Fuses needed on parallel solar panel connections?

[Incredulocious]

Aside from this question about fuses, I've completed adding three more 100W panels (from GreeSonic) to the original two 100W GoPower ones that came with my 2016 Unity TB. I've provided pictures and more details of my solar installation on my web page, RV Tips, Upgrades and Customizations:

    

Many thanks to TurboPilot, Klipstr and other contributors to these helpful threads: GP Flex 200 System Additional Panels (January 2016) and Installation of new solar panels (August 2016). And in particular thanks for the heads up about taking care of that 15 amp fuse between the battery and the charge controller.

One question I still need to resolve is whether I need to add in-line fuses for each panel at its parallel connection. I see a number of folk saying that it’s necessary when wiring more than two panels in parallel to protect from short circuits, others saying it’s not necessary and still others not mentioning it as part of their wiring instructions (as in the above two threads):

Here are two examples:

WindyNation: “When you have panels connected in parallel […] a short in one panel can draw all 60 amps towards that short-circuited panel. This will cause the wires leading to that panel to far exceed 30 amps causing that wire-pair to potentially catch fire. In the case of panels in parallel, a 30-amp fuse is required for each panel.”

AMSolar: “You aren’t going to get a surge from solar panels (unless maybe there was a very unusual solar flare, in which case you have bigger issues to worry about.) Because of that, our kits do not use fuses between the panels and the charge controller.”​

 

[Klipstr]

OK, everything I wrote below this line was written before I looked at your blog. Wow! Nice job. You should probably start another new thread and point folks to it. There is a ton of good stuff. I'm pretty type A but you covered me dude! I also see that you are using the breaker I recommend below on your install but maybe others will appreciate the link.

I don't think it's necessary to add a fuse on each panel. In the installs I did using the air conditioning disconnect it was natural to fuse both the charge controller input and output as there are two fuses in the disconnect and when you pull the plug it disconnects the charge controller from both. If you short the panels themselves that's not a problem. They don't generate any short circuit current. I don't think Windy Nation's description is accurate. The problem comes from the other direction: the batteries. But as the charge controller is in between you don't have a problem there either. On my current installs I'm putting a breaker like this on both the aggregated input and charge controller output:

http://www.ebay.com/itm/40-Amp-In-L...12V-24V-32V-/251742947572?hash=item3a9d0c88f4

Size it based on your configuration. They come in multiple sizes. I like these better than the A/C disconnect as they are smaller.

 

[icarus]

The basic rule, for parallel panels is if you have three or more panels in a string, then each panel should be fused. The idea being that any two panels can produce enough current that a short in the third (or more) could cause a problem. Most modern panels come with MC cables, and MC cable fuses and fuse holders are readily available and cheap. Older "J" box panels can simply have a fuse installed in the J box.

Personally, i probably wouldnt worry about it, but it is cheap insurance. The bottom line is, the more fuses the safer the installation, but the more problematic if there is a problem...to locate. I protect all the wire from all current sources, both ends of PV, simply because it is easy to do, and, for example battery current back feeding into PV could be a problem, or excess PV current into a badly wired battery bank could also be a problem, hence a fuse at each end.

Series wiring does not require a fuse.

Icarus

 

[Incredulocious]

Thanks, Klipstr. And thanks for your take on the need for panel fuses. Regarding the protection between the battery and charge controller, I'm actually using an in-line fuse holder that just looks a lot like that in-line breaker, but it's not a reset-able circuit breaker. Also note that your fuse on the combined input to the charge controller doesn't address what some describe as a need for fuses on each panel wired in parallel.

And thanks, Icarus. That is the rule (three or more panels in parallel) that I've seen explained occasionally but I'm not understanding how it is a short-circuit could develop in one of the panels. And of course I thought it was interesting that AM Solar, that is so highly touted, dismissed the need for fuses at the panel connections.

You also mention "excess PV current into a badly wired battery bank" but not knowing what that is, I guess it's about current going to the battery and not helped by having fuse protection on each panel? And of course there is the fuse between the charge controller and the battery. I haven't heard of "battery current back-feeding into the PV" either, but I don't know enough to know how that can happen or what would happen to the charge controller.

I look forward to further explanations from all...

 

[ablock]

I've never seen the parallelled PV fusing requirement before. Where would the fuse go? On the + leg of each panel connection, using an inline fuse holder like this one?

I'm series-parallelling my panels in pairs to bring up the voltage so I can reuse the existing 10/8 AWG wiring. That makes the current from a panel pair 200W/37V = 5.4A. Seems like a 10A fuse would be sufficient.

In addition, I am planning on protecting the wiring from the panel junction box to the MPPT controller (which I moved to the battery compartment) with a single fuse in the j-box. I also have a 30A Blue Sea breaker/disconnect on the PV side of the MPPT and a 60A version on the battery side.

 

[calbiker]

I wouldn't put major effort into installing in-line fuses. This issue only arises if one of the panels shorts. That's a fairly unlikely event.

Here's what happens if one panel shorts. The other two panels produce about 5A or 10A total that will now go through the shorted panel. The concern is that the 10A is too much for the cable. The cable will get hot and possibly start a fire.

That really isn't the case in your situation. You'll use at least 10 awg cable connecting the panels. 10 awg cable is good for 55A. 10A will not get near melting insulation or starting a fire.

Add the fuses if it helps you sleep at night.

 

[Klipstr]

My understanding and experience is if you short the leads of a panel there will be no current.

 

[calbiker]

Not correct. Solar panels are current sources. Short the panel and you will get Isc (short circuit current). For a 100W panel, that could be 4 - 5A.

My understanding and experience is if you short the leads of a panel there will be no current.