PV wire runs and mixing gauges

TJ King

New Member
This is kind of Electricity 101 question from a nube, but here goes. I'm trying to replace my cement roof off grid PV array with Nine 250W panels and previously I had a distance of about 22 ft from Charge controller with 10awg wire to my old array. I get that the gauge & distance has to do with not only loss but safety/fire hazard, but my question is,... if my array is ~90V and ~25A and I use the existing 22ft of 10awg to the first PV panel, how do I factor in the probably 20-30 ft of wiring that is attached to the other panels? I haven't purchased the panels yet so I dont know what gauge it is but since we are talking amps going through the "firehose", if I replace the main run from CC to the first panel with 8awg, isnt mixing gauges in a circuit going to create a bottle neck, meaning the resistance in the thinnest wire is its weakest link? Is the loss in power to the CC the same? Can I keep the 10awg? Forgive my awkward understanding of the concepts, but help would be appreciated

Numbers: MPP 24224lv MSD inverter/MPPT controller with 145V max input. Location never under 0 Centigrade. Operating Voltage range 30~80Vdc.
Probable PV modules (9-250w) (Not sure yet if 3P3S or 3S3P is best)- Each:
Open circuit voltage (VOC): 37.6 V
Max power voltage (VMP): 30.3 V
Short circuit current (ISC): 8.85 A
Max power current: 8.27 A
 

chrisski

Photon Sorcerer
Are these new panels going to be combined on the roof to be fed into the Charge controller with 10 AWG, if that's the case, the current 10 AWG will exceed the ampacity. If you're adding a second 10 AWG run into the house, then it probably to me, it's a little too close for comfort for the max ampacity for 10 AWG. I would like at least 8 on a second run. I like to stick to the 60 C° amp limit for a wire size:

1624579426661.png
For voltage loss, I use this calculator to find out the loss:


With voltage loss alone, 10 AWG is higher, but still acceptable. 8 AWG is even better.

If two lines is not how you're planning on installing, what is your idea?
 

TJ King

New Member
Are these new panels going to be combined on the roof to be fed into the Charge controller with 10 AWG, if that's the case, the current 10 AWG will exceed the ampacity. If you're adding a second 10 AWG run into the house, then it probably to me, it's a little too close for comfort for the max ampacity for 10 AWG. I would like at least 8 on a second run. I like to stick to the 60 C° amp limit for a wire size:

View attachment 54006
For voltage loss, I use this calculator to find out the loss:


With voltage loss alone, 10 AWG is higher, but still acceptable. 8 AWG is even better.

If two lines is not how you're planning on installing, what is your idea?
I had planned on combining the 9 panels in 3P3S (or 3S3P, not sure the difference) on the roof, with the final 2 pos/neg wires running 22 feet down to the Charge Controller (No 2nd set of wires). Wasn't even aware that was a thing. What Im hearing you say is the loss for the 10 awg is not a huge problem and according to your link, its about 1.4% loss in Voltage. But youre saying the safety issue is the area of concern, right? ie the temperature rating of the wire at 90V/25A. FWIW its worth the desert environment here can be 110F on occasion, which I imagine effects your wire temperature calculations, yes?
 

chrisski

Photon Sorcerer
I misread. I thought you were keeping your old array, but you're replacing it. So with about 27 amps total at about 90 volts, 10 AWG should suffice. This is where I might feel more comfortable with a a 4S2P with higher voltage so less amps, or even 5S2P with more voltage, but with the same amps as the 4S2P. I don't like the idea of coming close to that limit, but that doesn't mean its not a good idea. Since these are mounted on a roof, there's only so much space to put them.

Yes there is more resistance as temperature goes up, but I have not used that in my calculations. I've also not seen that in on line calculators.

I'm in central AZ so I get plenty of heat and sunshine. For solar wire on the roof of my RV wire I use this:

 

TJ King

New Member
I would like to go 4s2p or even 5s2p, but at 4s2p, my voltage goes over the CC 145v limit (37.6 VOC * 4 = 150.4), so my temptation was to start factoring in line loss, temperature and other things to imagine I will in the real world be below that 145 hard limit, but the risk of blowing my Charge controller brought me back from the brink. So Im playing it safe at 3s3p at ~90 volts and 27A. Still not sure difference between 3s3p or 3p3s. Thank you for the link. Im in Baja, gulf coast, so similar to phx, usually 8 degrees lower in summer and a bit more humid in late summer. Per my original question, on theory, is there any difference on these PV wire runs regarding resistance if certain segments are different gauge or is the thinnest gauge in the entire circuit the bottle neck that lowers the resistance and sets the loss for the entire segment? And how does one factor in the extensive wire segments that connect each panel to your calculation?
 
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