How to Fuse a Solar Panel Array (and why you may not need to)

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This blog post is going to teach you why you don’t need to place a fuse between your solar panels and your charge controller when the panels are wired in series.

For the first example, I’m going to start with panels with THESE stats:

Here is how it would look wired in a 3-panel series with 10 AWG (30 Amps Max Capacity) wire and a 15 amp fuse between the charge controller and the panels:

In the case of a short circuit or short to ground, the 15 amp fuse would not trip because the max amps that could flow to the short would be 9.78A; which would look something like this:

An active direct short would not pull enough current to trip the fuse.

It is not reasonably possible to protect this wire with an fuse/breaker because under normal operation, there is 9.33A flowing through the wire. To ‘protect’ the wire, the OCPD would, technically, have to be between 9.33A and 9.78A. In other words, higher than 9.33A to allow for normal operation of the panel and lower than 9.78A to protect from a short.

This margin between 9.33A and 9.78A is too small to reasonably place an OCPD to protect the circuit as you would technically be asking for something like a 9.5 amp fuse, which doesn’t exist (or at least, isn’t readily available). Also, as the temperature of the panel cools off and therefore gets more efficient, there is a potential for the panels to produce MORE power than what’s listed above, making nuisance trips possible. This is why a fuse is not required, nor useful in this location.

Fusing Solar Panel Arrays wired in Parallel

Now, in the case of 3 panels in series and 4 strings in parallel, if one string shorts, the max amperage to short is 39.12, which exceeds the 30 amp max current rating of the 10 AWG wire.

This means each series needs a fuse/breaker placed in the combiner box as shown here:

Now: on the OTHER side of the combiner box. Between the combiner box and the charge controller, the current flowing through the wire is:

  • Normal Operating Current: 37.32
  • Short Circuit Current: 39.12

This brings us back to the same point as earlier meaning that the fuse would have to be above 37.32A to allow for normal usage, but below 39.12A to protect from short, which is not reasonable. This reason is why no OCPD is necessary between the combiner box and the solar controller given proper wire size.

The code seeming to refer to this particular scenario is NEC 690.9 as an array of parallel-connected modules are capable of producing amperages higher than the short circuit voltage of a single panel. The code I’m referring to states:

NEC 690.0 Overcurrent Protection.

(A) Circuits and Equipment. PV system DC circuit and inverter output conductors and equipment shall be protected against overcurrent. Overcurrent protective devices shall not be required for circuits with sufficient ampacity for the highest available current. Circuits connected to current limited supplies (e.g., PV modules, dc to dc converters, interactive inverter output circuits) and also connected to sourcces having higher current availability (e.g., parallel strings of PV modules, utility power) shall be protected at the higher current source connection.

Exception: An overcurrent device shall not be required for PV modules or PV source circuit or dc-to-dc converters source circuit conductors sized in accordance with 690.8(B) where one of the following applies:

  1. There are no external sources such as parallel connected source circuits, batteries, or backfeed from inverters.
  2. The short-circuit currents from all sources do not exceed the ampacity of the conductors and the maximum overcurrent protective device size rating specified for the PV module or dc-to-dc converter

23 thoughts on “How to Fuse a Solar Panel Array (and why you may not need to)”

  1. 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.

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

    2. Guenter Beutter

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

  2. 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?

  3. 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).

  4. 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.

  5. 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?

  6. Alasdair Rendall

    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?

  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?

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