Adding to current solar system

phaeton10 said:

Outback Flexmax 80.  I am a newbee and when I bought the panels and controller he sales guy at Northern Arizona  told me the #6 cabling along as going in series would be sufficient for the 6 panels.

I'm not sure what you are trying to say...

phaeton10 said:

I am being told by others the problem is with the cable size #6  from the controller to the battery bank being 18 feet is not heavy enough, that it should be #1 or #2 AWG

I am not sure we are speaking the same language... which cable is 18 ft?  If it is from the controller to the battery, then yes, it needs to be heavier.  And how you configure the panels (series,parallel) is irrelevant. 

If you are referring to the cable from the combiner to the controller, then it does matter how you have the panels configured.

You have cables from the panels to the combiner.  These are probably 10 gauge and are part of the panel.  These cables run to a combiner.  From the combiner there is a cable that carries the combined current to the controller.  What is the length of the combiner-controller cable?

A group of panels when wired in series is called a string.  The voltage of the string is the sum of the individual panel voltages.  A string may consist of just one panel.

The strings of panels are combined in the combiner box.  That means the strings are parallel to each other.  Each string should have a circuit breaker in the combiner box.  The circuit breakers are desirable with any number of strings, and are required if there are more than 2 strings.

In your system, with 6 panels, there are several ways to configure the panels:

string length = 1  This means your panels are all in parallel.  You have 6 strings and 6 circuit breakers in the combiner.  The Vmp of the strings is 17.7 volts.

string length = 2  This means you have 3 strings and 3 circuit breakers in the combiner, each string being 2 panels in series.  The Vmp of the strings is 35.4 volts.

string length = 3  This means you have 2 strings and 2 circuit breakers in the combiner, each string being 3 panels in series.  The Vmp of the strings is 53.1 volts.

string length = 6  This means you have 1 string and 1 circuit breaker.  The Vmp of the string is 106.2 volts.

There are advantages and disadvantages to each of these configurations.  Shading is just one of many considerations.  Since you do have shading issues you should try to have as short a string length as possible.  This is because shading of one panel will reduce the output of the entire string, not just the shaded panel.  With a string length of 1, you have 6 parallel panels.  If one gets knocked out by shade, the other five panels can still produce.  If you have (for example) 3 strings, and 1 panel is shaded, you lose the output of 2 panels (the shaded panel and the unshaded panel in the same string).

On the other hand, there may be some downsides to a string length of 1.  In particular, there is the issue of voltage drop.

Tell us more about the distances involved between the panels and the combiner, the combiner and the controller, and the controller and the battery.

--vtMaps

phaeton10 said:

From my panel to the combiner box is 5 feet (combiner box mounted on the roof)

18 feet (#6) from combiner box to controller.

18 feet (#6) from a Flowmax 80 to batter bank.

OK, now we have some numbers to work with.   The first problem is the distance between the controller and the battery.  With 825 watts of panel, a 12 volt battery, and 18 ft (one way) of #6 copper, you are looking at almost 4% voltage drop.  That's too much.   The two most common solutions for this problem are to have the controller very close to the battery, and to use a higher battery voltage.   The other solution is to use heavier cable.  If you use #2 cable you can have a voltage drop of 1.5%.  If you use #0 cable you will have a 0.9% voltage drop.

Now, onto the second problem.  This is the cable between the combiner and the controller.  Here you have some choices.  Your controller is an MPPT type controller which means that it can efficiently step down higher voltages to battery voltage.  The higher your string voltage, the thinner your combiner-controller cable can be.  For example, if you configure your array as 3 strings, with a string length of 2, then the voltage drop with #6 cable is less than 1%.

As I mentioned previously, there are other considerations in your choice of string length.  The shading problem is less of a problem with a shorter string length, but with a string length of 1, you need some very heavy cable.

--vtMaps

phaeton10 said:

One last question,  what size fuses should I use for this setup?

Your panels have a 'series fuse rating'.  Probably about 15 amps.  That is the size to use in the combiner box for each string of panels.

The combiner-controller cable should have a circuit breaker at the controller end which serves as a PV disconnect switch.  Since that is #6 cable, you can use up to a 60 amp breaker, although your PV current will be only about 16 amps.

One of the most important fuses (or circuit breakers) is between the battery and the controller.  If you are using #0 cable, you could use up to a 150 amp breaker, but a 100 amp breaker would be safer. 

--vtMaps