AC Aggregation on Commercial Rooftops

The ac collection system in commercial roof-mounted PV systems generally extends from the inverter ac output terminals to the low-voltage point of interconnection (POI). The exception is when the system interconnects at medium-voltage levels, in which case the ac collection system ends at the low-voltage terminals of the medium-voltage transformer. From a design perspective, the goal of ac circuit aggregation is to field an efficient system that is cost effective; meets building, fire and electrical codes; and satisfies the client. Project stakeholders must locate equipment optimally, route circuits efficiently and size conductors properly.

While ac aggregation on commercial roof-mounted systems can seem challenging—or even aggravating—it is relatively simple if you break it down step-by-step. First, find the shortest path to route circuits from the inverters to the ac combiner, keeping in mind the safety of all those who will have to access the roof after the PV system installation, and then from the ac combiner to the POI. Second, size conductors to meet NEC requirements and limit ac voltage drop. Third, identify the least-costly way to interconnect the system, based on a careful examination of the service switchboard rather than any assumptions about its internal configuration. Lastly, specify equipment that meets all site- and system-specific requirements.

Equipment Placement

The location of inverters and ac combiners in relation to the POI is the starting point for ac circuit aggregation, as equipment placement affects circuit routing.

Inverters and ac combiner. In most cases, inverter placement is relatively inflexible as product selection or Code requirements tend to predetermine inverter grouping and placement. To satisfy the rapid-shutdown requirements in NEC Section 690.12, for example, designers generally need to locate string inverters at or in close proximity to the associated PV source circuits. Microinverter placement is even more limited, as it is fixed by the location of the modules.

Hence, ac combiner location often determines design flexibility with regard to equipment placement. Typically, you should start the design process by locating the ac combiner at the point where the ac circuit will leave the roof and head down to the POI.

Circuit routing. Most systems use conduit to route and protect ac circuit conductors, with a dedicated conduit feeding each inverter into the ac collection panel. The conductors between the ac combiner and the POI are generally much larger than those for the inverter output circuit; some installations require multiple conduits and paralleled sets of conductors. Unless the project can use an existing conduit between the roof and POI, installers need to add a new conduit run down the outside of the building or through its interior to accommodate these large conductors.

Designers want to identify the shortest possible conduit path across the roof and down to the service equipment. When routing circuits across the roof, however, take care to run conduit in a way that minimizes obstructions to other rooftop equipment such as air handlers. Technicians for other trades must be able to access any serviceable equipment. Also, avoid obstructing access pathways. If this is not possible, you may need to add walkway steps to allow first responders and tradespeople to move freely about the roof.

Conductor Sizing

Though Code-mandated ampacity calculations determine minimum allowable conductor sizes within the ac collection system, designers must also keep ac voltage drop below 1.5% or less to avoid inverter nuisance tripping.

Code requirements. Designers must size conductors in the ac collection system based on the inverter’s continuous output rating [690.8(A)(3)]. Article 310 details the conductor ampacity calculations for general wiring. Electricians and engineers should be familiar with the adjustment factors and ampacity tables that cover ambient temperature, the number of current-carrying conductors per raceway and the conductor installation environment. Depending on the Code edition, conductor height may also impact ampacity calculations.

Table 310.15(B)(3)(c) in NEC 2014 specifies ambient temperature adjustments for raceways or cables based on distance above the roof. In this scenario, it is best to keep the conduit more than 3.5 inches above the roof by using standard rooftop conduit supports. Under NEC 2017, a temperature adder applies only where you install raceways and cables that are exposed to sunlight at a distance of less than 7/8 inch above the roof [310.15(B)(3)(c)].

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