My installers have set up my 27 panel array in 2 strings; 18 panels and 9 panels (Sunpower 205BLK, SPR-5000x inverter). They say this increases efficiency. This seems strange to me, the inverter has the capability to wire 3 strings, and the current for 18 panels will be much higher than for 9. Is there something I am missing here? I always followed the belief that higher current means higher temp and more resistance = lower efficiency. The wire size they pulled is 10GA which is fine for the load, but wouldn't it have been even more efficient to have 3 strings of 9 panels each?

I'd think that each string needs to be matched, and that 18 + 9 will NOT be a good match.

I suspect the connections on the inverter are just parallel terminals, and that the higher voltage string will be pulled down to the lower strings voltage, wasteing a LOT of power.
The Xantrax sizer page claims :
http://www.xantrex.com/support/gtsizing/index.asp
Calculated PV String Configurations:
Min. # Modules: 7
Max. # Modules: 11
Max. # Strings: 4
With 3 strings of 9 being acceptable = max 473V & 15A [GT5 inverter]
5,535 STC watts

You might want to insist the installer confirm that, or get a new installer.

It all depends on the LENGTH of the wire runs, but they should plan on each string sourcing 5 amps, and either combine it on a box on the roof, or use "home run" wires to the inverter directly.

A solar length/gauge spreadsheet can be found here:
http://www.wind-sun.com/ForumVB/showthread.php?t=29

They are running the 2 combined strings in parallel, so the total volts are the same but the amps are 2x. My thinking is the same, that lower amps is better. But, their engineer insists that running the 18/9 config is better, that the 18 panels will push close to the maximum inverter allowed current per string and be more efficient.

They have done a lot of installs...and the wiring is now finished...so I guess I will have to defer to their judgement and see how the system runs.

Thanks,

I had my say. anyone else ?

You should have 3x parallel strings of 9 panels each (in series). So each string has the same voltage and current rating (individually).

They reason that they may have broke it into 2 parallel strings with one fuse, and one other parallel string with one fuse (or possibly no fuse)....

Generally, if you parallel more than two strings together, there needs to be a series protection fuse (to protect one shorted panel from being "set on fire" by a whole bunch of other panels driving current back into the shorted panel. The series protection fuse prevents the other parallel strings' currents from adding together causing problems with one lone shorted panel.

Electrically, you should have just three strings in parallel (of 9 panels each) and a combiner box with one or several protection devices (fuses/breakers).

The Sunpower (Xantrex GT series) Inverter has only one inverter solar panel input (as far as I know). Running 18 series + 9 series in parallel would not make any sense (as I understand your install).

-Bill

I suspect what they did is parrallel two string at the array ( so the 18 is really 2x9 ) and then did two home runs ???

Yes, you will have higher IR losses, but probably nothing that one will be able to measure in a home system

BB,

The GT5.0 has a common terminal connections for 3 series strings of panels ( 3 hots, 3 returns for series panel wires to connect to in the BOS box ) that do not require any fuses, not sure of the math, but did ask as was told it passes NEC having upto 3 strings parralled

SG

I think I may have miscommunicated what is being done:

The panels are being wired in strings of 9 panels. 2 of these strings of 9p are being combined in parallel in a single home run. The other 9p are on their own home run.

Would it have been better to run the 9 strings on their own home runs?

SG,

I realized that there were two (or more) physical inputs where wires can be attached to the inverter--I was trying to say that the inverter has only one physical inverter interface in that it cannot take a 18x1 and a second 9x1 parallel input and provide full power (I believe that there is at least one inverter that has several physical inputs that can use different voltage/current configured panels (or east/west type configurations).

But, in the end that does not matter--it was just an issue of how a 9x3 home run was configured... Which JZA confirmed.

JZA, you can use the solar calculator to see how much power loss there would be with three x 10 awg vs two 10 awg home runs... You did not say how long the runs were (and if all are 10awg?)--so either need that info, or you can download the Excel spreadsheet and do it yourself.

But, unless you have a very long home run, you could never really see the "extra loss" (probably less than 1%) vs the extra cost of copper/wiring.

Solar Guppy is the expert here on inverters/configurations.

-Bill