Wiring Panels in Series-Parallel | Is this diagram correct?

[Bud Martin]

Do you also already own the panels?
If you are buying the SCC I would suggest to get it from Watts247.com in case you have any issue with the unit.

 

[MisterSandals]

And also 450vdc sounds terrifying haha.

Having a healthy respect for electricity is a good thing. Its not just the volts, its the amps too. Consider a taser, its 50,000V and non-lethal (usually).
And, with big arrays, i don't touch any connections except at night!

Note this unit is only 1 big MPPT, not 2 side by side. So one large PV input only.

Whether its one input or 2 is just a minor detail.

Lets do some math:

450V / (37 x 1.25) = 9.7 panels.

So you could run 9S2P with 10AWG and 1.2% voltage drop.

 

[AZ Off-Gridder]

Do you also already own the panels?
If you are buying the SCC I would suggest to get it from Watts247.com in case you have any issue with the unit.

I don’t currently have any of it. Planning to buy soon, end of this month/early next month. Panels I’ll be getting local from SanTan Solar directly, they also sell the SCC unit so I was planning on getting it in person from them save on shipping and waiting. They seem to be solid to work with but I’ve heard great things of Watts247 I’ll check them out see if their stocking this SCC unit.

 

[WYtreasure]

And also 450vdc sounds terrifying haha.

If you stay terrified of 450 volts, you may live to be an ole man.

 

[AZ Off-Gridder]

Whether its one input or 2 is just a minor detail

Not when it means running extra lengths of the same wire side by side from array to inverter. One unit means half of the wire used as 2 units wired separately. Which I like! makes getting the bigger AWG cheaper and more attractive than I previously thought

 

[AZ Off-Gridder]

450V / (37 x 1.25) = 9.7 panels.

So you could run 9S2P with 10AWG and 1.2% voltage drop.
View attachment 79068

Can you help explain this math to me some. I want to understand it better. And I want to take a minute to seriously say THANK YOU. And a big thanks to WYTressure and Bud Martin as well! This is helping me immensely. I’ve got 40 tabs open on my iPad of research and information on wiring arrays and yet this conversation has taught me more then them all. Seriously thanks yall

 

[MisterSandals]

Panels I’ll be getting local from SanTan Solar directly, they also sell the SCC unit so I was planning on getting it in person from them

I would ask them some of these questions. You have a lot of ideas and numbers. Maybe they have some ideas or some products you have not considered or heard here.

And its REALLY refreshing to work with someone planning before buying.

Figure out what you need before you buy damn it.

[RANT MODE ON] I can understand when people don't understand the terminology. I can understand when people don't understand the technology. I can understand when people ask questions that show they are clueless about solar & batteries. I would even say I enjoy helping these people...

diysolarforum.com

 

[MisterSandals]

Can you help explain this math to me some.

450V / (37 x 1.25) = 9.7 panels.

Your panel Voc is 37V at 25 deg C. The voltage increases .3% for every degree C the temp drops below 25C (77F). The common fudge factor for temp compensated Voc is 1.25%. If you have a lowest temperature that you want your SCC to survive, pick a temp. 25% fudge is pretty good.

So, max temp compensated Voc is 37 x 1.25 = 46.25V

450V SCC max input voltage / 46.25V = 9.73 panels in series to stay below 450V (temp compensated).

 

[AZ Off-Gridder]

And its REALLY refreshing to work with someone planning before buying.

Measure twice, cut once.
I’ve got so many lists, spreadsheets, graphs…. I stay up most nights on the iPad in bed trying to learn something new, or add/adjust the budget. I try to stay on top of it all. We are building a tiny house and going off grid so there’s a lot to manage. We got the land, and the custom building is ordered and being built. (By the way I’ve lived off grid before don’t worry we know what we’re getting into )

Currently the whole solar system is sitting at about $6,650usd

 

[AZ Off-Gridder]

Your panel Voc is 37V at 25 deg C. The voltage increases .3% for every degree C the temp drops below 25C (77F). The common fudge factor for temp compensated Voc is 1.25%. If you have a lowest temperature that you want your SCC to survive, pick a temp. 25% fudge is pretty good.

So, max temp compensated Voc is 37 x 1.25 = 46.25V

450V SCC max input voltage / 46.25V = 9.73 panels in series to stay below 450V (temp compensated).

😀 very nice. So 9 panels series seems very safe for this inverter. That worked out darn near perfect! Because I’ve already been planning on 2 large pole mounts 9 panels each.

So, paralleling these sets of 9, has anything changed when we made all these adjustments? Or is pigtail MC4 connectors still viable? With fuses of course.

 

[MisterSandals]

Or is pigtail MC4 connectors still viable? With fuses of course.

I suspect they are fine since MC4 connectors are pretty standard for 600V arrays. I might opt for a better branded MC4 Y connector for a couple bucks more to be sure.
Technically you won't need the inline fuses for 2 strings in parallel. I think fewer connections is better.
You'll want a high quality breaker for shutoff and array wiring safety near your SCC.

That's the beauty of higher volts and much lower amps. The utilities transmit power long distances at really high volts for a reason.

 

[AZ Off-Gridder]

If you have a lowest temperature that you want your SCC to survive, pick a temp. 25% fudge is pretty good.

Lowest temp being early morning right as the panels begin to collect energy? Than I’d put the lowest temp at about 20 or 25 Fahrenheit to be safe. But the panels aren’t collecting very much energy at this point, on previous systems it was hardly any power coming in that early in the morning maybe 10% of the total array Wattage.

Or am I to plan based off lowest temp during peak sun hour on the coldest day of the year? That wound be 40 Fahrenheit and that’s rare.
And if so does this change your math results at all? Or is this within the 1.25% fudge? Haha thanks

 

[MisterSandals]

But the panels aren’t collecting very much energy at this point,

Agreed, but the voltage can spike at that time of day when cold and start getting good sun. The voltage can spike before the SCC starts charging at which point it draws the voltage down to panel operating voltage (Vmp).

20F = -6.7C

37V x (25 + 6.7) x .003 tcoef = 3.5V change (40.5Voc)

450V / (40.5V / panel) = 11.11 panels in series

 

[AZ Off-Gridder]

Found some bulk spool 8AWG solar PV wire at $0.91 per foot. (Ecodirect website) So if I’m understanding this 9S 2P correctly I only need to run 2 long cables to the house now, one being negative and the other positive. So that’s 270ft of wire at about $246usd that’s not bad and then I’d have better loss at 8AWG. Also this wire is UL listed.

 

[MisterSandals]

Found some bulk spool 8AWG solar PV wire at $0.91 per foot.

10AWG is plenty. Spools of solar wire are also usually sun/uv resistant.

 

[AZ Off-Gridder]

Agreed, but the voltage can spike at that time of day when cold and start getting good sun. The voltage can spike before the SCC starts charging at which point it draws the voltage down to panel operating voltage (Vmp).

20F = -6.7C

37V x (25 + 6.7) x .003 tcoef = 3.5V change (40.5Voc)

450V / (40.5V / panel) = 11.11 panels in series

Even more wiggle room now. I’m still staying at 9 panel strings but I feel even better about this with the added wiggle room! HAHA I’m getting very excited I can’t wait to start this project!

 

[AZ Off-Gridder]

10AWG is plenty. Spools of solar wire are also usually sun/uv resistant.

Ok than 10AWG it is!!! 10AWG is 31Cents cheaper per foot! Also cables will be underground, likely in conduit because we have pesky underground dudes here in the desert that like to go munch. Mmmm… 400Vdc roasted rat anyone? Yum!

 

[AZ Off-Gridder]

The Arizona Desert has underground bastards in all shapes and sizes!

 

[Hedges]

Lowest temp being early morning right as the panels begin to collect energy? Than I’d put the lowest temp at about 20 or 25 Fahrenheit to be safe.

Or am I to plan based off lowest temp during peak sun hour on the coldest day of the year?

Use the lowest recorded temperature ever seen in history for your location.
The idea is to ensure you don't kill the electronics, even during a cold snap.

A 2-pole disconnect, isolating both positive and negative of the array from inverter/charge controller is handy.
That can be per string, or for the array (especially if you use fuse per string not breaker.)
Then measure voltage on terminals of electronics, make sure it drops to safe level before touching.
Disconnect AC/battery side too. One guy was surprised he got bit with PV array disconnected.

High voltage strings up to 600VDC are no big deal to work with if they can be disconnected with a switch, and with touch-safe MC connectors.
Just check voltages before mating connectors at end of strings. Paralleling two strings of different voltages or hooking up backwards causes current flow.

 

[AZ Off-Gridder]

Use the lowest recorded temperature ever seen in history for your location.
The idea is to ensure you don't kill the electronics, even during a cold snap.

A 2-pole disconnect, isolating both positive and negative of the array from inverter/charge controller is handy.
That can be per string, or for the array (especially if you use fuse per string not breaker.)
Then measure voltage on terminals of electronics, make sure it drops to safe level before touching.
Disconnect AC/battery side too. One guy was surprised he got bit with PV array disconnected.

High voltage strings up to 600VDC are no big deal to work with if they can be disconnected with a switch, and with touch-safe MC connectors.
Just check voltages before mating connectors at end of strings. Paralleling two strings of different voltages or hooking up backwards causes current flow.

Is temp really a big deal with these kind of fancy all in one units? They’ve got so many sensors and gizmos don’t they have their own protections in place I thought?