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solar underground cable sizing

shadowmaker

shadowmaker

Solar Addict
Joined
Dec 4, 2022
Messages
544
Location
Finland
Total noob for solar, but trying to make DIY 44kW vertically installed system with bifacial panels here in Finland. I can tell you it is overwhelming sometimes and learning takes time as I don't have anyone experienced enough to turn to when I have questions. Unfortunately time is scarce just now.

I'm going to have 8 strings with 8 panels each (63 panels total so one string is 7 panels only, 4x15kW inverters). There's 4 underground aluminum cables buried 1m deep already in place , 4x25mm2 each. Lengths from field to inverters are 100m, 115m, 130m and 145m.

Panels are Bluesun 700W bifacials. Specs with optimistic 25% backside gain.
- Pmax= 875W
- Vmp= 39,6V
- Imp= 22,10A
- Voc= 47,8V
- Isc= 23,50A

Do I need more cables to keep losses down?
 
yodamota said:
start punching numbers into https://www.calculator.net/voltage-...ceunit=meters&amperes=62&x=112&y=30&ctype=nec

25mm looks to be somewhat in between 3AWG and 4 AWG sized wire

This tool will most likely be helpful. https://www.midnitesolar.com/sizingTool/index.php
From what i punched in your Open voltage circuit will be 452VDC at -30*C. Which you might experience colder than -30*C
Click to expand...
The problem being a noob is what numbers to punch these calculators? 8xVmp? 8xVoc? 8xImp/Isc? 1xImp/Isc?
Temperature coefficient for this panel is -0,24 IIRC and we rarely go under -35C here. To my understanding 25mm2 is just a fraction bigger than 4AWG.
 
My 2 cents... First I size wire and OCP based on Isc. Then for power losses I size wire based on NOCT. Imp x .8 if you can't find the published specs. People often (grossly) oversize wire for conditions that only exist a few hours a year. In most cases, and by most I mean none, you're never going to get that money of upsizing the wire to save a few lost kWh's back.
 
One calculator will tell you what your max VOC is. One calculator will tell you how much line loss there is.

Punch the max VOC into the line loss based on the length of the line.

Spend some time trying to figure it out. The midnite calculator will tell you quite a lot about your system.
You can post your results afterwards here and people can tell you whether or not your entered it correctly.

In my experience my panels mostly output what the VMP is.
 
If I use -35C and -0,24 temperature coefficient, first calculator gives Voc 438V. That and Isc 23,5A used on the second calculator with 4AWG aluminum wire gives voltage drop of 2,6% (145m) and 1,79% (100m).

I used Isc instead of Imp, because it makes as much sense to me than using Voc in these calculations. I would have guessed that Vmp (temperature corrected) and Imp are the numbers that can actually happen in a working system. Voc and Isc can't?

Also Voltage drop calculator doesn't take account of ground cable temperature. Cables are buried 1m down in clay ground so they should stay under 20C at all times. That should help a bit and it seems that 4AWG used in the calculator is actually 21,15mm2 and I'm using 25mm2, so I should be more than fine?:unsure:

I'm planning to have 8 strings so each of my four 4x25mm2 cable is used by two strings, two positive and two negative leads inside one cable. Is it OK to have plus and minus leads inside one cable, or should plus and minus be in their separate cables? I really don't know why I'm asking this, I just want to avoid noob mistakes if I can.
 
shadowmaker said:
If I use -35C and -0,24 temperature coefficient, first calculator gives Voc 438V. That and Isc 23,5A used on the second calculator with 4AWG aluminum wire gives voltage drop of 2,6% (145m) and 1,79% (100m).

I used Isc instead of Imp, because it makes as much sense to me than using Voc in these calculations. I would have guessed that Vmp (temperature corrected) and Imp are the numbers that can actually happen in a working system. Voc and Isc can't?

Also Voltage drop calculator doesn't take account of ground cable temperature. Cables are buried 1m down in clay ground so they should stay under 20C at all times. That should help a bit and it seems that 4AWG used in the calculator is actually 21,15mm2 and I'm using 25mm2, so I should be more than fine?:unsure:

I'm planning to have 8 strings so each of my four 4x25mm2 cable is used by two strings, two positive and two negative leads inside one cable. Is it OK to have plus and minus leads inside one cable, or should plus and minus be in their separate cables? I really don't know why I'm asking this, I just want to avoid noob mistakes if I can.
Click to expand...
numbers look good to me. I think ground buried cable will help.
Our wires are together in our tech cable. both positive and negative.

Any chance you could share the rest of your system? Is this a small village system? for a school?
 
shadowmaker said:
I used Isc instead of Imp, because it makes as much sense to me than using Voc in these calculations. I would have guessed that Vmp (temperature corrected) and Imp are the numbers that can actually happen in a working system. Voc and Isc can't?
Click to expand...
For safety you use Isc and Voc.

For performance modeling you use Imp and Vmp.

#4 Aluminum will be great and yes direct burial does get a "credit" for being in the ground.

1675029244306.png
shadowmaker said:
I'm planning to have 8 strings so each of my four 4x25mm2 cable is used by two strings, two positive and two negative leads inside one cable. Is it OK to have plus and minus leads inside one cable,
Click to expand...
Totally Ok.
 
yodamota said:
numbers look good to me. I think ground buried cable will help.
Our wires are together in our tech cable. both positive and negative.

Any chance you could share the rest of your system? Is this a small village system? for a school?
Click to expand...
?

No village or school, just 80 years old big house with two old huge garages. Here's my build topic: Vertically mounted bifacial 700W panels in Finland. This is my first take on solar, so time to time I feel a bit overwhelmed by it.

OzSolar said:
For safety you use Isc and Voc.

For performance modeling you use Imp and Vmp.

#4 Aluminum will be great and yes direct burial does get a "credit" for being in the ground.

View attachment 132128

Totally Ok.
Click to expand...
Thank you for a clear explanation. (y)
 
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