I started this series of articles after a friend asked some questions about overcurrent protection for the direct current (DC) output of photovoltaic (PV) modules, the DC output of combiner boxes, the location of disconnects for both the DC side and the alternating current (AC) side of the inverter, and the sizing of conductors on both sides.
Let’s begin with the DC installation from the combiner box to the inverter plus the AC connections from the AC side of the inverter to the service or feeder panel at the point of connection to the utility interface.
The parallel connections of modules will increase the available current. These connections can be accomplished at a combiner box located close to the modules or at the inverter. Some inverters include PV combiner functionality with overcurrent protection in their PV source input circuits (the output of the modules). The inverter/combiner overcurrent protection ratings must match the PV module’s maximum series fuse rating.
In this case, a set of conductors, sized in accordance with 690.8(B), from each series string could be run all the way to the inverter and would not require additional fuse protection. However, installing a combiner box at a location close to the PV array will permit the strings to be paralleled at the array with one set of larger conductors to be run to the inverter at a much lower cost. In most cases, the combiner box fuse holder is not designed to be opened under load conditions, so the inverter must be first disconnected before opening the fuse holder within the combiner box.
Based on 690.16(A), a fuse that is energized from both directions must be able to be disconnected from all sources of supply (a fuse in the combiner box can be energized from the module side and from the DC side of the inverter). A fuse in a PV source circuit (the source circuit is the PV module) must be able to be disconnected independently of other fuses protecting other PV source circuits—hence the reason for the individual fuse blocks within the combiner box. Blown fuses or tripped breakers in PV circuits are most often an indication of a serious wiring or module failure within that PV array string. Replacing a blown PV fuse or resetting a PV breaker should only be done after the failure within the array has been properly located and repaired.
Section 690.13 states that means must be provided to disconnect all current--carrying DC conductors of a PV system from all other conductors of the building or other structure. A switch, circuit breaker or other device shall not open a grounded conductor if opening it leaves the grounded conductor in an ungrounded and energized state. The grounded conductor can be opened where part of a ground-fault protection system or where the disconnect switch is for PV array maintenance, the switch is accessible only to qualified people, and the switch is rated for the maximum DC voltage and current that could be present during any operation of the system. Where the utility interactive inverter is installed at a readily accessible location, the PV disconnecting means, both DC and AC, must be installed at a readily accessible location either on the outside of the building or inside nearest the point of entrance of the PV system conductors. Where the inverter is not located at a readily accessible location, both DC and AC disconnects must be installed in sight of or in the inverter, and an AC output disconnecting means must be installed in accordance with 690.14(C)(1) at a readily accessible location either outside on the building or inside nearest the point of entrance into the building.
The sizing of the AC conductors at the output of the inverter is based on not less than the larger of calculations in 690.8(B)(2)(a) or (b). In 690.8(B)(2)(a), the inverter’s continuous output current must be multiplied by 125 percent without correction factors or in 690.8(B)(2)(b) by multiplying the inverter output at 125 percent with correction factors. Section 690.64 sends the installer to 700.12(D) for installation requirements for the connection of utility-interactive inverters to the line- or load-side of the service-disconnecting means. Each PV source interconnection must be a dedicated circuit breaker or fusible disconnect. The sum of the ampere (A) rating of overcurrent devices supplying power to a busbar cannot exceed 120 percent of the rating of the busbar. For example, a 200A panel with a 200A main breaker could have a 40A circuit breaker at the point of connection of the PV system to the panelboard. There will most likely be many changes for the 2014 National Electrical Code for PV systems, so stand by for more requirements.
ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and firstname.lastname@example.org.