Solar System Fuse Locations

[rs14smith]

Hi all,

I need to buy some fuses for my Renogy solar panel system, and I am trying to figure out which of the following diagrams do I need to follow:

The first one is the one Renogy recommends if you have 4 solar panels hooked in parrellel which is what I have (4 100Watt solar panels):
91a4IFvX6eL._SL1500_.jpg


The next one is from a website called Windy Nation: https://www.windynation.com/jzv/inf/...olar-pv-system
fusing_figure1wn.jpg

The main difference I see from the start is that Windy Nation recommends using a fuse after "each" solar panel, but Renogy is only using 1 fuse between the solar panels and charge controller.

Which way is right?

[inetdog]

Naturally, both are right under different circumstances.

When paralleling single panels rather than strings of panels the chances of damage from some of the panels backfeeding another defective panel are reduced. And a particular manufacturer may have evidence from testing that the worst case scenario would not cause a fire.
But the NEC has more strict rules.
Each panel will be rated by the manufacturer with a "maximum series fuse" rating. For a 50 watt panel (say ~2A at ~25V), that rating might be as high as 10A.
In a case like this, four panels could in theory be protected by a single 10A fuse after they are joined together. Section 690.9 allows specifically for this sort of condition, but you have to take into account both the fuse protected backfeed from a defective charge controller and the parallel contribution of the other panels. In the specific example I gave, this will not be easy to meet. (Current from three panels = 6A. so you would need a 4A single fuse. But that would not handle the normal 8A current. <sad>

The general rule is that for two parallel panels or strings an appropriately sized single fuse is acceptable, but for three or more individual fuses are required.

When higher power/voltage panels are used or panels are first series connected to form multiple strings it becomes more important for safety to fuse the strings individually, but the applicable rules are still not changed.

In many cases (multi-panel strings in particular) the advantages of using a DC rated circuit breaker, including the ability to use the breaker as a single string or panel disconnect, lead people to use them instead of fuses.

[rs14smith]

Naturally, both are right under different circumstances.

When paralleling single panels rather than strings of panels the chances of damage from some of the panels backfeeding another defective panel are reduced. And a particular manufacturer may have evidence from testing that the worst case scenario would not cause a fire.
But the NEC has more strict rules.
Each panel will be rated by the manufacturer with a "maximum series fuse" rating. For a 50 watt panel (say ~2A at ~25V), that rating might be as high as 10A.
In a case like this, four panels could in theory be protected by a single 10A fuse after they are joined together. Section 690.9 allows specifically for this sort of condition, but you have to take into account both the fuse protected backfeed from a defective charge controller and the parallel contribution of the other panels. In the specific example I gave, this will not be easy to meet. (Current from three panels = 6A. so you would need a 4A single fuse. But that would not handle the normal 8A current. <sad>

The general rule is that for two parallel panels or strings an appropriately sized single fuse is acceptable, but for three or more individual fuses are required.

When higher power/voltage panels are used or panels are first series connected to form multiple strings it becomes more important for safety to fuse the strings individually, but the applicable rules are still not changed.

In many cases (multi-panel strings in particular) the advantages of using a DC rated circuit breaker, including the ability to use the breaker as a single string or panel disconnect, lead people to use them instead of fuses.

From what I'm gathering, it appears that WindyNation put fuses in line with each solar panels to protect the "wires" from catching fire if one of the solar panels were short-circuited. However, they were using solar panels that if 1 did get short circuited, it could possibly draw 60 Amps. Well, my 100Watt solar panels are rated for only 5.75 (Isc), so that times 4 equals 23Amps. I'm using 10AWG wire, so even if 1 of my solar panels did short out, my wires should never have an issue handling the current the 1 panel might draw?

I'm just trying to confirm exactly what the fuses are being used to protect, and if they are even needed.

[Sunking]

From what I'm gathering, it appears that WindyNation put fuses in line with each solar panels to protect the "wires" from catching fire if one of the solar panels were short-circuited.

But that is impossible for a single panel or single string of panels. A solar panel is a current source, not a voltage source. If you look at a panel spec there are two specs called Imp (current maximum power) and Isc (current short circuit) Isc is just what it sounds like and is one test you can run to see if the panel meets spec or not. Example a typical 100 watt 36 cell battery panel has an Isc or 6.5 amps and a output wire of #14 AWG wire which can run 15 amps all day and night long. You would use a 10 amp fuse and can short that panel out all day and it could not even blow the fuse if it had to.

NEC does not even require fusses in the panel circuits until you get 3 or more parallel strings. It is where you have parallel strings where you run the risk of too much current in any one string if one string shorts out.

[Sunking]

Well, my 100Watt solar panels are rated for only 5.75 (Isc), so that times 4 equals 23Amps.

Hold the bus here. How are the panels wired up? Are they in Parallel or Series?

I hope they are wired in Series and you are using a MPPT controller. If wired in Series Isc is still 5.75 amps and no fusses or combiners are required. Only if you have them wired in parallel will there be 23 amps.

If you do have a PWM controller you kind of screwed yourself a few times.

1. It forced you to wire the panels in Parallel
2. It forced you to use combiners and fusses.
3. It forces you to use much larger wire.
4. It turns your 400 watt of panels into 220 watts.

[rs14smith]

Hold the bus here. How are the panels wired up? Are they in Parallel or Series?

I hope they are wired in Series and you are using a MPPT controller. If wired in Series Isc is still 5.75 amps and no fusses or combiners are required. Only if you have them wired in parallel will there be 23 amps.

If you do have a PWM controller you kind of screwed yourself a few times.

1. It forced you to wire the panels in Parallel
2. It forced you to use combiners and fusses.
3. It forces you to use much larger wire.
4. It turns your 400 watt of panels into 220 watts.

The panels are connected in Parellel:

None of the panels are hooked up in series. So I have 4x 100Watt panels, hooked up in parrellel going to a charge controller. And I do have a 30Amp PWM controller. My MC4 cables are rated for 30Amps, and so is the charge controller. So I do not understand how I would only be producing 220watts like you mentioned above, nor why fuses would be needed at the end of each solar panel. It makes sense to use say a 25Amp fuse between the solar panels and charge controller though.

What am I not considering here? I'm a bit confused.

[inetdog]

The panels are connected in Parellel:

None of the panels are hooked up in series. So I have 4x 100Watt panels, hooked up in parrellel going to a charge controller. And I do have a 30Amp PWM controller. My MC4 cables are rated for 30Amps, and so is the charge controller. So I do not understand how I would only be producing 220watts like you mentioned above, nor why fuses would be needed at the end of each solar panel. It makes sense to use say a 25Amp fuse between the solar panels and charge controller though.

What am I not considering here? I'm a bit confused.

The panels are going to have a Vmp of around 18V if they are "battery" panels. They will have a Vmp of 24v or more if they are "grid" panels.

When you use a PWM controller, the current out of the CC is identical to the current in, and the panel voltage will be a volt or so above the battery voltage at the time.

If you take a panel which produces 100 Watts into a load which sets the voltage and current to Imp and Vmp, and instead connect it to a load which pulls the voltage down to 12V, then the current will not rise much and the power will be 100 x (12/18).

An MPPT CC will convert the power in number to the same power out number at a different voltage and current. In the example above the output voltage will be 12/18 of
the panel voltage but the current will be 18/12 of the panel voltage.

[Sunking]

When you have more than two panels or strings in parallel you are required to have a fuse for each panel or string, plus the feeder cable for a total of five fuses. You need a combiner with 5 ports.

With PWM controllers Input Current = Output Current. Look at your panel Imp should be about 5 amps. With 4 in parallel is 20 amps. That means 20 amps on the output of the controller. Power = Voltage x Current. 12 volts x 20 amps = 240 watts.

MPPT output Current = Panel Wattage / Battery Voltage. 400 watts / 12 volts = 33.33 amps or 400 watts output. In addition with MPPT providing you buy one with a high enough Voc input you could wire the panels in series and not have to use a combiner or any fuses, and could use a much smaller less expensive cable.

What you did to yourself is bought a inexpensive PWM controller for around $100, which eliminated 33% of you panel power and requiring you to buy a lot of hardware for the fuses. You woul dhave saved a lot of money by buying the $300 MPPT controller. As it stands right now you have to buy two more 100 watt panels to equal a 400 watt MPPT system. What you have now with a inexpensive PWM controller is a 240 watt system and a lot of hardware you do not have yet and need to purchase.