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Old 01-26-2016, 10:16 AM   #85
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2015 30' Classic
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Originally Posted by switz View Post
Our GS-100 solar panels are sold as having a 5.5 amp output so five of them (like on our 23D) are capable of a 27.5 amp current which exceeds the continuous load rating of #10 wire which is 24 amps per the National Electric Code. On our Classic with nine of the same panels, they current generated would approach 49.5 amps which greatly exceeds the 30 amp maximum current rating of #10 wire.

The TriStar 60 MPPT remote display has shown 600 watts coming off the existing eight solar panels, Dividing by the nominal 12.5 VDC means 48 amps of current. Dividing by 18 Vdc would be 33.3 amps. So #10 wire lacks the amperage capacity for our installation before even considering voltage drop.
Let's forget for a moment that there's something called a MPPT solar charger in between the panels and the batteries. So in switz's case, he has connected all his panels in parallel, so yes the currents add for each panel in parallel. On his 23D the current from his panels is 27.5 A and I wouldn't recommend using 10 AWG wire because of the resistive losses. However, when you connect panels in series the current remains the same but you keep increasing the voltage, just like you do when you connect batteries in series. So in Wynn's case, they've connected 6 panels in series. Therefore the current remains the same, 9.6A, but the voltage is nearly 100 V. The total power is V*I = 100V * 9.6A = 960W.

Now the output of the panels is connected to a MPPT charger. This is actually a sophisticated DC/DC charger which in this case takes a higher DC voltage and converts it to a lower DC voltage with high efficiency among other nifty things I won't go into here. In Wynn's case the 960W at 100 V is converted to 960W at 13.7 V (just as an example), so at 13.7V the Wynns have 960W/13.7V = 70A of charging current available to the batteries if we assume that the MPPT charger has 100% efficiency. Typical efficiencies run in the range of 92% to 97%.

In Bold's example, IF he mounted his 4 panels in series, then yes, the total voltage would be 70V and the current would be 7.14A. If he connected this to a MPPT controller that can accept 70V, then the controller would deliver 70V*7.14A = 500W to the input of the MPPT controller and the controller would supply 500W/13.7V = 36.5A when charging the batteries, assuming 100% efficiency. I'd have to check with Bold but I'm pretty sure he did not connect his panels in series. This isn't a problem, it just means that the current from his solar array is 4*7.14A = 28.5A and therefore he had to use wire larger than #10 AWG to carry the current.

In my own installation, I connected my 4 100W panels in a series/parallel configuration and I'm able to reuse the existing solar pre-wiring while keeping the voltage losses due to the pre-wiring at less than 2%.

Al, K6IV
2015 30' Classic, "Chez Nu"
2014 RAM 2500 w/Cummins Diesel
ProPride Hitch, 400 W Solar
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Old 01-26-2016, 12:44 PM   #86
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Mine are in Parallel because I was worried about shading when we get back to the Pacific North West this year.

Thanks for the explainer, that makes a lot of sense and I totally didn't even think/realize the parallel/series exception with the panels. That's good information to know.

Family of 4 living, working & exploring the USA in our Airstream.
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