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Did I do my math correct?

Guda

Guda

Superstrut Strut
Joined
Nov 19, 2019
Messages
516
I have a Midnite 250 & 20pv.

  • Rated Power: 265W
  • Open circuit voltage (VOC): 44.20 V
  • Max power voltage (VMP): 35.33 V
  • Short circuit current (ISC): 8.20 A
  • Max power current: 7.50 A
  • Power Tolerance 0/+3%
  • Maximum system voltage: 600V (UL)
  • Fuse Rating: 15 A
If I hook 6 in series I'll have 265v 7.5a. Thats within the 250+ 48v because of the battery voltage is added correct?

I'll have 3 of those in parallel. Thats 265v at 22.5a correct?

The pv are used & I'm only going to use 18 of them.

Thanks!
 
Nope. 265 volts is greater than 250 volts. Never over volt your controller, it goes poof!
Not sure what you mean about battery voltage being added in? It's not added in to any calculation. You could do 5S4P for 221 volts and 30 amps instead.

Edit to add: I see why you want to add 48 volts, Midnight Solar has what they call Hyper VOC. That's only for cold mornings where your initial VOC is above 250. It's clearly listed as being non-operational at those voltages. So a constant 265 means you'll produce zero power. My 5S4P suggestion still stands.
 
Last edited:
Guda said:
I have a Midnite 250 & 20pv.

  • Rated Power: 265W
  • Open circuit voltage (VOC): 44.20 V
  • Max power voltage (VMP): 35.33 V
  • Short circuit current (ISC): 8.20 A
  • Max power current: 7.50 A
  • Power Tolerance 0/+3%
  • Maximum system voltage: 600V (UL)
  • Fuse Rating: 15 A
If I hook 6 in series I'll have 265v 7.5a. Thats within the 250+ 48v because of the battery voltage is added correct?

I'll have 3 of those in parallel. Thats 265v at 22.5a correct?

The pv are used & I'm only going to use 18 of them.

Thanks!
Click to expand...
265.2... way too high...

24.6A, not 22, you always want to size the wire and over current protection to the ISC...
 
Supervstech said:
265.2... way too high...

24.6A, not 22, you always want to size the wire and over current protection to the ISC...
Click to expand...

I didn't realize you should use ISC when calculating amps. So that make's 5S4P 221 volts and 32.8 amps which is still okay for your controller but A LOT of amps to run for any significant distance. With that many panels you might consider a bigger controller that accepts more volts or using two controllers and each get half the amps. You'll run twice as much wire but it'll be about the same cost cause you can use smaller gauge wire.
 
While calculating peak input voltage, don't use the Voc on the panel's label, since this is at 25 C cell temperature. Instead, estimate the Voc at the lowest temperature the panels are likely to see. Use the panels temperature coefficient for voltage for this calculation.
 
Bogart Engineering said:
While calculating peak input voltage, don't use the Voc on the panel's label, since this is at 25 C cell temperature. Instead, estimate the Voc at the lowest temperature the panels are likely to see. Use the panels temperature coefficient for voltage for this calculation.
Click to expand...

Great point. And specific to the controller he's looking at here a what MS recommends.

Screenshot_20200514-132337_Drive.jpg

So if it gets to 0 farenheit you'll need to multiply my 221 volts by 1.18 which gets you to 260 volts. That's okay for a cold morning with the MS250 but not okay for a full day because again it falls into the non-operational range of 250-298v.
 
I really love this forum.

Okay so I am on the very edge of possible. I think. The pv are used poly so they might be lower actual specs. But maybe they used to be 300w or or something else & they just down graded them to 265w & they will really put that out. Have to see.

A really cold clear day could be a issue. I did not know the pv temp was a consideration. Why I love this place.

Should I.
Use 5 pv instead of 6?
Or is there a part that would lower the A &/or V on a cold day?
Or rely on a loss of current from a long wire run?
Or unplug the 6th pv on cold days?
 
sed6 said:
Yep. That's what I mean by 5S4P, 5 panels in series and 4 (groups of 5) paralleled.
Click to expand...
Ah, so I could use all 20 in that configuration. Thank you for explaining 5S4P. I was not understanding that.

Oh ya those numbers are all within tolerances. 4 directions instead of 3. So like east, west, up east & up west.

MBR said:
How long is your wire run from your panels to your charge controller?
Click to expand...
2 options. A 60'+ run or 10' ish. If I put the array on the ground it will be a long run. 1st I was looking a my shack roof, then I was looking at the ground. If I have another fire I'd wished I'd have put them on the ground.

If I install on the roof I can use 8g wire. If I put it on the ground, 2g. huge difference.
 
Guda said:
Oh ya those numbers are all within tolerances. 4 directions instead of 3. So like east, west, up east & up west.
Click to expand...

No, not 4 directions, 4 groups of 5 panels. You said parallel three together in your first post, I'm saying parallel four together.
 
Like this. Then you face each group of 5 what ever direction you want. Like all 4 south or 2 south and 2 west.

20200514_155432.jpg
 
Guda said:
Ah, so I could use all 20 in that configuration. 5S4P.

If I install on the roof I can use 8g wire. If I put it on the ground, 2g. huge difference.
Click to expand...
How did you calculate 2AWG for 60 feet at 32 amps?
35volts Vmp each so 175 volts for 5 panels.

I get 10 AWG
Voltage drop: 4.55
Voltage drop percentage: 2.59%
Voltage at the end: 171.45

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.
www.calculator.net www.calculator.net
 
Last edited:
MBR said:
How long is your wire run from your panels to your charge controller?
Click to expand...

Doesn't matter, no one should encourage him to undersize his wire and burn off extra energy as heat because part of his system (charge controller) is undersized for his system (panels).
 
sed6 said:
Doesn't matter, no one should encourage him to undersize his wire and burn off extra energy as heat because part of his system (charge controller) is undersized for his system (panels).
Click to expand...
The rule of thumb is under 3% voltage drop so where did you get the idea anyone was encouraging undersized wires?
I asked because he seemed confused on all aspects of panel layout for a SCC so i figured that his wire sizing was also suspect.
 
MBR said:
How did you calculate 2AWG for 60 feet at 32 amps?
35volts Vmp each so 175 volts for 5 panels.

I get 10 AWG
Voltage drop: 4.55
Voltage drop percentage: 2.59%
Voltage at the end: 171.45

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.
www.calculator.net www.calculator.net
Click to expand...

Nope. 10ga wire is only good for 30 amps if you're using it for power transmission.
 
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