AC Aggregation on Commercial Rooftops: Page 3 of 3
Inside this Article
Supply side. On the face of it, a supply-side connection seems like the easiest way to interconnect a PV system. The Code language on the subject is brief. Though 705.12(A) provides little guidance, it also imposes few restrictions: “The sum of the ratings of all of the overcurrent devices connected to power production sources shall not exceed the rating of the service.” All you need is room on the busbar to land the PV conductors, right? Not so fast—your first goal is to protect the switchboard.
In April 2010, UL’s regulatory services department published an article, “To Tap or Not to Tap?,” providing guidance on this topic. You cannot attach tap conductors to unmarked holes in a busbar. Unless a switchboard has existing holes in the busbar marked “Tap” or the manufacturer’s installation instructions detail how to make a tap, the switchboard is not listed for that purpose. In the absence of a manufacturer-identified field tapping method, AHJs should treat taps as a modification of UL-listed equipment.
The UL paper details a number of potential problems associated with the most common methods of making a supply-side connection. Drilling a hole in a busbar, for instance, reduces its cross-sectional area, which is the basis for its current-carrying capacity. Removing busbar material could lead to elevated operating temperatures or weaken the busbar. Adding lugs at unlisted locations, such as existing holes or terminals, could reduce the air gap required to insulate energized components, increasing the likelihood of an arc flash.
It is up to the AHJ to determine the appropriate recourse when installers modify listed equipment in the field. The AHJ could accept the modification, reject it or require that UL’s field engineering staff evaluate it. While a field evaluation provides the highest level of assurance that the modified equipment will continue to operate safely, it is also costly and time-consuming.
Utility coordination is another potential issue with supply-side connections. To safely connect the PV system, you need the utility to disconnect the power. This is a scheduling issue, especially if the distribution feeder supplies multiple customers or the client has significant restrictions regarding loss of power.
The last ac aggregation step is to specify electrical equipment that meets project-specific design criteria. The primary focus is on the ac combiner panel and the PV system ac disconnect.
AC combiner panel. The things requiring attention when specifying the ac combiner panel are the ampacity requirements under normal operation, the available fault current from the POI and the busbar OCPD. In most cases, ac combiner panels are not sized according to the 120% rule but rather based on the sum of the load and supply breaker, excluding the busbar’s OCPD rating [705.12(B)(2)(c)]. Another important—and often overlooked—consideration is the available fault current at the ac combiner in relation to its fault-current rating. This calculation rarely impacts equipment specification in rooftop applications because the length of the conductor between the ac combiner and the POI is generally quite long. If the utility service has a particularly large available fault current, however, and the circuit to the ac combiner is a short one with low voltage drop, take care to avoid specifying a panel and circuit breakers that the process of clearing a fault could damage.
PV disconnect. To specify an ac disconnect for the PV system, select a disconnect with the appropriate voltage and current ratings. However, it is easy to overlook more-complex requirements. If the system interconnects at a circuit breaker on the load side of the service disconnect, for example, many designers specify an unfused ac disconnect. But what about the short-circuit rating? Most unfused disconnects are rated only for 10 kAIC, and the available fault current from the main OCPD could exceed this value.
Most commercial services have an available fault current higher than 10 kA. Since most designs mount the PV system ac disconnect close to the service switchboard, there is little conductor length to lower the fault current value before it gets to the disconnect. To remedy this problem, you can use a fused disconnect with a Class R fuse, or use components that are series rated as a system. In most cases, a Class R fuse provides a higher short-circuit rating than the average commercial service, but you need to verify that the manufacturer’s rating exceeds the fault current available from the utility. To deploy a series-rated system, a Nationally Recognized Testing Laboratory must have evaluated the combination of the disconnect model with the upstream circuit breaker model and, based on the combination, assign a greater AIC rating to the disconnect. Since this setup does not provide the same level of protection as a Class R fuse, cross-check the series rating against the available fault current.
—Marvin Hamon / Hamon Engineering / Alameda, CA / hamonengineering.com