Looking for a switch to go between my combiner box and the charge controller

jmiller968 · May 2020

Ran into a bit of a problem this weekend and am looking for the best fix.

My panels (4x 100Watt Renogy) are about 150' from my charge controller in my off-grid system. I have them in series going to my Tristar MPPT 45 controller. When I put the system in 5 years ago, I put an AC switch in the line coming from my combiner box so that I could disconnect the solar panels from inside the cabin (for startup, troubleshooting, etc). This weekend I added 2 more panels for a total of 6. This is where the fun starts...

I was doing some troubleshooting with the charge controller...possibly related...and i turned off the AC switch in order to reset the charge controller. The switch started to glow, which I'm guessing is bad? I shut everything down, pulled the disconnect out of the combiner box, and am now home looking for a better option.

I did some looking and discovered that the AC switch was a bad idea. Not sure if it failed today because I added the extra panels, or if it was just its time. But I need to replace it, and want it to not fail. If I'm doing the math correctly, 6 panels in series will be about 130 volts and 6 amps. Is there a simple switch out there that will work for this? All the DC switches I'm finding seem to be 12v/50 or 100 amp.

Thanks!

mike95490 · May 2020

How are your panels wired ? all 6 in series ? 3S2P ? all in parallel ?

If you have more than 2 strings or panels in parallel, you really should be using a PV Combiner box. With a 7A DC breaker ($15 ea) on each string, it prevents a bad panel from burning up from the power of the parallel good panels. And then the breakers give you a very handy A-B-C compairson between your 2 or 3 strings of PV for troubleshooting, And of course, they can all be turned off. Safely, since the Midnight breakers sold by the webhost, are DC rated to 150V
https://www.solar-electric.com/mnpv6.html
https://www.solar-electric.com/midnite-solar-mnepv-7-amps-circuit-breaker.html
The right size breaker will be listed on the sticker on the back of the PV panel, Max Series Fuse/breaker __ Amps
so that would be the proper size to get.

Additionally, any MPPT controller should have battery power present before PV array is connected to it, so you could use spare slots in the combiner panel to wire that breaker (50A ballpark ? ) Here's how I did mine:

Image: https://us.v-cdn.net/6024911/uploads/editor/0r/v9qorxiw7e6k.jpg

jmiller968 · May 2020

All 6 panels are in series. they then go into a junction box with a serviceman disconnect similar to this:
https://www.homedepot.com/p/GE-30-Amp-120-240-Volt-240-Watt-Fused-AC-Disconnect-TF30RCP/100576894

From there I have 12 gauge wiring running to the cabin where my charge controller is. I did this to keep the amps low so I wouldn't go broke buying 150' of heavy gauge wire. that circuit breaker you showed would certainly work...but I don't really need the combiner box, do I? Is there a small box that that breaker could go in.? I don't really want to have another large box on the wall. Thanks for the input!

mcgivor · May 2020

When dealing with DC, all overcurrent protection devices, or disconnects should be rated for DC, the use of AC equipment will result in arcing of contacts in the case of a switch, particularly when opened under load, or jumping the air gap in the case of a fuse.

The current rating of an AC fuse will generally be derated to 50% if permitted for use with DC, however it's best to choose the correct equipment if in doubt. With DC breakers it should be noted that some are polarity sensitive, being marked with a + symbol on one terminal, it is actually more accurate to describe it as a potential sensitive device, the + marked terminal should be connected to the side with the greatest energy potential, usually the battery in the case of solar.

So with regards to the 30A disconnect, it needs to be replaced because although it may work in the closed position, it has the potential to fail when opened under load, much like the AC switch failure you described.

jmiller968 · May 2020

Ok, so does this make sense...buy the midnight solar combiner box along with the 7 amp DC circuit breaker. Install it down at the solar array in place of the AC disconnect that I currently have. Then buy another 7 amp circuit DC circuit breaker and install it in the cabin before the charge controller?

mike95490 · June 2020

Mostly, yes. Please verify what size breaker you need, 7A is my guess. The sticker on the panel is right.
You need some sort of box to hold the wires and breaker, and there are 2 styles of breakers, DIN mount and panel mount

jmiller968 · June 2020

Just as an update. I went ahead and ordered 2 of the breakers Mike recommended from the mothership. The panels said 15 amps, so that is what i went with. I put the one on the beach by the panels in a midnight solar combiner box, and used a baby box in the camp before the charge controller. Wired them both to the positive wire. Everything seems to be working correctly, and I'm sleeping better at night now! Thanks for the help everyone!

Looking for a switch to go between my combiner box and the charge controller

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