For this sizing example, assume the following:
- 6.8 kW grid-direct PV array
- Forty 170 W PV modules—four series strings of 10 modules each
- Module Voc: 21.6 VDC
- Module Isc: 8.7 A
- Series fuse rating: 15 A
The PV modules are wired in four series strings, so we will need to use series fusing (see the “Why Series Fusing?” sidebar). We also want to combine output wires to simplify the wire run (three output wires are much easier to work with than nine).
To determine the needed maximum combiner box voltage rating, find the maximum system voltage of a series string by multiplying the Voc of one module by the number of modules in the string, times a temperature correction factor. The module Voc temperature coefficient is commonly used in this calculation, but for simplicity we use NEC Table 690.7’s worst-case temperature correction factor of 1.25:
21.6 VDC x 10 modules x 1.25 = 270 VDC
The PV modules have a series-fuse rating of 15 A, so we need a combiner box that can accommodate at least four 15 A OCPDs. For four 15 A OCPD, our PV combiner box must have a maximum rated output current of at least 60 A.
Given the above, we have many options, including the AMtec Solar Prominence 6R, Blue Oak HCB4, groSolar ReadyWatt 10x10HV, MidNite MNPV12-250, OutBack FWPV8, or SMA SBCB-6.