panel array to inverter wiring Question

autoxsteve
autoxsteve Solar Expert Posts: 114 ✭✭✭✭
I received the plans from the firm spec'ing out and installing my PV sys. I have a question regarding the wire gauge from the array to the inverter.

I am going to have 22 evergreen 190W panels on 2 strings feeding a Fronius 4000 inverter.

their design calls for awg 10 wire from the array to the inverter. I ran

[url=http://www.solarray.com/TechGuides/Downloads/WireSizer.xls
]this wire sizer spreadsheet[/url] and came up with awg 6 as ideal.
I put in about a 40 foot run (that's the shortest I can see) from the array to the inverter.

Am I correct in seeing that awg 6 is the best size (it will also allow me to add 2 panels - 1 per string later to get a little more power)?

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: panel array to inverter wiring Question

    The Evergreen specs:

    Max Power Voltage - Vmp 26.7 Vdc
    Open Circuit Voltage - Voc 32.8 Vdc
    Voltage Temp Coeff - Vtoc -0.1148 V/°C
    STC Rating - Pmp 190 Wstcdc
    Max Power Current - Imp 7.12 Adc
    PTC Rating 167.2 Wptc

    Or somewhere around 7.12 amps per string...

    Using the default settings from the excel solar wiring calculator... 2 strings x 7.12 amps x 267 volt system voltage...

    With 40 feet of 10 awg wire, the total power loss is 0.13% (0.045 ohms for 40' of wire at 122F/55C for 0.33 volt drop from arrays to inverter).

    Even if it was a 40' run and 80' of wire, the drop would still only be 0.24%--still well under the 3% maximum that everyone tries to stay under...

    If this was one run of 40 foot (80' of wire) 10 awg wire (14.14 amps--external combiner at the solar array and a single 10 awg run to the inverter) you would still be under 0.48% power loss...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: panel array to inverter wiring Question

    Your local inspector will probably refer to the NEC to determine code compliance. Accordingly, the wire size calculation should be based on NEC requirements, wire specs, and conditions of use.

    * The sub-array string STC Isc is 8.05 A. The NEC maximum circuit current spec is 8.05 X 125% = 10.0625 A.
    * The conductor ampacity must be 125% of the value above, so the design value is 10.0625 A x 125% = 12.6 A
    * Assuming running four wires (two per sub-array) in conduit, an 80% adjustment factor is required.
    * Ambient temperature near a PV array can be as much as 35 C above ambient. The 71 C to 80 C condition-of-use multiplier for derating for wire rated for 90 C applications (i.e., THHN and THWN-2) is 0.41.
    * The wire ampacity should therefore be 12.6 A / (0.8 x 0.41) = 38.4 A.

    #10 THHN or THWN-2 is rated for 40 A per NEC 310.16.

    Separate calculations may be warranted to achieve a specific maximum voltage drop based on distance. Remember that the wire length is twice the distance between the array and the inverter.


    HTH,
    Jim / crewzer
  • BB.
    BB. Super Moderators, Administrators Posts: 33,422 admin
    Re: panel array to inverter wiring Question

    PS: I slightly messed up on the working voltage... I did 10 panels x 26.7 VDC, it should have been 11 panels x 26.7 vdc = 293.7 VDC.

    The correct voltage actually even reduces your percentage losses to a bit less than I posted earlier.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Roderick
    Roderick Solar Expert Posts: 253 ✭✭
    Re: panel array to inverter wiring Question

    If the wire goes in conduit, and it probably needs to for Southern California, it will be considered a wet location, so don't use THWN wire - you'll need THWN-2, or USE-2 is ok. This is assuming you let them use their original #10 wire. At #8, it should no longer be an issue, and certainly will not be an issue with #6. The thing is, you would need to derate the wire for maximum temperature (gets hot where you are), and also for peak current of the array with an additional 25% safety factor, and also for a 25% safety factor required by the NEC. As I recall, our array should have had a peak current of about 14A, but after all the deratings for northern california, we didn't make it in conduit. The inspector tagged it, and I had to pull all the wire and replace it.

    It was also difficult to find THWN-2 wire in #10, but #8 was readily available, so we went with that.

    With our flagging economy, copper prices have gone up, so expect wire prices to be non-trivial. Except for cost, there isn't a downside to larger wire, within reason.

    One other consideration: your inverter terminals may not accept wire as big as #6 - better check.
  • autoxsteve
    autoxsteve Solar Expert Posts: 114 ✭✭✭✭
    Re: panel array to inverter wiring Question

    Thanks. They are calling for USE-2 and I think I'm ok with AWG 10. Thanks for the reply tho.