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4 400W panels, 2S2P. Am I thinking about this correctly?

Madcodger

Solar Enthusiast
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Hi Everyone. I've been lurking on the site and learning, and really appreciate both the knowledge and friendly attitudes of people on the forum. My question today is about how to size and connect the wiring from the solar panels to the charge controller, which I'll get into in a moment. I think I have it right, but figured it made sense to ask the gurus here. Here's the overall system:

Totally off-grid, 200SF (10x20), insulated building (R51 ceiling, R19 walls, R21 floor) with a south-facing roof. We live in Maine (temps as low as about -20F but hasn't hit that for several years now) and I'm figuring only about 2.5-3 hours of sun per December day as theres a little bit of afternoon shade on the building, late in the day. Inverter charger will be a Victron Multiplus 24 volt, 3000KVA inverter-charger even though my load requirements are only about 1100 WH per day in winter, and perhaps 2200 WH per day in summer. I went with a larger inverter than I needed because I wanted enough excess capacity to run a coffee maker or some power tools now and then, and I may also use the building for another purpose one day. Battery will be 24V, either 100AH or 200 AH, which I'm still trying to figure out based on how often I want to fire up a generator (I'll probably post a second question about that, later).

I plan on installing four solar panels on the roof of this building, anywhere from 340-400W each, depending on the best deals and shipping/pickup locations I can find, and the vertical space limitations of the building. I really need to keep the panel size under about 70-72" long to fit it on the roof of this small building. For planning purposes, I've been using the upper end of specs for panels I'm interested in, which are (about):

35V VPM / 42V VOC
11A IMP / 12A Short Circuit


I'm planning to hook these up in a 2S2P configuration using 30A MC4 connectors at the panels and then running two wires (one positive, one negative) down to a 30A fuse or breaker (comments on which is better appreciated), then a marine cutoff switch, and then into the solar charge controller (will also be Victron). By my calculations, that should give me a max of 84Volts, 24 Amps INTO the charge controller. When I run the calculations through a solar charge controller calculator and account for a -20F temperature, it tells me that this could be a voltage as high as 105V. So.....

1) I'm thinking that a 150 volt, 60 amp charge controller should be fine. Correct?
2) I'm thinking that 14 AWG wire should be fine all the way down to the controller. Correct?

Thanks in advance for help, advice, and guidance.
 
1) I'm thinking that a 150 volt, 60 amp charge controller should be fine. Correct?
4x 400W / 28V charging = 57A

Looks good.

2) I'm thinking that 14 AWG wire should be fine all the way down to the controller. Correct?
Need to know volts, amps and distance to calculate wire needs.
Looks like 15' is max distance for 14awg
Screen Shot 2022-08-24 at 9.16.09 AM.png
 
Two 12 Amp panel in parallel is 24 Amps which is a little over nominal Amperage for 12 Awg. I would consider 10AWG especially if the distance is over 20 feet.
 
Distance will be about 30 feet' max from the furthest panel and down inside the building (a bit less), but it's certainly easy to go to 10AWG. Many thanks for the input. Happy to hear from others as well with ideas and input.
 
Also... Any reason not to go up another 10 amps on the charge controller, to a 150/70? It's only about a $50 difference and it seems like it might give me a little buffer given that we're not close on voltage but amps are getting near 60.
 
but amps are getting near 60.
My calculation on Amps is 24 Amps based on two 12 Amp panels in parallel. Has the configuration changed from 2S2P? For most controllers Voltage is the critical constraint and allowance for increased voltage when panels are cold is why you want headroom there.
 
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Also... Any reason not to go up another 10 amps on the charge controller, to a 150/70? It's only about a $50 difference and it seems like it might give me a little buffer given that we're not close on voltage but amps are getting near 60.

Just in case there is any confusion, the amp rating in the 150/60 or 150/70 etc models refers to the max output it can charge the batteries with, not the max pv input current.


The data sheets show that both the 150/60 and 150/70 both have a 50a pv input limit, but you will be well below this running 2s2p. I've read it's ok to surpass the input rating and the controller will just ignore the extra current, but again it won't be an issue with a 2s2p configuration
 
I've read it's ok to surpass the input rating and the controller will just ignore the extra current, but again it won't be an issue with a 2s2p configuration
And to add to that it is often not okay to exceed voltage limit because that can damage controller. So calculating increased panel voltage from lower temperatures is an important reason to have more headroom on voltage.
 
My calculation on Amps is 24 Amps based on two 12 Amp panels in parallel. Has the configuration changed from 2S2P? For most controllers Voltage is the critical constraint and allowance for increased voltage when panels are cold is why you want headroom there.
The configuration is still 2S2P. My logic is the same as @MisterSandals , in that 4x400W of panels divided by a charging voltage of 28 V (or a bit more) for a 24V system puts me at almost 60 amps.
 
It's perfectly ok to overpanel most MPPT controllers to some extent. People do it for poor exposure/poor atmospheric situations. Also, you should not expect more than 75 - 80 % of panel ratings in real world conditions. That being said if you don't anticipate adding any more PV on the roof due to space limitations then staying with the 150/60 should be just fine.
 
Just in case there is any confusion, the amp rating in the 150/60 or 150/70 etc models refers to the max output it can charge the batteries with, not the max pv input current.


The data sheets show that both the 150/60 and 150/70 both have a 50a pv input limit, but you will be well below this running 2s2p. I've read it's ok to surpass the input rating and the controller will just ignore the extra current, but again it won't be an issue with a 2s2p configuration
Hmmm... With respect, that does not seem correct to me. The data sheet (see https://www.victronenergy.com/solar-charge-controllers/smartsolar-250-85-250-100) shows that 50A is the max short circuit amperage. However, the max nominal PV power for a 150/60 on a 24V system is 1720, while the max nominal PV power for a 150/70 is 2000. It's the power over either of those limits that would be ignored and not used, per the data sheet. So, there's not a reason for me to go up to a /70 unless my panels will exceed 4x430. But it does not appear that 50A is the limit the charge controller will use. Please help me understand if you still think that is incorrect.
 
Hmmm... With respect, that does not seem correct to me. The data sheet (see https://www.victronenergy.com/solar-charge-controllers/smartsolar-250-85-250-100) shows that 50A is the max short circuit amperage. However, the max nominal PV power for a 150/60 on a 24V system is 1720, while the max nominal PV power for a 150/70 is 2000. It's the power over either of those limits that would be ignored and not used, per the data sheet. So, there's not a reason for me to go up to a /70 unless my panels will exceed 4x430. But it does not appear that 50A is the limit the charge controller will use. Please help me understand if you still think that is incorrect.
I was just pointing out that the amp rating in the model number (the "70" in the "150/70" for example) was the max charge current the unit is capable of, not max pv input. Sorry if I added any confusion!
 
My logic is the same as @MisterSandals , in that 4x400W of panels divided by a charging voltage of 28 V (or a bit more) for a 24V system puts me at almost 60 amps.
Perhaps @MisterSandals can clarify his calculation since you are relying on his logic. He may have been referring to the potential charge Amperage coming out of the charge controller, not the Amperage going in. If you do have a 2S2P then the maximum voltage and Amperage going into the charge controller would be double the max of a single panel or 24 Amps and 96 Volts. But a 4P array would be 48 Amps but only 48 Volts. Conversely a 4S array would be 192 volts and 12 Amps. My assumption is the Voltage of a single solar panel is 48 Volts and the Amperage is 12 Amps? It is important, for clarity in these conversations, to distinguish between the voltage and current going into the charge controller from the panels and the voltage and current going out to the battery.
 
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Perhaps @MisterSandals can clarify his calculation since you are relying on his logic. He may have been referring to the potential charge Amperage coming out of the charge controller, not the Amperage going in.
Yes charging amps. I thought that was clear given that I mentioned charging (volts) in the same bit...

4x 400W / 28V charging = 57A

The other bit i mentioned did not consider the amapcity of the wire so your recommendation of 10awg was good (i appreciate it when others follow up with better answers).
 
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