Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connector

Hi all!

We are installing three 270 watt 24v panels on the roof of our RV. We know branch connectors are approved for 2 panels in a parallel, but can you use branch connectors to run the 3 panels in parallel? We have a downstream DC breaker before the Solar Charge Controller. Here are the specs of the panel.

Voltage Maximum Power 31.5
Current Maximum Power 8.58
Voltage Open Circuit 38.5
Current Short Circuit 9.17

We know we could buy one of the pre-wired Midnite Solar boxes and combine the solar arrays there with fuses on each and then run to the DC breaker and then charge controller. But wiring with branch connectors would be a lot simpler and cleaner for our install. Using something like this:

http://www.solar-electric.com/mumc4colam.html

What do we risk by using the Branch Connector vs. the combiner box? Is this dangerous? I have even found 3 input 1 output branch connectors, but i wanted to get some other opinions.

Thanks!

Comments

  • PhotowhitPhotowhit Solar Expert Posts: 5,905 ✭✭✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    You can, and there is a small risk. Some can even legally be connected 3 in parallel and meet NEC code (I can't recall the situation). There are also some inline fusing options available. Search 'MC4 inline fuse', when I first heard of these I thought they would be good in a situation like this.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    There are two reasons why you shouldn't use branch connectors for three panels in parallel.

    1). If one panel shorts it will become a conductor for the combined current of the other two. in this case 9.17 Amps * 2: 18.34 Amps (excluding any increase from enhanced insolation). The maximum series fuse rating for the panel is probably 15, meaning it is not meant to have more than 15 Amps through it as a conductor.

    2). As you combine panels current goes up. That means the connectors used on the last panel inclusion will be subject to Isc * 3 or 27.51 Amps. Chances are these connectors are not designed for 30 Amps of current. This could lead to premature failure of the connector. Bother scenarios could lead to fire.

    The same goes for the down-lead from the array to controller: it would need to be at least 10 AWG wire to handle the current.

    BTW given the Vmp of the panels I hope you are connecting them to an MPPT type charge controller on a 12 Volt system. If not you will either be wasting power (loss from V difference between panels and system) or not fully charging a 24 Volt system (V drop over wiring resulting in less than sufficient Voltage to properly charge batteries).
  • KiomonKiomon Registered Users Posts: 13
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    Thanks guys! this is what we were looking to know. the connector we found is rated at 30 AMPs continuous, but the shorting issue is the problem. We did find some inline fuse holders for MC4 connectors so that could tackle that. And yes we were using this in a 12v system wired to a Tristar MPPT 60 AMP controller. So if we fused each of the connectors to the branch connector AND ensured the branch connector is rated for 30 AMPS or more, then this is safe?

    Alternatively, does anyone know of a roof mounted junction box with MC4 prewired connectors?
  • unyalliunyalli Solar Expert Posts: 121 ✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    Why not wire in series?

    -Jeff
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto
    unyalli wrote: »
    Why not wire in series?

    -Jeff

    Two reasons:

    Voc 38.5 * 3 = 115.5 Volts. Add some Voc increase for cold temp and you could exceed the controller's input maximum (depending on how cold it gets there).

    Vmp 31.5 * 3 = 94.5 Volts. Down-convert to a 12 Volt system and lose some power to heat in the charge controller (approximately 7%). Not good for power or controller.
  • unyalliunyalli Solar Expert Posts: 121 ✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    Coot, I respect your opinion and knowledge. I've learned a lot from your posts.
    Two reasons:

    Voc 38.5 * 3 = 115.5 Volts. Add some Voc increase for cold temp and you could exceed the controller's input maximum (depending on how cold it gets there).
    Really? Cold temps could send this array over 150 volts?
    Vmp 31.5 * 3 = 94.5 Volts. Down-convert to a 12 Volt system and lose some power to heat in the charge controller (approximately 7%). Not good for power or controller.
    Now I'm very interested. The Morningstar Tristar and Midnite Classic MPPT controllers are rated for 150 volts input. There is endless documentation stating the benefits of MPPT down conversion and I've not read one thing discouraging it. Are these manufacturers lying to us?

    -Jeff
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto
    unyalli wrote: »
    Coot, I respect your opinion and knowledge. I've learned a lot from your posts.

    Well I'm glad somebody does! :D

    Really? Cold temps could send this array over 150 volts?

    Yes. Depending on where you are you could get cold pushing the Voltage that high. Now if you're down there in sunny Mexico this is probably not going to happen. In Canada on the other hand ...

    Now I'm very interested. The Morningstar Tristar and Midnite Classic MPPT controllers are rated for 150 volts input. There is endless documentation stating the benefits of MPPT down conversion and I've not read one thing discouraging it. Are these manufacturers lying to us?

    -Jeff

    It isn't anyone lying. It's just a matter of physics; put more heat to components and they won't last as long. This isn't a "it will hold for exactly 20 minutes" scenario. It's more like more time at higher temperature inevitably leads to shorter lifespan of components. Will it work? Yes. Is it better not to do this? Yes. Anyone who tries to convince you otherwise, well they're lying; no one's stuff is built out of magic parts that can take top end heat indefinitely without any chance of failure.

    Besides that heat is power you are losing.
  • unyalliunyalli Solar Expert Posts: 121 ✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    Coot you make me want to revisit Bluesky and limited down conversion.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto
    unyalli wrote: »
    Coot you make me want to revisit Bluesky and limited down conversion.

    They have some quirks too. :p

    Even though the big controllers can use up to 150 Volts (or 200/250 for the two HV Classics) it's still best to keep the array Vmp down around 2X system normal for best performance.

    Unless boB says otherwise, and I haven't heard him say it.
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: Three 270 Watt Solar Panels on RV Roof, Must we use Combiner Box or Branch Connecto

    1. Series will create more shade related issues
    1a. with series higher controller losses = more heat inside RV
    1b. with series lose one panel to mechanical damage, branch, stone etc, whole string shuts down
    2. Combiner allows you to isolate panels for testing or redundacy
    3. Breakers are good, resetable without fuss or spares

    Why make things diffcult? The industry standard method is a midnite combiner box, fitted with CBi breakers. On an motorhome id just run all three wires inside, direct to combiner.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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