Solar Energy 101: Part IV

a guest post by Ken Whiteside, Director of Business Development at ONTILITY, LLC.

Solar 101

In the first three installments, we talked about the PV modules and the inverter. Now let’s look at the other parts.

Combiner Boxes: Circuits of PV modules need to be combined into home runs to take power from the array to the inverter. Combiner boxes, NEMA-4 rated enclosures containing terminals and fuses for each circuit do that job. They come in a variety of sizes depending on the number of circuits and the output from each. Larger systems can make use of sub-arrays where the output from a number of combiners feeds another combiner, called a sub-combiner,which in turn sends power to the inverter. Like all components, combiners must be sized according to the current and voltage they are expected to handle and like all components on the array side of the inverter, they must be designed and approved for DC use.
DC Disconnects: In most jurisdictions a disconnect is required on the home run before it reaches the inverter. The NEC contains specific rules about the specifications, placement and labelling of disconnects, just like it does for all other components.
AC Disconnects: Most jurisdictions also required an AC disconnect between the inverter and the load center. With disconnects on both the DC input to the inverter and on its AC output, the inverter can be completely isolated.
Back-feed Breakers: These are installed in the load center and must be sized according to the output from the inverter. They must also be designed and rated to be back-fed, installed in specific locations inside the load center and labelled according to the NEC.
Surge Protectors: These devices are typically installed on the DC side of the inverter so current spikes from lightning are diverted from the inverter and other vulnerable equipment. It is also recommended that an AC surge protector be installed on the output side of the inverter to protect it from power surges originating on the utility grid. Surge protectors come in a variety of forms and are rated according to the amount of voltage and current they can handle.
Ground Lugs: These also come in a variety of forms. The two most common for PV use are lay-in lugs and WEEBs.
Wire and Connectors: PV wire is used to connect modules together and each module comes equipped with two leads, one positive and one negative. The leads are terminated with male and female locking connectors. These leads can be extended to make the homerun to a junction or combiner box. Extension leads are either made ahead of time or on the job site, with appropriate locking connectors installed. Sometimes the PV wire from a circuit of modules will land in a junction box where the current is passed to wire enclosed in conduit to make the run to either a combiner box or the inverter. In other cases, the PV wire is run directly to a combiner box. Wire and connectors used throughout the system must be sized and rated for the current being carried and the environmental conditions in which they are being used.
A good system design will include a site drawing showing the location of the array, the equipment, and the utility interconnection, a one line electrical drawing, a structural drawing, a detailed three-line drawing, and specifications for every component and wire run, including type, rating and size, just like any other electrical job.
I hope you’ve found this helpful and that you’ll come back often for more solar industry and product information, market trends and other information that will help you better understand solar energy.


Ken Whiteside photo Ken Whiteside has been a fan of solar energy for decades. His first hands-on experience was installing solar on off-grid houses around Telluride, Colorado in the 1990’s (summer in the San Juan Mtns. - somebody had to do it). From his home in Austin, Ken writes and works for widespread adoption of solar electricity, smart energy production and use, and sustainability.

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