Wire sizing question

rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
I'm helping a neighbor install a solar array. It consists of 21 280 watt panels. I've wired them into 7 strings of 3 to the Midnight combiner box. Now I need to run a wire to the Outback load center. It is 35' one way. Each string of panels will produce 15 amps at about 110v. for a total of 105amps. The combiner box is set up to handle #2 wire but I was afraid that might not be large enough. What size wire would be appropriate for that amp load and wire length?
Thanks
Larry

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question

    Hit the brakes.

    21 * 280 = 5880 Watts * 0.77 / 48 Volts nominal = 94 Amps.

    Not even a MidNite Classic can handle that. Time for redesign.
  • softdownsoftdown Solar Expert Posts: 2,403 ✭✭✭✭
    Re: Wire sizing question

    Check into virtual tracking. Pointing some panels to the west and to the east will reduce the overload to the charge controller. That is the key benefit of virtual tracking.

    http://forums.energymatters.com.au/solar-wind-gear/topic5064.html
    First Bank:16 180 watt Grape Solar with  FM80 controller and 3648 Inverter....Fullriver 8D AGM solar batteries. Second Bank/MacGyver Special: 10 165(?) watt BP Solar with Renogy MPPT 40A controller/ and Xantrex C-35 PWM controller/ and Morningstar PWM controller...Cotek 24V PSW inverter....forklift and diesel locomotive batteries
  • BB.BB. Super Moderators, Administrators Posts: 29,511 admin
    Re: Wire sizing question

    What is the Vmp of the panels/array? 110 VDC might be a bit high if high desert/cold nights (Voc-cold-array might be over 140 VDC). Have you used the Manufacturer's string calculator to double check?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Wire sizing question

    That wasn't the question, but since you are curious, there are 2 fm 80's for charge control.
  • mike95490mike95490 Solar Expert Posts: 8,373 ✭✭✭✭✭
    Re: Wire sizing question
    21 * 280 = 5880 Watts * 0.77 / 48 Volts nominal = 94 Amps.

    Well, it will be a problem if the sun comes out from behind heavy clouds at noon, and trys to slam 5.8Kw into the batteries, but normally, the morning current rise will slowly bulk up the batteries, and they may not demand the full 94 amps..

    And I think you can program the classic to limit the input and output amps, so you can over panel a charger, and not cook it. (instantly)
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • gww1gww1 Solar Expert Posts: 963 ✭✭
    Re: Wire sizing question
    That wasn't the question, but since you are curious, there are 2 fm 80's for charge control.

    The way you phrase the question in your first post, that was the question. You didn't say you were running two arays you ask about over 100 amps on the run.
    gww
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question
    rplarry wrote: »
    That wasn't the question, but since you are curious, there are 2 fm 80's for charge control.

    May not have been your question, but it is a problem.

    This is very important: you do not connect one array to the inputs of two MPPT charge controllers.

    Therefore your original premise is also wrong as the current from the total panels must be divided between the two controllers and carried on separate wires.
  • rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Wire sizing question

    Oops, now that I have recovered from my brain fart, let me try again. 7 strings of 3 panels, I will run 4 strings on one leg in the combiner box and 3 strings on the other leg so I will have a potential of 60 amps on one leg and 45 on the other. Is #2 wire large enough to handle 60 amps on a 35' one way run?
    Thanks for rattling my cage enough to wake me up.
    Larry
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question

    2 AWG will handle 60 Amps no problem.
    Now about that Voltage ... 110? Is that possible Voc * 3? Because you should be calculating using Vmp, and on a 280 Watt panel that's usually around 30 so X3 would be around 90.

    In any case the V-drop on 35' of 2 AWG even at 60 Amps and 90 or 110 Volts is going to be less than 2%. You could probably use 6 AWG in fact.
  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    Re: Wire sizing question

    I see another anomaly, Panel amps...... 280 W / 30v= ~9.3A not 15 as stated in post #1. Amps are not cumulative in a series configuration only Volts
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question
    westbranch wrote: »
    I see another anomaly, Panel amps...... 280 W / 30v= ~9.3A not 15 as stated in post #1. Amps are not cumulative in a series configuration only Volts

    He is probably going by the maximum series fuse rating, which is of course not right. That rating does not enter in to figuring V-drop (and therefor power loss).

    If you take the maximums given and multiply them out you get 110 Volts * 60 Amps = 6600 Watts, which is well above 21 panels * 280 Watts each - 5880 Watts.

    You have to use the right numbers to begin with or else the calculations all fall down.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question

    Oh and if the panels really are 36 Vmp (110/3) then the current is <8 Amps which times four strings brings it down to 32 Amps of current from the combiner to the controller and that's even easier to handle.

    But it then brings up a new problem of Voc being about 44 per panel and X3 gives 132 per string and you're now in the realm where a bit of cold will push the Voc up to where the FM60 will shut down.

    There's always something looking to trip you up.
  • westbranchwestbranch Solar Expert Posts: 5,142 ✭✭✭✭
    Re: Wire sizing question

    Ya, the CC specific string sizing calculators sure make it easy, and avoid mistakes:roll:
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • rplarryrplarry Solar Expert Posts: 203 ✭✭✭✭✭
    Re: Wire sizing question

    Yes, I am using the max rated amperage of the panel that is how I came up with a 15 amp potential in the string of three. I would rather over wire it than under wire it. also my neighbor happens to have some #2 wire left over from another project so Looks like we can use that. Thanks for your patience with me.
    Larry
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Wire sizing question
    rplarry wrote: »
    Yes, I am using the max rated amperage of the panel that is how I came up with a 15 amp potential in the string of three. I would rather over wire it than under wire it. also my neighbor happens to have some #2 wire left over from another project so Looks like we can use that. Thanks for your patience with me.
    Larry

    This is unnecessary.
    The series fuse rating has nothing to do with the maximum output of the array. It's about protecting the array (and your house) in case something goes wrong and one string shorts, thus becoming a conductor. At that point it would be subject to the combined Isc of the other strings. In this case you could be looking at 9 Amps Isc * 3 'good' strings dumping a total of 27 Amps to one 'shorted' string which is only capable of handling 15 Amps. Thus the circuit breaker trips and protects against the shorted string from heating up to the point of ignition.

    A panel or string of panels with an Isc of 9 Amps is never going to produce 15, even under super-optimum conditions.

    So in reality you have 280 Watt panels with a Vmp of ~30 and an Imp of ~9.3 configured as four parallel strings of three in series giving an array with Vmp 90 and Imp 37.2 which are the numbers used when calculating wire size and Voltage drop. V-drop matters most at this maximum power point, and all wires/conductors can take momentary surges above their continuous rating. As such using the 110 Voc is not right, nor is using the maximum series fuse rating X4.

    But the 2 AWG wire is going to handle anything that array can produce. The downside is having to work with the heavy wire, especially as the charge controller won't accept it on its input terminals. You will probably need to transition from 2 AWG down to 6 AWG using crimped lugs & bolts or saddled bus bars.
Sign In or Register to comment.