RV system - type and location of combiner box?

vsheetz

On a previous motorhome I installed six 100w panels on the roof - all in parallel wired to a roof mounted combiner / junction box, and from the combiner box ran a large gauge wire down to a Morningstar PWM solar charge controller.

Now have a different motorhome and looking to install a new, and larger, system consisting of sixteen 100w panels and a MidNite Solar Classic 150 controller.  I am thinking of having the panels in 4 strings.

The question:  Whether to do a roof mounted combiner box setup similar to my previous motorhome *or* to run multiple smaller (#10) wires from each string to a combiner box with 4 DC breakers located near the solar charge controller near the battery bank.  The wire run from the roof would be approximately 30-35' long.

Suggestions?  Pros/cons?  Is #10 wire size ok?

thx!
Vince

BB.

Exactly what panels do you want to use? Have you looked at larger >200 Watt Vmp~30 volt panels (used for GT Inverters)--They will work nicely with a your Classic controller.

What size battery bank (voltage and AH rating)?

-Bill

vsheetz

I am considering are Renogy 100D panels.  These are a physically narrow panel, which can work with the limited space I have on the roof of the motorhome.  Yes, larger > 200w panels have been considered - to include racking and lofting them over objects on the roof.

Even if I were using larger > 200w panels, the question of the combiner box type and location remains the same.

thx
Vince

BB.

Where to put the combiner is your choice... The wiring costs will be about the same (6x 10 AWG wires vs 1 x 4 AWG set--Same amount of copper).

The actual wiring, number of circuits, awg, etc. depends on how you series/parallel connect the panels. For a 12 volt battery bank, 2-3 of your 100 Watt (Vmp~18 volts) in series, then combine those in parallel (at the box) will be more efficient for your Classic.

You can run your panels in series up to Vmp-array~100 VDC (check Classic String Controller, your local temperature to be sure) if it makes wiring better for you (classic will run a bit hotter, higher voltages into MPPT controller has more losses).

If you have 6 panels, and they are Vmp~18 volts--Your ideal mix would be 3s x 2p or 2s x 3p. The advantage of 3s x 2p is that you really do not need a combiner box... For three (barely) and more parallel strings, you need the combiner box to reduce the chances of fire if one panel is shorted and fed by the other parallel strings.

Many people still like a switch (on combined strings) to turn off the controller for service. Or you still put a 2x combiner box with breakers in so you can turn off one string and see if it is carrying is share of current (easier/quicker debugging).

To get exact answers, need to know the Vmp/Imp of the panels and how many you will use. Note that >200 Watt panels (Vmp~30 volts) are cheaper (about 1/2 the price) of typical 1xx watt panels. And less wiring too.

But you have to make do with the space and hardware available.

-Bill

vsheetz

Thanks Bill.  The combiner box placement matters if just a junction box or if a breaker box - I don't think one would usually put a breaker box on the roof.  I noted I was planning to use sixteen Renogy 100D panels.  These are Vmp~18 volts, 100w physically narrow panel - for space available reasons.  Config will be four strings.  Your comment about having a disconnect per string is relevant - this would be a good thing. With my previous system with all panels wired in parallel the disconnect was all or nothing - leaving no way to know if one 'string' was under performing. 


Thanks
Vince

BB.

Sorry--16 gives you the option of 2s x 8p; 4s x 4p (8s x 2p is too high of voltage for a Classic 150)... Either will work fine. 4x4 will be simpler wiring (fewer breakers) and allow you to use smaller awg wiring.

The Classic does have fans which can be noisy... Need to place near battery bank (short/heavy battery leads to low voltage drop) and good airflow. Having it near your work space (for example) in the RV may be distracting from the fans running during the day.

That is a good size array for a 12 volt battery bank(?). What AH rating will it be?

-Bill

vsheetz

Battery bank is currently six GC2 6 volt batteries - looking to upwards of doubling this capacity in the future.  I have a good feel for our usage from the previous motorhome and PV system that was there.

The 4x4 configuration I agree, will provide for wiring ease and economy allowing #10 runs from the roof to the combiner box / breaker box, and fit within the capacity parameters of the classic 150.  As well as some shade mitigation.  The controller and combiner box with breakers will be located adjacent to the battery bank - with 2/0 as the wiring there.

Good to note about the fan noise of the classic 150 - it will be located below in a storage bay where I don't anticipate this to be a problem.

Thanks for the help,
Vince

BB.

You might want to look at larger (AH) batteries... I am not a big fan of lots of parallel strings of batteries--1 string of batteries is best (in my humble opinion), 2-3 parallel strings can work OK. More than that, I would suggest avoiding.

Trojan L16RE-2V 1110 AH 2-Volt Deep Cycle Battery $353.00 each

Trojan, as I understand, actually has three cells (three caps) in parallel--Not ideal (I would prefer one cell)--But to give you an idea. 6 of these batteries in series would be your bank (and like 5x parallel 6 volt GT batteries. (note: weight of lead will be about the same--no matter the configuration--I.e. a 12 volt @ 1,200 AH battery weighs about same if a bunch of small GC batteries or single string of large AH batteries).

The next question I would ask you--At that size of battery bank, I would be suggesting a 24 volt (or 48 volt) battery bank. Fewer parallel battery strings and smaller awg wiring (24 volts is 1/2 the current; 48 volts is 1/4 the current of a 12 volt battery bank). More or less, I would suggest any battery bank >800 AH should look at the next higher voltage battery bus (12 volts @ 800 AH to 24 volts @ 400 AH--as an example).

And with that much weight--Perhaps look at LiFePO4 (version of Li Ion chemistry). Smaller and lighter weight.

-Bill

scrubjaysnest

The down side to a 24 or 48 volt battery bank in an RV is everything that uses DC is 12 volt; water heater, furnace, lights etc.