Even though it’s not a calculation required by the NEC, ignoring voltage drop—caused by a conductor’s internal resistance as current flows through it—can result in sizable energy losses as well as increase the possibility of the array output falling below the inverter’s minimum DC input voltage.
Good system design limits total voltage drop on DC circuits to 2% or less. Voltage drop between the inverter and its connection with the grid should usually be 1.5% or less to ensure that the inverter has enough voltage to push back the AC voltage from the grid. Voltage drop on this circuit will result in the inverter “seeing” a voltage above that measured at the point of connection; if this is too high, it can exceed the AC operating voltage window of the inverter, causing it to go offline. (See “Back Page Basics” in this issue and “Voltage Drop after NEC Requirements” in HP80.) Numerous voltage-drop calculators, such as those on inverter manufacturer Web sites, are available—though it is always wise to double-check results.