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IAEI MAGAZINE WINTER 2023 2 BACK TO BASICS S o many interconnections, so little time. Where to begin? To start, it should be noted that all National Electrical Code (NEC) references mentioned are taken from the 2023 cycle unless noted otherwise. Any time most of us in the electrical industry discuss Article 705, it involves photovolta- ic systems. Article 705 provides guidance on interconnecting any power source parallel to a primary source. The most com- mon example would be a PV power source paralleled with THE ART OF THE INTERCONNECTION Safely paralleling power sources for beginners FEATURE Article 705 provides guidance on interconnecting any power source parallel to a primary source. Many of the requirements can best be understood with a quick 1st-year apprenticeship review. by Chris Papp an electric utility. We will begin our interconnection journey with a few basics. Many of the requirements found in 705 can best be understood with a quick 1st-year apprenticeship review. Beginning at Article 240, per Section 240.21, a fuse or circuit breaker provides overcurrent protection when installed in series at the origination of supply for con- ductors, equipment, and loads. By doing so, it assures the total circuit current is forced to travel through the ---PAGE BREAK--- WINTER 2023 IAEI MAGAZINE 3 device, which in turn guarantees the OCPD can perform as intended and effectively clear any overcurrent that may be present on the circuit. Recall that the current only has one path in a series circuit; this simple rule also applies to parallel wiring when locating the OCPD per NEC Sec- tion 240.21. [See Figure Notice that the total circuit current divides at the first node or parallel path introduced. However, the placement of the OCPD forces the total circuit current to flow through the 15-ampere circuit breaker. This simple review of our early studies in the electrical trade lays the footing for our venture into Article 705. For our venture, we will only be focusing on load-side connections, leaving connections to a service and use of energy management systems for another time. PARALLELED POWER SOURCE CIRCUIT PATH Part of what has made PV inspections and plan reviews so fascinating for me lies in the unorthodox, for most electricians’ current flows in one direction, at least for learning purposes anyways. Current travels from the utility generation source all the way down to the last outlet and back. Article 705 gives us guidance on how to parallel power sources safely. Let the fun begin! This is where our brief recap of OCPD placement will come in handy. A power sources circuit or “loop” can be completed in a large variety of pathways. Power source current will be bi-directional to the primary source and complete its circuit path via loads within the building or structure. When the power source is generating more energy than the connected load throughout the building, it can complete its path by traveling further down the distribution line. For educa- tion purposes, this path will stop at the sec- ondary winding of a utility transformer, where the power source current completes its circuit traveling through the secondary winding and back to a power source. [See Photo Understanding the parallel power source circuit path helps with load-side interconnection comprehension. Now we can better understand the reasoning behind the calculations and code text laid out in Article 705 as they pertain to load-side interconnections. WHERE CAN INTERCONNECTIONS BE MADE One question I am often asked is where interconnections can be made. Section 705.12 begins with the “where” load- side connections can be made. Parallel power sources are permitted to be interconnected at any distribution equipment throughout the premises wiring distribution system. This provides installers with many options; it also gives me much to discuss in the remainder of this article. Safely intercon- necting sources that can simultaneously supply feeders and branches is achieved by compliance through NEC Sections 705.12(A) through Before we get into a deeper dive, one more question I often receive needs to be answered. Is it permissible to interconnect multiple power sources through- out the premises wiring system? My response, it certainly would appear that way when I read 705.12. There has been a large amount of debate here surround- ing dedicated OCPD, as previously referenced in first level sub-division to 2020 NEC Section 705.12. Connecting the power source to a dedicated OCPD assures there is no Figure 1. Depicts a simple combination circuit; loads wired in parallel and series. Photo 1. Typical utility transformer supplying a building service. ---PAGE BREAK--- IAEI MAGAZINE WINTER 2023 4 combination of load and power source on a branch circuit where the power source has no overcurrent protection; it was never intended to limit the number of sources interconnected. With the large reorganization of Article 705 for the 2023 cycle, the dedicated OCPD require- ment has been removed. End users can now simply reference Section 705.30 for overcurrent protections of power source output conduc- tors and equipment. Also, 110.3(B), to ensure micro-inverter fans have not been forgotten, were listed and tested to have multiple inverters protected by a single OCPD, NEC compliance is achieved. Always important to follow listing installation guidelines. Time to break out the exhilarating stuff, the math portion of the article. Fear not; there will be many visual aids accompanied by very simple math. INTERCONNECTIONS TO BUSBARS Section 705.12(B) provides the calculations necessary to en- sure that busbars fed simultaneously by both sources do not become overloaded. It may surprise some that many parallel power sources can produce a maximum current for three hours or more. Shocking, I know; with this in mind comes a 125% multiplier. When determining the power source output current, we simply multiply the nameplate output cur- rent by 1.25 or 125%. This will be necessary for the proper application of the busbar calculations per 705.12(B)(1), and Connecting to busbars (100% option). When intercon- necting to a bus, NEC 705.12(B) can be followed for per- missible options. Utilizing engineering analysis as permitted per NEC 705.12(B)(6) will not be covered in this article. Beginning with 705.12(B)(1), better known as the 100% option. When the 100% option is correctly utilized, the sum of the ampere rating of the OCPD protecting the busbar and 125% of each power source output connected to the busbar is less than or equal to the ampacity of the busbar. Where 705.12(B)(1) compliance is achieved, there is no possibility of overloading the bus bar, as stated in the informational note. [See Figure It is worth noting that with the 100% option, there are no limitations where the power source OCPD is located on the bus. Connecting to busbars (120% option). NEC 705.12(B) is the most frequently used interconnection as it permits a healthy amount of power source contribution while using the always time-efficient and safe method of simply popping Figure 3. (36 X 1.25) + 200 is greater than (200 X 1.2), in turn violating 705.12(B)(2) Figure 2. The 100% option has been successfully applied where the 40-ampere con- tinuous power source in sum with the 60-ampere fuse protecting the busbar does not exceed the ampere rating of the busbar. in a back-fed breaker. This option can best be explained mathematically with the following formula, [(power source output current X 125%) + ampere rating of OCPD pro- tecting the busbar] < busbar ampacity X 120%. When utilizing the 120% option, the primary and paralleled power sources are required to be located at opposite ends on the busbar and labeling adjacent to the back-fed breaker stating “WARNING: POWER SOURCE OUTPUT CONNEC- ---PAGE BREAK--- WINTER 2023 IAEI MAGAZINE 5 TION-DO NOT RELOCATE THIS OVERCURRENT DEVICE.” [See Photo Bi-directional sources delivering power simultaneously create the potential for busbar “hotspots.” Bi-directional power source currents will oppose the primary source cur- rents; locating primary and paralleled power sources opposite each other will prevent the sum of these supply currents from flowing at a single spot on the bus, in turn preventing the dreaded “hot spot.” Important to note that the “opposite end” can be achieved in center-fed type panel boards as per 705.12(B)(4), and any confusion was addressed and provided clarity in the 2017 NEC cycle. Thank you, William Brooks, for the great public input and CMP 4! It is a true state- ment; there is more than one opposite end with center-fed panel boards. However, it would still be a violation of the NEC where the sum of the currents from each power source Photo 2. Example of the labeling required per NEC Section 705.12(B)(2). Figure 4. provides an example of the 100% option in a sub-panel and the sum option in the main panel. located at each end of the bus of a center-fed panel is more than the current permitted by 705.12(B)(2). [See Figure Connecting to busbars (sum option). The sum option is very simple to follow. Howev- er, it can present many challenges utilizing where a bus has many existing branch and feeder circuits. The sum option can be utilized in conjunction with other interconnection methods. Many clever installers have utilized several methods where “all in one” metering and distribution-type equipment would prevent a supply side interconnection as it may require terminations that are prohibited by the manu- facturer’s installation instructions, which would violate NEC Section 110.3(B). [See Figure INTERCONNECTIONS TO FEEDERS What is a feeder? Interconnecting to feeders can be a great means to parallel a large amount of current to the primary source and, when following NEC 705.12(A), a safe method. Let’s dive right in, beginning with the NEC definition of a feeder. The NEC defines a feeder as “all circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch circuit overcurrent device.” Per the NEC defined term, a bus bar would most certainly be a feeder; for clarity, when applying Article 705, feeders should be thought of as wire-type conductors to avoid possible confusion as to when to apply 705.12(A) or 705.12(B). Remember, conductors can come in many differ- ent shapes and sizes; just look at NEC 250.118. Connecting to feeders Parallel power sources can be interconnected to the primary source via a feeder conductor. This type of connection has some similarities with the tap conductor; let’s review this definition. The NEC defines a tap conductor in Article 100 as “a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value permitted for similar conductors that are protected as described elsewhere in 240.4.” There are no NEC injunctions as to where the tap can be made along the length of a feeder; however, where the feeder is only protect- ---PAGE BREAK--- IAEI MAGAZINE WINTER 2023 6 .1 X 90 = 9 Amperes vs 705.28(B)(1) or 32 X 1.25 = 40 amperes ed by overcurrents by an OCPD at one end of the feeder, there is a possibility of overloading the feeder. Remember, paralleling another power source presents unique considerations, unlike tradi- tional taps in a system with only a single power source. The analogy I often use for explanation is that of a 3-way intersection. Drivers have the option to turn right or left; either way brings them home. However, there is a police officer monitoring speeds via radar to the left and no officer to the right. The driver is the power source current, the officer is the OCPD, and the speeding hazard is the overload condi- tion. Drivers making a right turn can cause a hazardous condition when speeding, but with no officer witnessing the speeding driver, the hazard cannot safely be mitigated. Any power source current creating a closed-circuit path by traveling through loads associated with the dis- tribution equipment supplied by the feeders will never pass through the feeder OCPD. Here in lies the overload hazard, and the safe way to eliminate the overload hazard can be accom- plished with either a or b of 705.12(A)(2). Im- portant to note here that this hazard only exists along the length of the feeder from the power source tap to the end of the feeder and not from the feeder supply OCPD and power source tap. [See Figure This is precisely why the code text in 705.12(A)(2) reads, “that portion of the feeder on the load side of the power source output connection shall be protected by one of the following,” either a or b. Option a, 705.12(A)(2)a, increases the size of the feeder to accommodate parallel power source contributions. Option a reads, “The feeder ampacity shall be not less than the sum of the rating of the primary source overcurrent device and 125 percent of the power-source output circuit current” This assures the ampacity of the feeder from the tap to the end of the feeder is greater than or equal to any current which may potentially flow along that portion of the feeder. Option b reads, “an overcurrent device at the load side of the power source connection point shall be rated not greater than the ampacity of the feeder.” Using the three- way intersection analogy again, subdivision b places a police officer at all circuit pathways to mitigate the hazard. With an OCPD where the feeder originates and ends, there is no possibility of overloading the feeder. Although the place- Figure 5. The overload hazard exists from the tap to the panel board bus. Figure 6. In this case the tap would be selected from NEC Table 310.16 to carry a minimum of 40 amperes. ment of the OCPD on the load end of the feeder will be most conveniently achieved in the distribution equipment supplied by the feeder, placement of the OCPD nearest the power source tap eliminates any further taps from potentially being made between the power source and feeder supplied distribution equipment creating possible overload hazards. ---PAGE BREAK--- WINTER 2023 IAEI MAGAZINE 7 SIZING POWER SOURCE TAP CONDUCTORS Just as with traditional feeder taps, NEC rules for sizing the tap eliminate the possibility of creating a fault return path of high impedance and, in turn, causing insufficient amounts of current for the feeder OCPD to clear the fault. Again, recalling apprenticeship studies, the impedance of a conduc- tor is inversely proportional to the area of the conductor. As a conductor increases in size, the impedance decreases; this is evident when viewing Chapter 9, Table 9 of the NEC. To ensure the power source tap conductors are of sufficient area to allow enough current to flow and effectively open the feeder OCPD, 705.28(B)(3) provides guidance. Section 705.28(B)(3) directs the end user back to 240.21(B). We are all familiar with the 10’, 25’, and outdoor tap rules, but we will do some examples together just to be certain. 240.21(B)(1) or the 10’ tap rule. Power source tap > 0.1 X feeder supply OCPD 240.21(B)(2) or the 25’ tap rule Power source tap > 0.33 X feeder supply OCPD Where the following tap formulas result in an amount that is less than the calculations per 705.28(B)(1) or 705.28(B)(2), the power source conductors will be select- ed based on this amount of current. For our purpose, no adjustment factors for ambient temperature or more than three current carrying conductors will be necessary for sim- plicity, so we will only be comparing the calculations from 705.28(B)(1) and then selecting the larger for conductor size. [See Figures 6 and IN CONCLUSION These installations will only continue to enhance the reli- ability of the electrical grid, further providing more energy sources to contribute when demands are at their highest. This is not a trend that will be going away, and it is imperative that all in the electrical industry have a fundamental grasp of the basics when applying NEC Article 705, and hopefully, this brief article has done just that. THANKS This concludes our NEC Article 705 journey. I hope you have enjoyed the read, and it has provided clarity in navigating the often-challenging text surrounding Article 705. Chris Papp is currently a Plans Examiner for the City of Arvada, Col- orado, and conducts a large amount of PV reviews daily. He is a licensed master electrician in the State of Colorado as well as a certified electrical inspector by the International Association of Electrical Inspectors and in- structs the 2nd year IEC apprenticeship program for the Rocky Mountain Chapter of the IEC. Figure 7. In this case, the tap would be selected from NEC Table 310.16 to carry a minimum 40 amperes. .33 X 90 = 29.7 Amperes vs 705.28(B)(1) or 32 X 1.25 = 40 amperes