Advice for use of combiner box or No....

Textrout

Backstory: I am planning to run 600 watts of solar from four 150 watt 12v panels on top of my RV.  I only have one 10 ga wire to get this power to the charge controller, as was prewired when I bought it.  I'm going to run the 4 panels down to the charge controller by way of 2 panel pairs connected in series (creating 24 v system), and then those two pair joined in parrellel so I can send 24 volts to the Charge Controller without over Amping the wire....  This makes a total max load of 17.64 amps 24 volts thru the 10 ga.wire, and it should handle this fine, from everything I've seen.    
This brings me to my question:  Should I use my combiner box as all, since my single wire from the roof has more amps (17.64 max) than any one input can handle as my current combiner box only has inputs to handle 12 amps each (I think I could go as high as 15 amps with right fuses, but if I could use 20 amp fuses, I suppose that would be ideal).  My combiner box is an Eco-worthy 6 pair box.
SOOOOO, Would it work if I used Y splitter to reduce the amps coming in by using two inputs (each with a 12 amp fuse, or 24 amps with the pair?  to effectively get around the single combiner box fuse limit of 12 amps?   Or do I just forget the combiner box, which offers additional protection, and go with inline breaker/disconnect combo (40 amp?) to the charge controller?

BB.

You do not need to use a combiner box or fuse/breaker per string here (generally). Check the panel specifications for series fuse rating (if Isc short circuit current from the array is less than the series fuse rating, you are OK).

With solar (and really any electrical project), details matter.

How long is the 10 AWG wire run (looking for voltage drop--Too much can kill solar production).

What solar charge controller (PWM controllers need around Vmp-array ~ 35-40 volts from your panels)--The solar panels themselves should be Vmp (standard test conditions) of ~17.5-20 volts... HOWEVER, there are some panels that are not "12 volt" panels.

And what type of solar charge controller would you be using. A PWM (pulse width modulated) solar charge controller is typical for smaller systems and would use Vmp-array ~36 volts.

There are MPPT (Maximum point point tracking) charge controllers that are nice for larger systems... And ideally you would need a Vmp-array >~45 volts or so for optimum operation (and need to know Vpanel-max input voltage, will you be in subfreezing climates--Solar panel Vmp/Voc rise as temperatures fall).

Some of the questions that need to be answered before you get too deep into the system.

-Bill

Photowhit

I think Bill's reply is spot on...
A friendly reminder to use DC rated Breakers for your installation.

Textrout

Thanks for the help.... and critical information!  I'm going to use a MTTP 40 amp rated controller (12/24V) (EPEVER Triron4210N model).  *
The run is only about 12 to 14 feet max.  Our Texas and occasionally Colorado camping temps are virtually always above 0 degrees F.
I will have a look at Vmp/Voc is -(80+/-10)mV/*C data on the panels. Other panel specs are  "Maximum power: 150W, Voltage at Pmax: 17.0V, Current at Pmax: 8.82A".  Max open V is 20.23v, and short circuit current is 9.36 amps,   Apparently the panels have diodes in them, but do you ever advise adding them (as well as a cutoff switch) between panels and controller.  IF I install a breaker/disconnect from the panels, what size would you recommend?  Something like the This?

Bussmann 187 Series Marine Rated 30 Amp Circuit Breaker with Manual Switch listed on this site.

mike95490

If you are running a MPPT controller, be advised the 40A rating is for the output.    The reason to use the MPPT controller is to be able to run the panels in SERIES.    What is the max input input voltage for your controller.   I would guess the 4 panels in series would be about 100V open circuit - what's the VOC spec on the panels

Textrout

The controller is only a 12/24V controller, with max input of 100V.  I am hoping I'm within those parameters, as VOC is 20.23v * 4 panels = 80.92 V, (if this is the correct way to add the voltage).   Wait...the 12/24V designation is for the battery bank?... not input voltage, correct?  I was thinking I was limited to a 24v input into the controller, thus my arrangement.  So, I could put my array in series (17V*4=68v with 80.92 max open volts) at 8.82 amps and send it to the controller?  That would be easier to me... would the 10 ga wire at 12 ft run still work for that?  (seems like "yes".)  

mike95490

4 panels in series is correct,  However, the PV voltage goes UP when it gets colder, so now the coldest record temp needs to be factored in, Many charge controller parent sites have caculators, or you could use any calculator and input your panel Voc
http://www.midnitesolar.com/sizingTool_kid/index.php

BB.

Yes Textrout, you have the series/parallel for solar panels (batteries, etc.) correct...


Using Mike's link for a nice tool to calcuate solar array min/max temperature vs voltages:

Voc~20.23, 4x in series.
Vmp~17.4 volts
Imp~8.62 Amps
Isc~10.78 amps (guess)
0F minimum temperature
104F max
4s x 1p array
defaults for rest of questions

PV Array
Rated PV Array Power:	600	Watts
Anticipated Array Power @ 104F:	560	Watts
Rated PV Array Current:	8.62	Amps
Battery Charging Current @ 28.8 V:	20.8	Amps
VMP (Maximum Power Point Voltage) :	69.6	Volts
VOC (Open Circuit Voltage):	80.92	Volts
VMP @ 0 F°:	83.2	Volts
VOC @ 0 F°:	92.4	Volts (under 100 VDC for your controller)

They don't list Vmp-array-hot... But worst case you can assume 0.8x Vmp-array:

17.4 volts Vmp * 4 series panels * 0.8 hot panel derating = 55.68 volts

For a 24 volt battery bank that charges near 30 volts.

The minimum Vmp-array should be 1.3 x charging voltage for an MPPT input

30 volts charging * 1.3 MPPT fudge factor = 39 volt Vmp-standard-temperature

So, everything looks good.

-Bill

Textrout

Wow, you guys are great!  Thanks for the information/education.... and patience.   There's a lot to learn but I enjoy it.   And Thanks Bill for the break down and reasurance!
I'll get to work now!  

BB.

Your are very welcome Textrout.

We always suggest that you do the calculations yourself too... I know I am not perfect have been known to make some 'interetsting" typos.

Have fun and let us know how everything works out.

-Bill