Replacing solar panels and inverter

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mickri
mickri Registered Users Posts: 17 ✭✭
I recently bought a home where the owner had died and his heirs removed the existing panels and the grid tie inverter.  I am going to install new panels and a new inverter. This will be the second time the panels and inverter have been replaced. 

A little history.   The system as originally installed in 2003 on the garage roof.  The owner of the house at that time was a solar installer.  Don't know if he had his own company or worked for someone.  When he moved he took the solar panels and inverter with him.  There is a record online of permits for the original installation.  The owner that I bought from had the previous owner come back and install new panels and inverter.  These panels were also installed on the garage roof. No record of permits that I can find online for this reinstall.  The roof is leaking from the previous installation.  Also neither the garage roof or the house roof are optimal for solar panels

I plan to do a ground mount in the backyard with the panels at the proper tilt and oriented as close as possible to due south.  My system will have 2 strings of 6 panels each.  One string of 340 watt panels and one string of 300 watt panels.  3840 watt system.  Why the different size panels.  I bought the panels at an auction where the company was going out of business.  I would like to be able to install another string of 6 panels in the future.  Probably never happen but I like to plan ahead. 

Any problems with using an oversize 6000 watt Sunny Boy inverter to allow for future expansion?  This inverter allows up to 3 strings.

Grounding issues.  The ground mounted system will be installed on steel pipes set in concrete.  Will I need to have a copper ground rod or will the steel pipes be a sufficient ground.   Will having a copper ground rod and steel pipes create a ground loop?  I already have the copper ground rod and the steel pipes.

Wire size.  The inverter will be located at the array.  The run to the disconnect at the panel is approximately 70' to 75' from the inverter.  What size wire should I use to connect the inverter to the disconnect? 12 ga, 10 ga or ????

Any issues that I should be on the look out for.

I am familiar with low voltage solar systems.  I lived off the grid for 9 years with all of my electricity provided by solar.  That was a 12 volt system.   


Comments

  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
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    I'm NOT the grid-tied person, but look at the voltage of a string of panels if 60 cell panels with a VMP of 30-32 a string of 6 would only be in the 240-250 volt range perhaps too low to work effectively. If 72 cell panel with a vmp of 35-40 you might be okay. 

    You will likely need a new permit for the install, you will want to check with power company.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
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    Depending on where you live--The new NEC/Solar codes can be a much bigger pain in the neck, vs times past. Ground Mounted Racking does save some of the Home+Roof issues of access, remote shutdown, firefighter access, etc.

    If this was a "repair"--You might be able to bypass the permitting and/or get grandfathered in for some Code issues.

    You need to also look at your utilities present rate structure... In some areas, GT solar has new rate plans which can dramatically reduce return on investment (time of use peak charges into the night) and some areas are prohibiting new installs---Actual prohibitions, or rate plans that just do not make economic sense.

    In theory, your utility has already "engineered" their network for the size system already permitted. It is possible that if you go with a larger system that they may require new engineering and possibly even new network upgrade costs for your system today.

    I would get these issues worked out first before buying any hardware.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mickri
    mickri Registered Users Posts: 17 ✭✭
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    From Google earth pictures the prior system had 18 panels mounted on a metal frame work of some sort  titled to the southeast on the garage roof.  The garage roof is almost flat.  1 in 12 to at most 2 in 12 pitch.  The roof angles to the east.

    I am in California with PG&E.  The rate plans for solar are not great.  But then I am only looking to cover some of my electric bill.  This house is all electric including heat.  Most of the heating comes from a wood stove in the living room.

    I already have the solar panels and piping to ground mount the array.  There is an existing solar disconnect next to the panel with a 30 amp double pole breaker in the panel and 30 amp fuses in the disconnect box.  The only thing that I am missing is the grid tie inverter.  Will need to run wire from the array to the disconnect box.  I doubt that my proposed system is larger than the prior system.  Most likely it is smaller.  The existing electrical meter is a reversing meter. 

    I wonder if PG&E even knows that the prior system has been removed or will notice my replacement. 
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
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    As long as you are still a GT Solar/Net metering customer (make sure you are "registered" as a GT customer)... PG&E will not care that you "disappeared" and "reappeared" with your GT system (that I have seen with my system and PG&E--Which failed for a while until I could get new parts).

    Longer term... Insulation, double pane windows help. And mini-split heat pump systems can be a big help. Both for A/C usage (no ducts to lose heat on--Modern heat pumps are more efficient) and for heating. In reasonable weather (higher than 32F or so)--A mini-split heat pump is about 3x more efficient that a pure resistive (electric) heating system. And they can do surprisingly well below freezing (efficiency numbers keep changing--I am not in the business--But can be worth looking at).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mickri
    mickri Registered Users Posts: 17 ✭✭
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    I have looked into mini split systems.  My biggest complaint is the big ugly box hanging on the wall.  And they only heat/cool the room they are located in.  I have been researching heat pump heat/ac systems too.  What I am seeing is below 35 degrees their efficiency drops off so much that you need backup electric heat.  It is a little sketchy on how well they do in really hot weather.  My winter lows average 30 degrees give or take 5 degrees with summer highs ranging from 105 to 115.  My house only had a small wall heater and a small wall a/c.  I have removed both of those and will be installing a fau/ac system.  Still debating heat pump verses traditional electric heat/ac.

    To my knowledge I am not a GT solar net metering customer.  I am thinking about getting my system up and running then contact PG&E to let them know that the solar system is working again.  Ask if that changes my rate plan.  See what they have to say.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
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    Here is a busy Mini-Split thread you can look thru... Some members use them even with snow on the ground.

    https://forum.solar-electric.com/discussion/23007/mini-split-update-for-offgrid/p1

    Before you spend the money... I would get PG&E's approval first (hi guys, I am just repairing the existing GT System--Can you put me bak on the Net Metering plan?).

    Don't spend the money on hardware until you find out what their requirements may be.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mickri
    mickri Registered Users Posts: 17 ✭✭
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    I would like to get this thread back to my original question about wire size.  The Sunny Boy inverters that I am considering list the max output amps for the 3.8  at 16 amps and for the 6.0 at 26 amps.  My understanding of wire sizing is that 12 ga wire is ok up to 20 amps and 10 ga wire is ok up to 30 amps.  I don't know if the approximately 75' run would require going to a larger wire size.  Or if I used aluminum wire what size the wire should be.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,468 admin
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    Do you have a link for the Sunny Boy Inverter(s) you are thinking about? I think you are talking about 3.8 kWatt @ 16 Amps @ 240 VAC or 6.0 kWatt @ 26 amps @ 240 VAC (just to be clear).

    Depending on the size of the array--If you have a full/over sized array:
    • 16 Amps rated * 240 VAC * 1/0.77 panel+inverter deratings = 4,987 Watt max "cost effective" array
    • 26 Amps rated * 240 VAC * 1/0.77 panel+inverter deratings = 8,104 Watt max "cost effective" array
    GT inverters will limit their output power to maximum rated current/energy safely and part of normal operation. More or less, the average array output energy on warm to hot days under full sun derate their output energy (and losses of the inverters) by around 77%. So, over sizing the array a bit is normal for the "best bang for your buck".

    Because the GT inverter can output their rated current for several hours under full sun, the NEC has a derating for maximum "continuous" current of 1.25x (or 80% of wire rated continuous current). Wiring can get "warm" and circuit breakers (in North America) are designed to Not Trip at 80% or less of nameplate current. And will trip (minutes to hours) if operated at 100% ore more of rated current. So, I would highly recommend that your branch circuit/breaker ratings be:
    • 16 amps * 1.25 NEC derating = 20 Amp rated branch circuit (minimum)
    • 26 amps * 1.25 NEC derating = 32.5 Amp rated branch circuit (minimum)
    Using the NEC table (this is a simplified version of the "real NEC" tables:

    16 Amp GT inverter => 12 AWG minimum in copper (90C) or 12 AWG copper clad aluminum
    26 Amp GT inverter => 10 AWG minimum in copper (90C) or 10 AWG copper clad aluminum

    I am not the NEC solar code expert... Depending on how and where you run your wiring/conduit--The can be more deratings (conduit fill, ambient temperatures in hot attic, etc.). Refer to NEC/Code for your building department requirements.

    Note that 14/12/10 AWG wiring is already "derated" as max current in code anyway at 15/20/30 Amps (I have never figured out why).e
    Also check the manuals for your inverters. The manual will list wiring requirements and whether or not they can take CU/AL aluminum wiring or not.

    Aluminum for house wiring has a poor history. In older homes (built in the 1960-1970's or so) Aluminum wiring was used in homes whose sockets and such were designed for copper wiring. Aluminum wiring had a habit of "creep" where it would get warm/hot in normal operation and expect--And squeeze out from under the screw a bit. Then cool and contact exposing a bit of the raw aluminum to air--And oxidize the surface (in seconds). Then next cycle of hot/expand/cool contract/oxidize a bit more... And after years of operation, the contact area would become high resistance and burn out the connection or even start fires... So make sure everything is rated for CU/AL or stick with copper.

    https://inspectapedia.com/aluminum/Copper_Clad_Aluminum_Wire.php (copper clad aluminum wiring--Should be OK)

    Then there is aluminum wiring that is typically used for drops from pole to home and other longer distance runs. It is still used today, but use compression/crimp connectors (with a special grease--I think) to make gas tight/reliable connections. Generally use a transition compression connector to connect AL cable to copper cable for final run into the home.

    I don't have a quick link for rating of Aluminum cable, but it is something like 2 AWG heavier vs the Copper version (as I recall).

    Then there is sizing the wire for voltage drop (or voltage rise in this case) from GT inverter output to your main panel to the pole... Generally try for 1% to 3% drop (rise).

    For example, 16 Amps @ 240 VAC @ 75 feet of copper cable will be (using voltage drop calculator):

    https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=1&necconduit=steel&necpf=0.85&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=240&phase=ac&noofconductor=1&distance=75&distanceunit=feet&amperes=16&x=36&y=25&ctype=nec

    Result

    Voltage drop: 4.19
    Voltage drop percentage: 1.75%
    Voltage at the end: 235.81

    So less than 2% voltage rise (really 240 VAC + 4.19 volt rise = 244.19 volts at the GT inverter terminals).

    There have been issues with folks at the end of rural electrical systems where the voltage regulation may not be as close as in the city. More or less, the maximum 240 VAC line voltage should be around 264 VAC. If your utility power is 260 VAC + 4 volt "rise", the GT inverter could shutdown because of "over voltage" line condition. Typically, you should see around 240 to 250 VAC (at least at my home)--So not usually a problem for most folks (and you can call the utility if you have high or low line voltage and they should fix the problem).

    In this case, "over sizing" the wire by 1.25 NEC continuous current derating also helps with keeping voltage drop/rise down to "acceptable" levels.

    Again I am not an NEC code guy... Confirm with your local code book (and electrician/inspector if needed). be careful on getting help. Not all electricians or inspectors will understand solar power requirements (or anything outside of their day to day work)--And they can confuse the issues sometimes.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Photowhit
    Photowhit Solar Expert Posts: 6,003 ✭✭✭✭✭
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    mickri said:
    To my knowledge I am not a GT solar net metering customer.  I am thinking about getting my system up and running then contact PG&E to let them know that the solar system is working again.  Ask if that changes my rate plan.  See what they have to say.
    Please understand that grid tied, and off grid are completely different systems.

    If you connect to the grid without the power companies knowledge, Lots of ugly things can happen!
    The power company can disconnect you!
    If you have a new electronic meter, you may be charged for electric passing through it in either direction!!! Means you could be paying to send electric to the grid!!!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • mickri
    mickri Registered Users Posts: 17 ✭✭
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    This is my current plan.  I am going to get everything ready to go but won't throw the switch..  Talking with my neighbor today one of his best friends is the PG&E field supervisor (probably wrong title) for where I live.  He'll ask his friend to check out my system.   I will then contact PG&E to switch over to a GT rate plan.  That's the plan at this moment.