Proper Fusing Of Three Parallel Panels

shinybeast wrote:
K Mena661, just wish I was born with a solar spoon in my mouth, lol!

LOL! Sorry man, I got a little rough on that one but that wasn't directed at you. There some are on the forum that overdo it on safety measures. Even if those safety measures have no possibility of working like they think it will. LScamper says why perfectly.

K Mena661, just wish I was born with a solar spoon in my mouth, lol!

Thanks for the replies. BFL13, the cooperbussman link was just what I was looking for. If I have it right for my stuff...
(3-1)*7.5=23.4
I'm using #10 wire (30A max) which is over the 23.4A calculated above so I don't need fuses at panels, correct?
I read somewhere that if one panel in a string of three or more in parallel failed somehow, the current from the others could/would start a fire as it overloaded the bad one. Being new to solar I felt compelled to seek out the safest way to setup and use my new gear with my new TT.

Trying to make sense of panel fusing.

A fuse is a current protection device. A fuse at the output of a panel is to protect the wires from the fuse to whatever it is connected to, mainly the controller.

An array is the parallel connection of two or more panels (strings).
A string is the series connection of two or more cells. A panel can be say 30 cells or 90 cells. A string can be one panel of say 90 cells or three panels of 30 cells in series, same exact thing!

Say a panel is rated at 10A short circuit current and the fuses are 15A.

So what happens if there is a short directly at a panel before the fuse?
If there is only one panel the panel output goes to zero and the controller gets no input. The fuse will not blow, no current through it.

If there are two panels in parallel and one shorts directly at a panel the output the voltage goes to zero and the controller gets no input. The short circuit current supplied by the other panel, 10A, now goes through both fuses, the one on the good panel and the one on the shorted panel. Neither fuse blows because they are rated to handle the current from one panel. The wires are safe because they are rated to handle at least the short circuit current from one panel.

If there are three panels in parallel and one shorts directly at a panel the output voltage goes to zero and the controller gets no input. The short circuit current supplied by the other two panels is added, now 20A total, and flows through each of the good panel’s fuses, 10A each, and through the shorted panel fuse. This fuse now gets two times its normal current, 20A, and should blow from over current. When the fuse blows the voltage goes up at the controller and will still charge the battery from the two good panels.

If the wire is rated to handle the current from the two good panels no fuse would be required. This would be the case in most of the installations talked about on the forum; most use much larger wire than needed to keep voltage drop at a minimum.

This brings us to a reason not to use fuses at the panel, voltage drop!

In order for a fuse to blow it has to dissipate some power to heat it to the melting point. This means that the fuse must have resistance. A 7A fuse resistance is about .013 Ohms and a 10A fuse resistance is about .008 Ohms. A 10-gauge wire has a resistance of about .001 Ohm per foot. So adding a 7A fuse is like adding about 13 feet of 10-gauge wire from the panel and a 10A fuse is like adding about 8 feet of 10-gauge wire. Plus there are all the connections that add resistance and could become loose or not a make good connection.

Bottom line is if the wires from each panel can handle the total current from all the panels, most likely they can, there is no reason to use fuses and they introduce voltage drop plus added connections.

BFL13 wrote:
Here is the correct info--wade down to the strings fusing part

http://www1.cooperbussmann.com/pdf/9df1f7ec-8c62-4210-8cf8-9504927394f0.pdf

X2 You only need fuses if you have 3 or more strings connected in parallel (a string is panels in series). So unless you have 6 panels minimum connected in a series/parallel array, you don't need fuses. In fact, I'll even state you're wasting your time and money by fusing panels not configured in this manner. And you're not protecting anything. Why? Because panels are DESIGNED TO BE SHORT CIRCUITED. Only in the series/parallel array mentioned could you exceed not only the wires capability but the panels capability. If you cannot grasp this, perhaps you should rethink solar.

Panel fuse up in the combiner box should be avoided when possible as it adds at least 12 more connections in a difficult place to service.

RJsfishin wrote:
Quote:
Normally fuses are chosen for twice the normally expected full current.
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Where did that info come from ? That would most times exceed the wire rating, and we'd be up in smoke for certain !

Thats quite typical to fuse at twice the operating current to cover in-rush in electronics.

But I ask again: What is the OP trying to fuse against? The panels will run into a short all day long, and 10ga wire (I'm using #8) could care less about a 7-7.5A load, and thats all you're going to get, even on a good day. So what good is the fuse other than a place to disconnect a panel, and why do that?

The fuse, or breaker belongs where Smkettners diagram shows it, where there is serious current from the battery. Not on the panels.


(edit: iPhone spelling)

I would use a 8 amp fuse on EACH panel.

Panel--fuse-|
Panel--Fuse-|--controller-battery & loads
Panel--Fuse-|

This is small enough to cover the wire (30 amp in walls) and to protect the panels (less than 120% ISC)

You can LED across the fuses to indicate a blown fuse or if the system current drops... VOlt meter both ends of the fuse in full sunlight to see which one popped.

Quote:
Normally fuses are chosen for twice the normally expected full current.
------------------------------------------------------------------
Where did that info come from ? That would most times exceed the wire rating, and we'd be up in smoke for certain !

What are you trying to fuse against?