What is the best way to configure 3 strings of 12 panels to combiner box? Keep all leads short as possible or group panels to mim. shade?

    Here are the options I am considering:
    Option 1 - Minimize all leads both positive and negative to combiner box.
    Option 2 - Minimize only the positive leads
    Option 3 - Organize panels into groups so that morning sun shade moves across the the array in such a way that power is maximized.

    Comments

    Well I went with Option 2 by making the positive leads not only as short as possible but also exactly the same length.  My biggest concern is voltage drop in this system.  I did get an interesting result because when I tested individual strings before being combined I got a Voc of 415 but after combined I got a Voc of 430.  Is this normal or maybe the solar radiation changed that much in the few minutes I was working on the connections?
    Your Voc is most likely the result of higher irradiance.  Isn't voltage drop affected by the length of the whole circuit? Negative plus positive?
    You're right, the voltage drop is calculated on the length of the wire.  It is definitely true for AC current, but I still think DC may have different current characteristics for voltage drop calculations. The negative wire in DC does not act like the neutral wire in in AC, it is more like the ground in AC, thus I still think it is advantageous to shorten the positive DC as much as possible. Is there an electrical engineer out there to enlighten us?
    Voltage drop is directly proportional to the impedance of the circuit(wire).
    Impedance = capacitive reactance + inductive reactance + resistance.
    In a DC circuit impedance is equal to resistance.
    In an AC circuit (at 60 hertz) there is not much capacitive reactance, or even inductive reactance.
    The DC PV circuit consists of both +  and - wires (plus whatever is in the circuit).
    Therefore all wires must be shortened to reduce voltage drop.
    The Electrical Code limits voltage drop in any one circuit to 3%.
    That is intolerable in PV systems. Limit voltage drop to 1% or less by using larger wire sizes.
    Remember, you buy the wire once, but the power loss goes on forever.
    Max, Thanks for the reply.  My array is 260 ft from my inverter so I was very concerned about voltage drop.  I used AWG 6 wire from the combiner box at the array to the inverter.  #6 wire was the largest size wire that the combiner box and inverter could handle.  I calculated voltage loss at about 2%.  Not great, but I had limitations due to equipment.  I think a #4 wire would of had me down to around 1% drop.

    On another subject, after researching how current flows in a DC system, I realized that current flows negative to positive which is opposite of the "convention" positive to negative.  It would had been to my advantage to shorten ALL wires both positive and negative rather than shorten just the positive.  You're absolutely right on the advice to shorten all wires as much as possible on a DC system.  Just wondering if this is the reason some systems have positive ground.
    Dan,Another way to reduce voltage drop is to use larger wire as you know.A trick I picked up about 40 years ago is to remove a few strands of wire from a large conductor so that it fits the terminal.The length of the conductor will still give you lower resistance, less voltage drop and therefore less power loss.Max
    Higher irradiance produces more current not more voltage, in fact it caused higher cell temperature and therefore lower voltage.
    After doing a little more research, it is recommended to have strings set up in a parallel array (ie parallel to the ground ) as opposed to vertical to the ground, since snow shading will affect the panels next to the ground or the lower portion of the roof in snow prone areas.  Thus it is better to have two strings operating at full power and one snow shaded string, than to have all three strings shaded somewhat do to vertical arrangement.
    Submitted
    10 years 6 months ago
    Asked by
    Dan Schantz
    Support topics
    Installation
    System sizing
    Wiring and circuits
    Support tags
    solar panel
    combiner box
    configuration