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Can anybody suggest an affordable SCC for this setup?

Lt.Dan

Solar Wizard
Joined
Dec 25, 2020
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Location
Tulare, Ca
Hey guys, I need some help with temp calculations. Heres what I have going on;

I have 12x Sunpower 240w (spec sheet attached to this post). They are wired in a 3S4P fashion. I have a wire run of 150ft to the battery. VOC is 145.8v. Temperatures never get below 0*C where I live, but it has gotten close. My calculations show VOC could get up to 157v at 0*C. Now here is where I have a hiccup and need help...

At 150ft, and with the peak current at 23a (rounding down a little), a voltage calculator spits out 148v. Does this mean its safe to use a SCC that has a 150v VOC? Or can I not count on the voltage drop because first thing in the morning there will be very little current, thus, very little voltage drop?

I have a feeling I need to step up to a much more expensive 200v SCC.
 

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The voltage potentially can go over 150 because if your batteries are full the load will stop and the open current voltage will be essentially un affected by the length of your wire. Why don’t you just go with 2s6p?
 
Because they are already wired and installed. The labor to remove the panels and rewire it is enough for me to just pay more for a larger SCC. Its not an easy task in this situation.

Also a 6p. Eww.

Also, a 6p makes it close to 35 amps, more than 10ga PV wire is rated for, and then the voltage drop over 150ft is 17.34%! Unacceptable
 
I agree with both Bobert, you are so close to the peak voltage that it's not worth risking killing the SCC. I would jump to the larger unit.
 
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Because they are already wired and installed. The labor to remove the panels and rewire it is enough for me to just pay more for a larger SCC. Its not an easy task in this situation.

Also a 6p. Eww.

Also, a 6p makes it close to 35 amps, more than 10ga PV wire is rated for, and then the voltage drop over 150ft is 17.34%! Unacceptable

Voltage drop may be acceptable. It is only efficiency loss, which is a trade off with the cost of Cu and Si. And impact is greatest when you have excess production, less during marginal times.
Ampacity, operating temperature, derating, and insulation temperature rating are more important.

Most of the higher voltage SCC are less "affordable". Midnight makes 150V, 200V, 250V (and they have "HyperVoc"; the 150V model would handle occasional higher excursions.)
I of course like AC coupled, with 600V typically supported. That only works if your battery inverter supports it. Various older model GT PV inverters can be economical. Some are heavy for a portable system.
 
Lt. Dan - the IEEE standard for panels data sheets is to report VOC at 25C (77 Fahrenheit). At 0C, I would not gamble with the theoretical voltage drop on the wire. This is because wire also changes conductive values with temperature.

In your case (According to the data sheet), your panels VOC was measured at 45C not 25C, so the VOC is going to be significantly higher at 0C.

Disclaimer: I'm new to solar, but work as a EE in the industry. I know enough to sound convincing, but realize I can be wrong :)
 
Because they are already wired and installed. The labor to remove the panels and rewire it is enough for me to just pay more for a larger SCC. Its not an easy task in this situation.

Also, a 6p makes it close to 35 amps, more than 10ga PV wire is rated for, and then the voltage drop over 150ft is 17.34%! Unacceptable

2s3p facing SE parallel with 2s3p facing SW (much more labor than just rewiring) would reduce peak current to perhaps 0.7x as much.

If this is for your fixed location setup, better to get SCC for higher voltage and/or current, keep existing array as 3s4p or rewired 2s6p, and support future expansion with another 12 panels of alternate orientation.

The 150V Midnight Classic will cost less per watt than the 200V or 250V model, and operate at higher efficiency with less voltage reduction in its switcher. But higher current is more loss in wire. You can check the math for your length and gauge.
 
Because they are already wired and installed. The labor to remove the panels and rewire it is enough for me to just pay more for a larger SCC. Its not an easy task in this situation.

Also a 6p. Eww.

Also, a 6p makes it close to 35 amps, more than 10ga PV wire is rated for, and then the voltage drop over 150ft is 17.34%! Unacceptable
If your time is worth anything $300 is well spent for you situation.
 
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