DIY Manual

10 | P a g e 10-543-1 REV A
DIY Manual
Another perspective on series strings and Voc …
From the preceding discussion of the maximum number of 250W panels using the Classic 150 in
12V, we can use 4.6 panels. Let’s round that up to 5 panels for this discussion. And let’s also
assume each panel has a Voc rating of 37V and Isc (short circuit current) of 9A.
So, your CL150 in 12V can handle 5 panels. How to wire them up?
If you wire 3 in series to keep the Voc below the CL150 limit of 150V, that presents a problem in
that you will have 3 panels in the first series string and 2 panels in the second series string. Can’t
do that! Must have same number of panels in each string to keep the string voltages the same.
You must either round down to 4 panels or round up to 6. Now 6 panels will be more total
wattage than the CL150 (in 12V) can process, thus you are wasting panels (6 x 250W = 1500W;
the CL150 in 12V can only process 1152W). On the other hand, most PV arrays are about 80%
efficient, losses due to panel orientation, line resistance, variable sunlight intensity, etc. So, 80%
of 1500W (if we use the 6 panels) = 1200W, very close to the max Classic 150 limit of 1152W in a
12V system. Therefore, 6 panels will actually be very productive considering the inherent losses.
You can wire all 5 panels in series for a Voc of 185V and an Isc of 9A. Having only 9A of current
allows your PV array to be placed a good distance away from the Classic. Assume the best spot
to build your PV ground-mount is 100 feet from the charge controller. 9A over 100 feet with a
3% voltage drop requires a #4 AWG wire. If you had wired 4 panels in series/parallel, the total
current would be 18A; therefore, over the same 100 feet, you would need #2 AWG wire.
Now, 5 panels in series gives a Voc of 185V. That will not work with the CL150. The CL200 will
work in warm environments, as will the CL250. 185V into the CL200 offers little wiggle room,
though, for cold temps, which raise the Voc.
NOTE: The above examples of current, wire length, and voltage drops are generic. Refer to the
Wiring and Circuit Breaker Chapter of the DIY Manual for more information on calculating current
and sizing wire.
Voltage Drop = Decrease in voltage due to higher resistance in longer wire lengths.
V (Volts) = I (Current) x R (Resistance) … If resistance increases and current stays the same, then V
must lower (drop).