solar and battery config with 12/2 wire bottleneck?

OffGridInTheCity

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In my city - electrical codes forDC 48v (or higher)require red/black wires inmetal conduit within the home (in attic, walls, etc) andAC does not (can just run the wire). I think this might affect DC vs AC 'bias' beyond just wire/amps/voltage. I've read that this is one of the pros of 'solar panel' inverters - to run AC from panels on roof into attic, you don't need conduit in attic. Codes.... (sigh).
 

daromer

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You dont need your panel voltage to be 1.5times larger just to the most efficient. That is old saying for pwm controlles and cheap chargers. Mine is hvdc and is around 800vdc....
 

wattwatt

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Wow, lots of useful info. I'm lost AF now, but I'll re-read it again with Google and get up to speed.

However, I did just remember that I analyzed my past electric bills not too long ago to get some usage stats and discovered that depending on the config I go with,I can just barelypower the whole home withjust solar panelson the garage roof alone! I used as a reference for dimensions (and I guess wattage and budget),Renogy's 24v 300W 10 pack panel kit - I can fit 12 total panels on the garage roof alone (and down the roadpotentially 12 to 15 more panels on my main house roof). That's 3600W/3.6 kWh (if I go with a 24v system instead of the preferred48v) and according to my electric bill I'm averaging 2.64 kWh per hour (64 kWh per day, 1977 kWh per month). I said barely earlier becauseI'm assuming panel efficiency will bring that energy down somewhere between 3.6 kWh and 2.64 kWh and then there's watts lost during theconversion. Of course I know this is only during peak sun hours, but still.

Is my work correct on this? If so, can I push this 24v systemdown a 12/2 wire if I followed the 240V ACsuggestion orthe "pushDC over the 12/2 instead of AC" suggestion? Of course, that's a $4000 cost for just the panels alone so this is a high end, "do it more right than dirty" option.


P.S. - In case anyone's wondering - my original plan was buy some 100 watt panels usedoff Craiglist just because I havea spare cheap charge controller and inverter laying around... just because. Then I remembered that 12/2 wire coming from the house and our$250/month electric bill and started taking this a little moreseriously
 

Wolf

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Time to dig a trench with 10/3 underground wire.
Quick and dirty has just become more complicated

Wolf
 

Redpacket

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daromer said:
Mine is hvdc and is around 800vdc....

Wow 800V is pretty high! Are you worried about PID "potential induced failure" ** - some panel might get this, hopefully not yours!
(** where there's creepage due to the high voltage between the cells & the frames & you get ground faults, some panels here in Australia have had this issue)
 

wattwatt

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Okay, so to wrap things up... if I wanted to stick with that 12/2 wire and stay within it's spec and if I have all DC stuff in the garage then I should aim to keep ACpushedunder roughly1800 watts to avoid constantly spiking/peaking and stressing that 12/2 wire??

And there's no consensusthat pushing DC instead ofAC orpushing AC at 240 volts instead of 120 volts over that 12/2 wire is safe orworth it?

Otherwise, if I did want togo with the high end plan of12 300 watts solar panels, then I need to dig another trench and run at least10/3 wire? Should I just run 8/3 instead to future proof things?
 

Redpacket

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DC is out for the safety reasons above.
With AC you can run the rated current for that cable allowing for buried underground conditions.
You don't have to worry about short bursts for higher current, ie you could easily get double the rated current along it for a few minutes safely before temperature rise started to kick in.
Cables like this are rated for average current & won't blow or melt suddenly.
Suitable breakers should be used in the garage gear anyway to protect the cable.

In reality, will you have more power available in the garage than the existing cable can handle? Probably not :)
 

daromer

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Redpacket. Most home owneres here have 300-800HVDC systems on the solar side.
No I dont worry about issues there. That would be like worrying going over the street every time you do that :p
 

Wolf

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daromer said:
You dont need your panel voltage to be 1.5times larger just to the most efficient. That is old saying for pwm controlles and cheap chargers. Mine is hvdc and is around 800vdc....

Eh well shoot me and paint me red. First I did not say "most efficient" I said "For your MPPT controller to work at the best efficiency your solar panel voltage should be at least 1.5 timeshigher than the nominal battery voltage."

Whether that is an old saying or not you are not really going to charge a 48V battery with a 35V PV string no matter whatthe controller is whether PWM or MPPT expensive or cheap.
1.5 timeshigher is a good starting point and a good rule of thumb especially if you are getting started and remember his original post was "quick and dirty" not expensive and complex.
Also If you consider MidNight Solar Classic 150 cheap which recommendsthat thePV input voltage should be at least 30% higher than the highest expected battery voltagethen Boy Id like to see your controllers :)Oh wait I have.

image_rooxfh.jpg

My system runsat 2 to 2.5 times the battery voltage maybe a bit more on occasion and if the OP would put 3 panels in series he would realise that also.
Not all of us can and for that matter of fact want a800VDC PV array.

Redpacket. Most home owneres here have 300-800HVDC systems on the solar side.
No I dont worry about issues there. That would be like worrying going over the street every time you do that

Well in Sweden I guess HVDC is common and that is great!!Asfar as crossing the street oh you bet I'm gonna worry especially if you are on the road and driving. :p

image_kggbtm.jpg

image_vmswbf.jpg

Wolf
 
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