On the south-facing rooftop, the PV modules are mounted on Direct Power & Water Power Rail mounts and wired in two series strings of five modules each. The two strings are wired in parallel in a combiner box mounted to the roof. The west-facing array of eight modules is mounted and wired similarly, but the series strings contain only four modules each. Six-gauge, bare, stranded copper wire was used between the modules for equipment grounding. The equipment-grounding conductors were transitioned to 10 AWG in the combiner boxes.
A single conduit run carries a pair of #10 conductors, plus the #10 equipment ground wire, from each array through the roof overhang and down to the balance-of-system components mounted on the house’s exterior. The positive wire from each array passes through the DC disconnect switch before the pairs terminate at the two Fronius inverters. One inverter processes 1,950 watts (Pmax) at 276.5 volts from the south-facing array, and the other processes 1,560 watts (Pmax) at 221.2 volts from the west-facing array.
On their output side, each inverter produces 240 VAC. A quartet of wires exits each inverter—two hots, a neutral, and an equipment ground. The four hot wires pass through two, two-pole, 15-amp breakers that act as the main AC disconnects and overcurrent protection for the PV system. On the line side of these breakers, the four hots are paralleled into a single pair of hot wires and join one neutral wire for the journey to the production meter.
The production KWH meter is an additional component. In many grid-tied PV systems, a single, bidirectional KWH meter measures net production from the PV system as well as electricity consumption from the grid. In Kathleen’s case, her utility meter doesn’t deduct the PV-produced electricity from her utility electricity purchase. Instead, the designated production meter keeps track of the electricity produced by the PV system, which she is paid for. (See the “The Performance Connection” sidebar on page 32 for more information on how Kathleen’s system pays her back.)
From the production KWH meter, the two hot wires, a neutral wire, and an equipment-ground wire continue to the AC service entrance. The hots enter a standard household AC distribution panel through a two-pole, 30-amp, 240 VAC breaker. There, the neutral and ground wires terminate at their respective bus bars. The energy produced by Kathleen’s PV system either contributes to the mix of electricity powering her household loads or, if the system is producing more electricity than she’s using, enters the electric utility grid through her utility KWH meter.