Solar panel fuses and wire gauge

Topic: Solar panel fuses and wire gauge (Read 283 times) previous topic - next topic

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February 03, 2021, 09:43:48 pm

In a different thread, majortom asked a secondary question about if a fuse was required between solar panels and the solar controller. RonB correctly answered that the need for a fuse from the panels to the controller was determined by the manufacturer of the panels. I thought this important topic required further discussion and should have it’s own thread. So here goes a long post.

Update: To be clear, this post is about fusing and wiring between the solar panels and solar controller. Fusing between the solar controller and battery is very important too, but was beyond the scope of this post.

Is a fuse needed between solar panel and controller

The need to place a fuse between the solar panel(s) and controller is situational and often determined by the manufacturer and installer. There are a few things to understand in order to know why this is true.

1. The purpose of a fuse is to prevent excessive current from flowing through a wire causing it to overheat and possibly catch fire. Wire gauge (thickness) and insulation type determines the maximum continuous current (amps) that is safe to flow through the wire.

2. Solar panels by nature are current limited. In other words, a given solar panel at best can only provide a specific maximum amount of current and no more regardless of demand. The solar panel manufacturer will rate Isc (short circuit current) of a panel based on a standardized test. Actual short circuit current may be higher under ideal conditions, so a common practice is to add 25% to Isc to obtain the maximum current that a given panel can produce.

3. Multiple solar panels may be connected in series or parallel or a combination of both. Panel voltages are additive in series connection while panel currents are additive in parallel connection. Most RV applications use parallel connections to minimize shadowing effect, such as from a tree limb or air conditioner. In a parallel configuration, each panel can produce a maximum of 125% of Isc current flowing from one panel to the connection in the combiner box. These individual wire runs from solar panel to combiner box are called branches and the current through each branch is called a branch current. Therefore, in ideal conditions, four 100 watt panels connected in parallel with an Isc of 5 Amps would produce a maximum branch current of 6.25 Amps (Isc + 0.25 x Isc) and a total of 25 Amps ((Ics + 0.25 x Ics) x 4) from the combiner box to the solar controller.

4. In the example given in #3 and under normal operation any individual branch would only see a maximum of 6.25 Amps and you would think that the wire gauge for the branch could be based on that. But no... in case of a short in the wire of one branch, all branch currents will take the path of least resistance. Meaning that the current from the faulted branch would flow from it’s solar panel through the faulted branch wire and through the short. The other three branch currents would flow from their respective solar panel to the combiner box where they would then be added together and flow up through the faulted branch wire and through the short. This would result in a total of 25 Amps flowing through the short and possibly causing a fire.

5. Bottom line: If the maximum current possible from all solar panels (see example in #3) is equal to or greater than the safe continuous current capability of any branch wire, then a fuse is required. Actually, if it’s anywhere close, then installing a fuse would be a wise precaution. In the chart below you can see that 12 gauge UF wire is rated at 25 Amps continuous while 10 gauge UF wire is rated at 30 Amps continuous. UF wire is individual insulated wires wrapped together in an outer sheathing, suitable for underground/outside use and common in solar panel wiring. Given the parameters in #3 (25 Amps maximum), 12 gauge wire would require fusing but not 10 gauge wire. However, I would go ahead and fuse 10 gauge because the numbers are too close for my comfort.

What wire gauge to use

There are things other than maximum continuous current and fusing to consider when determining wire gauge. So far we’ve been discussing current flow, but there is also voltage and more specifically voltage-drop to consider. Voltage-drop is what occurs when a current flows through a resistance and the measured voltage on either side of the resistance is different.

In our case the current flow comes from the solar panel and the resistance is the wire gauge. The bigger the wire gauge the less resistance, which means less voltage-drop. A solar panel that starts out at 20 Volts and then drops to 18 or 17 Volts by the time it gets to the solar controller is not an efficient system. The factory uses 12 gauge wire, which is ok for 100-200 watts of solar panels. 10 gauge is better and should be used if you are upgrading. BTW, this is only for the branches from the solar panels to the combiner box. The down run from combiner box to the solar controller should be even heavier gauge wire depending on the total current flow to the controller. See the wire gauge chart below for suggestions.

Where should the solar panel fuses go

Solar panel manufacturers will sometimes add a fuse right where the pigtail attaches to the solar panel. This is convenient for them because, after all, they’re the ones building the panel. But this is exactly the worst place for the fuse to be placed. If you were to follow the manufacturer’s advice and the short circuit scenario mentioned in the first section #4 were to happen... the most any fuse would see is 6.25 Amps, while the shorted branch wire would see the full 25 Amps, potentially overheating the wire and catching fire.

The best place and should be the only place for the fuses is in the combiner box. One fuse for each branch and possibly one larger fuse for the down run to the solar controller. Fusing the down run from the combiner box to the solar controller would of course depend on total solar panel current and wire gauge. By placing the fuses in the combiner box, the short circuit scenario mentioned before would blow the fuse in the shorted branch and prevent any possibility of an RV fire.

Here’s a couple of fuse blocks from BlueSea that would work well for branch fuses in a combiner box:
ST Blade Compact Fuse Blocks - 4 Circuits
ST Blade Compact Fuse Blocks - 8 Circuits

Here’s a fuse block from BlueSea that would work well for the down run fuse in a combiner box:
ANL Fuse Block with Insulating Cover - 35 to 300A

Last thing... you may be thinking that adding a high current switch in the down run may be convenient in case you need to work on the solar charger and want to disconnect the solar panels. Personally, I don’t think so. For one thing it would introduce, however small, a voltage-drop to the system and for another, it’s one more thing that could fail. If I want to disconnect the solar panels, I can climb a ladder, open the combiner box and pull the fuses.

Thank you for reading this extremely long and maybe overkill post. Comments, corrections and opinions are always welcomed 😃

- John