Array wiring

11 Posts
Jan 26, 2015 03:17 pm
Array wiring

Question I have 12 Canadian solar 255 watt panels. Is it better for me to series fewer panels to increase amps, or more panels to have a higher voltage. I have 12 Rolls S530 batteries configured for 24 volt. It was suggested that I wire 4 sets of 3 panels, as opposed to 2 sets of 6. wire size for voltage drop is not an issue. What would you suggest
47 Posts
Jan 26, 2015 04:41 pm
Re: Array wiring

Specs for the panels: 255 Watts

Max Power Voltage (Vmpp): 30.5 Volts
Max Power Current (Impp): 8.35 Amps
Open Circuit Voltage (Voc): 37.7 Volts
Short Circuit Current (Isc): 8.74 Amps

I usually recommend higher voltage to reduce voltage drop. The MPPT charge controller will drop the voltage to match the battery bank and increase the current, so the volts and amps on the output will be the same regardless of how you wire it. The advantage is you can use smaller wire with the higher voltage.

If you are doing 3 nominal 20V panels in series, you will have 20V x 3 = 60V nominal in / 24V nominal out = 2.5 ration of voltage dropping and current increasing.  So if you have 4 parallel strings with 8.74Isc x 4 strings x 2.5 ratio = 87A out. NEC says you have to add another 25% when selecting a charge controller, so you would need a charge controller that can handle 87.4A x 1.25 = 109A. I don't know of any that can handle that, so you will need to break it into 2 circuits, with 2 strings of 3 going into each charge controller.
8.74Isc x 2 strings x 2.5 ratio=43.7A x 1.25 NEC requirement = 54.6A minimum charge controllers, so probably two 60A charge controllers needed.

Note that if you were doing a 48V battery bank, you could use just 1 charge controller. 60V in / 48V out = 1.25 ratio.  8.74Isc x 4 strings x 1.25 ratio x 1.25 NEC = 54.6A charge controller for the whole system.
11 Posts
Jan 26, 2015 10:32 pm
Re: Array wiring

Amy, Thanks I see what you mean and agree. I do happen to have a extra flex 80 that I haven't opened yet but with only 1 set of 1/0 wire going from my combiner to the CC a second CC would be useless, I do not have room in my conduit for a second run I also would prefer to use the higher voltage as opposed to 4 strings. My old Battery bank suffered a major problem last year with all the positive post terminals swelling and deforming the cases. ie. I was only using 6 evergreen 200 watt panels and believe I had outgrown my previous battery bank of 8 215  6volt batteries  wired for 24 volt. I have purchased 4 sets of mc4 cables but won't receive them till Thursday. With what Batteries cost I want to make sure I do the best I can. So with 2 sets of 6 panels wired for higher voltage you believe I will get enough amp to keep the batteries topped. I am currently using a flex 60 assuming I need to change to the 80?
47 Posts
Jan 27, 2015 10:31 am
Re: Array wiring

The FlexMax 60 can handle 6 of the Canadian Solars, the FlexMax 80 can handle 8 of them. The 6 you could wire in 2 parallel strings of 3 in series, or 3 parallel strings of 2 in series, either way, you are right, with 1/0 cable, voltage drop shouldn't be a problem. If you switch to the FM80, the 8 can be 4 parallel strings of 2 in series.

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