How/When to Fuse a Solar Panel Array

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This blog post is going to teach you how to determine when and if you need to add in-line fuses when designing a camper solar array. Sometimes you need to fuse your solar array, and sometimes you don’t. There’s quite a bit of reasoning behind the decision, but ultimately… the NEC (National Electric Code) makes the decision for us. By the end of this blog post you will know when, where, and what sizes of fuses you need in your solar array.

Fusing a Solar Array

The National Electric Code is pretty straightforward on this issue in NEC 690.9 (A) (2020 Edition) and says (Paraphrased for clarity):

Solar arrays having higher current availability (parallel strings of solar panels) than the maximum overcurrent protective device rating specified for the panel shall be protected from overcurrent. EXCEPT when the short circuit currents from all sources do not exceed the ampacity of the wires and maximum overcurrent protective device size rating specified on the PV module nameplate.

KEY TAKEAWAY: This means that if the Short Circuit Current of the entire solar array is GREATER than the Maximum Series Fuse Rating on the solar panel label, each parallel connected panel (or series string) must be fused.

This means you need two things to determine if your solar array needs to be fused:

  • The Short Circuit Current of your Solar array.
  • The Maximum Series Fuse Rating of your Solar Panel.

With those two pieces of information, follow the steps in this infographic to determine if you need to use fuses in your solar array:

Why Must you Fuse a Solar Array?

In the event that your solar array needs to be fused because the Array Short Circuit Amperage is greater than the Panel maximum series fuse rating… you must fuse your array at the point where the panels or series strings get combined to prevent potential fires or overheating due to a faulty panel. Here is what happens in an un-fused array if a short were to somehow happen inside of the panel:

In the event that panel #3 develops an internal short or fault of some kind… panels #1 and #2 would seek out the path of least resistance (the point of the short) to complete their circuits). This means that there would be 20.4A flowing to panel #3, combined with the 10.2A FROM panel #3 where there could POTENTIALLY be 30.6A flowing through the short, which is over 15A higher than the max amperage rating of the panel and is more amperage than the panel is designed to handle.

Now… What if there were fuses attached to each of the positive wires from each solar panel where they connect to the MC4 Combiner?

In the event that panel #3 develops an internal short or fault of some kind… panels #1 and #2 would seek out the path of least resistance (the point of the short) to complete their circuits). This means that there would be 20.4A flowing to panel #3 EXCEPT, that since we installed a 15A fuse protecting panel three’s circuit… that fuse would blow and isolate the problem panel to a short circuit that is within the maximum short circuit parameters of that panel.

That is why, by code, fuses are required in this array.

Why Would A Solar Array NOT Need to Be Fused?

If the Short Circuit Current of the solar array is less than the Maximum Series Fuse Rating of the solar panel, the array does NOT need to be fused. Fusing this type of array adds no additional protection or benefit, and here’s why:

The diagram above shows 3x 200W panels wired in series. Each solar panel has a short circuit current of 10.2A, and operating current of 9.8A, and a Maximum Series Fuse Rating of 15A.

Since the Maximum Series Fuse Rating is 15A, we know that the wires, diodes, connectors, and other internal components of the actual solar panel can handle a max of 15A.

If a short circuit or other malfunction were to happen inside of one of the solar panels, since the short circuit current of the array is 10.2A, it’s safe to say that the panel itself is designed to handle this short circuit event as the short circuit current cannot exceed the maximum fuse rating of the panel.

Now… What if you just wanted to add a fuse ‘just to be safe’:

Ultimately… it wouldn’t do anything… In the event of a short circuit, 10.2A would flow. Under normal operation, 9.8A is flowing. Sure, a 10A fuse would indeed be between those two values, but the difference of .2 amps in either direction would either:

  • Never allow the fuse to blow, or…
  • Make the fuse constantly experience nuisance blows under normal operating consitions.

This is why a fuse, by code, is not required in this scenario.

Electrical Code vs Real World Discrepencies

Sometimes, there will be a discrepancy between what would happen in a real-world scenario and what is required by code. When this happens… ALWAYS abide by electrical code. Electrical code is sometimes a bit more conservative but is there for a reason.

Here is an example of a setup that would NOT benefit from fuses, but DOES require fuses according to electrical code.

This scenario WOULD indeed require use of fuses in this array because the array short circuit current of 20.4 is greater than the 15A Maximum Series Fuse rating of the panels used.

In the event that panel #2 had an internal fault, the power produced in panel #1 would seek out the path of least resistance and flow into panel #2. This would mean that the area of the panel experiencing the fault would potentially have 20.2 amps flowing to the affected area.

Although there are 15A fuses in the proper locations, the current going from panel #1 into panel #2 would not be enough to blow either of those fuses.

Although this is not ideal, it’s nearly impossible to avoid when using two panels or two series strings in parallel and this setup is NOT forbidden by the National Electric Code so it’s one of those things you just have to sit back and say; It’s fine. If it WASN’T fine… there would be an electrical code or standard saying something about it.

Fusing a Camper Solar Array – Final Thoughts

Faults in solar panels are quite rare. There are diodes in MOST solar panels that do not allow most of the above scenarios to happen. Fusing is required for a redundancy in the event that one of the diodes fails or something else goes wrong with the panel. There isn’t much to a solar panel. It’s pretty much just a bunch of Silicon cells fused together between some laminated glass connected to some diodes and wires. Even if a cell burns out, usually the panel will still work. But… Since the National Electric Code specifically makes a call on the issue of fusing an array, we really should follow that in our attempt to design and build a high end camper electrical system.


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38 Responses

  1. Great blog Nate, well written explanation of when and not to add PV fuses. I have a question. I see a lot of combiner boxes available on amazon that have the PV fuses, breaker AND a surge protector. If solar is on a camper is it necessary for the surge protector or recommended?

  2. Hey Nate,

    Thanks again for diagramming this out for us solar noobs! I see that I will need a solar disconnect switch to meet code regulations. I have (4) 100 watt Rich Solar panels in series-parallel like in your diagrams. What size/type of switch should I get to be code compliant?

  3. Hey Nate, thanks for all the info.

    I am setting up my solar system for my bus build and am going through all your posts.

    I have 6x 330watt panels. Planning on setting them up in 2 groups of 3 panels (series/parallel). The fuse rating for my panels is 20amps, the total stc short circuit current will be 20.6 amps.

    Will I be ok with not adding extra fuses?

    Many thanks

  4. Very helpful blog and vids. We have 4 200watt 12 volt panels, 11.11amps and 4 250 watt 12 volt panels, 13.89amps all 18 volt maximum power voltage at the moment wired in parallel. We would like to wire them in two sets of series and then join them in parallel. We have just purchased a Victron 150/100 amp solar controller. Would this work efficiently and would we need a fuse? Any suggestions would be great.

  5. Hi, first of all it’s a great blog with all the magic stuff well explained. And now to my question: I have a small solar instalation on a boat. Due to limited space I only managed to squeeze 1x60W panel (max amp 3.3A, short circuit amp 3.6A, let’s call it F1) a 2x20W panels (1.18A and 1.3A respectively, let’s call it F2 and F3). They are all connected in parallel.

    My calculations are that F1 fuse should be 10A but what about F2 and F3?

  6. Hi!
    I’m series connecting three 200Watt solar panels to Blue Sky SB3000i charge controller.
    Then to the batteries.
    What size amp circuit breakwe or fuse do I need between panels and charge controller, and what size do I need between charge controller and batteries?

  7. I’m setting up solar on a sprinter, I have two 100W (Imp 5.29A, Isc 5.75A) PVs that I’m wiring in parallel using 10AWG, if I’ve done my math correctly I don’t think I need a fuse anywhere before my mppt ( max input 25v 26A)
    This is only my second time doing this, does that sound correct?

  8. I have 2 x 100w solar panels running to a 30 amp solar controller feeding 2 x leisure batteries. 1200w peak inverter being fed off battery.
    What size fuse / breaker do I need between panels – controller, controller – battery and battery to inverter?

  9. I ordered a Renogy 4 panel, 400watt solar kit with the 40 amp controller. I am going to wire in series/parallel. I am assuming that I do not need any fuses on the panels or between the panels and controller because the amperage and voltage will probably be well below the limits of the charger. Is this correct?

  10. What might cause a circuit breaker to “go bad” but not be thrown? I had a “40A Audio Inline Circuit Breaker Reset Fuse 12V-24V DC” (typical on Amazon) that was getting warm to the touch, and the Charge Controller was “stuck” in FLOAT mode at a relatively high voltage (>15V on a 12V battery system), however the batteries were really at ~11.8V (i.e. not being charged). The same style of circuit breaker (but 20A) is working fine between the PV and the CC. I removed the 40A breaker, and the CC is now working properly (in BULK mode and the batteries have increased from 11.8 to nearly 13V, as measured by a meter across the terminals). Is it the breaker that has gone bad or am I miss-using it?

    1. The breaker could, indeed, be going bad; but without knowing much more about how your system is designed and wired, it’s tough for me to troubleshoot. Was your system based off of one of my diagrams? If so, which one? I haven’t put DC breakers in any of my wiring diagrams in about 2 years.

      1. Oh, it’s a off-grid system for a shed. 2x300W Renogy panels, Victron SmartSolar 100/20 CC, 2x Duracell Deep Cycle Marine batteries (SLI24MDC), with 1500W inverter connected to the batteries. The circuit breakers were intended as a convenient “quick release” to disengage components. But I’ve read more about the ANL fuses and maybe that’s the way to go.

  11. I have 2 100w panels wired in series and subsequently wired in parallel with 2 other 100 wat panels, (400w total) , operating at about 40v 10amp if I am calculating this correct. Should I fuse them for 15 amp prior to using a 2 to 1 combiner before the mppt ( so two 15 amp fuses).

  12. Hi, I’ve followed all your videos, you do a great job, Thanks! I have a mossiest system and used your amp hr spread sheet. i use just over 100ah now but it will grew just a little. I have 2 100 Ah battle Born lithium batteries and a MTTP 100/30 in case i want to add more. Id like to add a smart battery protector so i don’t ruin the batteries. how do i size and use it? Also how do you use the alternator device you suggest?

    1. I have 6×300 wat solar panels
      Wired in parallel what size fuse do I need between solar panels and charge controller

    2. I have two strings of three 150Watt solar panels wired in parallel. What size of fuses do I need and what size of disconnect?

  13. What if I am going from two portable 100 watt suitcase panels wired in series and going a Victron 100/30 solar controller? Do I need to fuse BEFORE the controller? I was going to use a 40 amp after the controller.

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