25 to get the fuse size, which comes to 104.1 amps (83.3 x 1.25 = 104.1). This rating will need to be rounded off to a fuse size that’s ready and available.

Conclusion

Proper solar fuse sizing is an important aspect of keeping your power system safe. As you can see, there are different types of solar fuses used in different parts of a solar energy system. These will usually be sized differently, depending on the location. Be sure to consider that when setting up your PV system. Also, always use the manufacturer’s instructions where and when provided.

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