Page 9 - 2020 January E-issue

Technical Corner

via Kathleen Connolly, NABCEP Certifed PV Installation Professional, Bergmann
George G. Landberg, PE, Mixing Equipment and Jonathon Walton, Bergmann
Company; Dr. Paul F. Pagery, Taylor Instrument
Companies; and John F. Schickler, Rochester PV Interconnection Methods
Products Division of GM. Te organization Solar PV is a hot topic in many engineering circles and many homeowners are looking to take advantage of potential cost
of the new RES Solid Waste Committee, saving that come along with installing a PV solar system on their home to offset their electric usage. Because of this, it is
Operation RESOURCE was announced, with important for engineers to be aware of code compliant ways to interconnect these systems. Here we will go through two of
its eight sub-committees all having chairs, from the most common interconnection methods and the applicable 2014 National Electrical Code sections.
local Rochester industry and consulting frms.
Additional support was invited from the community at large. Common Interconnection #1 Backfed Breaker
The first common method is commonly referred to as a “backfed breaker”. This refers to installing the inverter output breaker
May 5, 1971 (Board of Directors Meeting, Taylor
Instrument, Division of Sybron) in an existing electrical panel. This allows the solar system to provide power directly into the existing electrical panel that
Te Board approved applications for three Regular Members can then be used by the existing loads on site. Because you now have two sources of power feeding the electrical panel,
and one Associate Member. Reports from the 1971 Info the electrical utility and the PV solar system, there are concerns about overheating the busbar. In the National Electric Code
Expo Committee indicated that, while booth reservations 702.12(D)(2)(b) refers to this method. The language of the code are as follows:
had been slow, there had been much progress in assembling a
strong seminar program. James D. Row, Rochester Telephone 705.12(D)(2)(b) states that Where two sources, one a utility and the other an inverter, are located at opposite ends of
Corporation, Chair of the RES Solid Waste Advisory a busbar that contains loads, the sum of 125% of the inverter(s) output circuit current and the rating of the overcurrent
Committee, reminded the group of a scheduled open meeting protective device protecting the busbar shall not exceed 120% of the ampacity of the busbar.
with representatives of Greely-Hanson for the next day to
discuss landflls, and the organizations eventual report to the This is commonly referred to as the 120% rule. This is best described by the following example. Let’s envision we need to
City of Rochester. Subsequent meeting on Waste Conversion interconnect a single phase 7.6kW inverter to a 200A, 120/240V service with a busbar rating of 200A. The 7.6kW inverter
and Waste Incineration were also announced. Te subject has a continuous current rating of 32A. To decide if we can backfeed this inverter we would need to complete the following
of possible encroachment of the project into the feld of the calculation:
consulting engineer was also discussed, and in this connection, (125% x inverter continuous output current) + MCB rating ≤ (120% x busbar rating)
Edwin Anthony declared that it was his conviction that (125% x 32A) + 200A ≤ (120% x 200A)  40A + 200A ≤ 240A  240A ≤ 240A
since the operation was a research project, and not a design
recommendation, it does not infringe on the feld of consulting Since the statement is true, we can interconnect the PV solar system by using a backfed breaker. If the panel instead was
engineering. George Landberg reported that the evening meeting 150A with a 150A busbar rating, the inverter would not be able to be backfed because the following statement is not true.
to hear a presentation on the National Science Foundations (125% x 32A) + 150A ≤ (120% x 150A)  40A + 150A ≤ 180A  190A ≤ 180A
Deep Sea Drilling Project, by Dr. Alan Rufman of the Bedford
Institute, had been cancaled due to lack of interest. Tis brought This method can only be used in situations where the MCB and the backfed PV breaker are at opposite ends of a busbar. The
about extended discussion of the attempted focus of future RES- rational for this is to eliminate hot spots on the busbar. If the PV breaker and MCB are installed next to each other, there is the
Afliate coordinated meetings. It was announced that Mr. Oliver possibility for one place on the busbar to see the full MCB allowed amperage along with the full solar PV amperage.
Angevine, Sr., RES executive Secretary for 40 years (1924-64),
had passed away. Te Board approved a resolution, to be sent to Common Interconnection #2 Line Side Tap
Mr. Angevines family, and the establishment of a memorial fund The second common method for interconnecting a PV system is a line side tap connection. While confusingly referred to as
in his honor. a “tap,” this method of interconnection does not qualify as a tap according to the NEC. This method is typically used when
you have a PV system that cannot be interconnected by a backfed breaker because it is too large for the service doesnt
May 27, 1971 (Special Meeting of the Board of Directors, abide by the rule above, or because there are no available breaker spaces in the electrical service. The NEC refers to this
Taylor Instrument, Division of Sybron) method in 705.12(A) Supply Side. When utilizing this method, the only limitation to the size of the PV solar system is that
Te Board approved a salary increase for the RES Executive the output cannot be larger than the existing service size it is connecting to. This means that if you need a 200A breaker for
Secretary to $13,500, annually, and an increase in the weekly your PV solar system, you need at least a 200A electrical service to be able to interconnect it. When doing a line side tap, the
salary of the RES ofce secretary to $115 per week ($5,980 conductor for the output of the inverter is physically connected to the conductors between the electrical meter and MCB. This
annually). is allowed by 230.82(6) which states PV solar systems are one of the nine pieces of equipment that are able to be connected
on the supply side of a service disconnect.
Hopefully this article finds you well and can be used as a reference for your project
Subsequent articles in this series will describe the RES' continuing outreach to other
technical societies as it considered its role in this and the larger community, along needs.
with more of the activities of the RES as it moved to be of greater service to its
membership, especially those sufering from current economic crises, and adopted a If anyone would like to contribute to the RES magazine and add an article or would
greater role in shaping the future of the City and its environs. Noted also, will be the like to request information on a specific topic (not limited to Electrical) just email me at
contributions made by RES members in the struggle to meet the challenges coming beliasz@bergmannpc.com. As always, any comments are appreciated! Thank you
out of World War II and the Korean Confict, as well as a hoped-for period of post- for reading.
war growth and prosperity. Tese articles will also feature an impressive array of RES
activities in support of post-war re-emergence of Rochester area industry. Brett Eliasz, P.E., LEED AP BD+C , Kathleen Connolly, NABCEP Certified PV
RES Director Installation Professional, Bergmann
We welcome your questions and comments on this series.

res news - history res - technical corner JANUARY 2020 The ROCHESTER ENGINEER | 9