3 groups of 4 panel each for a total of twelve 205 watt panels

I interpret that as 3 groups consisting of 4 series connected modules. Each of the three groups of 4 then connected in parallel. Here's how I calculate the numbers....

The Voc of each module is 22.8 volts x 4 = 91.2 volts for the series string

Apply NEC and cold weather safety factor; 91.2 x 1.25 x 1.25 = 142 volts

That's within the absolute max of 150 VDC for the CC. So that's okay, though close.

Ioc (Isc?) of 12.1 amps per module and series string x 3 = 36.3 amps

Apply the irradiance safety factor 1.25 x 36.3 = 45 amps

That is within the 48 amps the manual suggests. So that's okay, though again close.

And then taking the 142 volts max. x 45 amps max. = 6390 watts which is within their figure of 6720 watts.

So it would appear the numbers are getting close to maximum with all the safety factors applied.

It's too bad the Xantrex Sizing Tool does not include the SunElec modules in its available selections, and they don't seem to allow manual entry for non listed modules.

http://www.se-renbu.com/support/xwsizing/Default_SE.aspxOutback's String Sizing Tool does permit manual entry

http://outbackpower.com/resources/string_sizing_tool/I plugged your values into the Outback Tool as it does permit manual entries.

That tool also says 4 series x 3 parallel is okay. (I used the FM60 which has the same maximum voltage and amperage limits as the Xantrex XW MPPT60-150

I like the Outback tool as they include minimum and maximum environmental temperature sliders and a graphical display or what module combinations work. Fun to play with. The bottom left corner indicates one module, going up adds modules in series and across to the left adds series strings in parallel.