Most residential electrical services are not designed to accept two sources of power, but they are often utilized for the purpose of connecting PV systems in addition to utility power. NEC
Article 705 provides accommodation for such connections, but sometimes these accommodations cannot be used without replacing the service enclosure, or adding a distribution panel, or making a supply-side connection, all of which entail additional cost. Occasionally installers get “creative” by tapping directly into feeder conductors coming off the service to a subpanel in the house. Article 705 is silent on this specific method, and controversy exists as to whether it is a compliant method at all. Jurisdictions must decide for themselves whether this is compliant; and if they do allow it, examiners and inspectors must be aware of how such an installation triggers the provisions of Article 705 which are meant to prevent the overloading of conductors or busbars. This article explores the specific issues encountered in such an installation.
For a broader treatment of NEC 705.12(D) regarding requirements for making load-side connections for PV systems, see the papers and articles on the subject written by John Wiles of the Institute for Energy and the Environment at New Mexico State University. Those articles are referred to at the end of this one. Mr. Wiles has written extensively on the fundamental intent of 705.12(D). The focus of this article is how 705.12(D) comes into play only in the case of a load-side tap. This specific issue brings the fundamental intent of this code section to bear and so requires a brush up on the basics of these provisions. See John Wiles’ article in the January-February issue of IAEI magazine, “Unraveling the Mysterious 705.12(D) Load Side PV Connections,” which demonstrates the basic intent of these provisions as they pertain to connections at a load center bus bar with a breaker. Mr. Wiles’ article does not address the issue of tapping into feeders; however, the same principles that come into play at bus bars with breakers also apply with feeders and taps.
When a PV system is connected to a load center by a breaker, the problem that can occur is overloading the busbars (see figure 1). For example, if the main disconnect and busbars are rated at 200 amps, and the PV system is capable of producing 40 amps, it is then possible to pull 240 amps off the busbars via feeders and branch circuits, if the users of the system connect enough equipment simultaneously. Section 705.12(D)(2) requires that the “sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.” So, 240 amps are allowed onto busbars rated at 200 amps. But, 705.12(D)(7) requires that PV breakers be located at the opposite end of the busbar from the utility feed. Figure 1 shows why. If the PV 40-amp breaker is located adjacent to the utility feed, the sum of amperage on the busbars immediately after that breaker can be 240 amps, too high for the metal of the busbar. But, if the breaker is located at the opposite end, this cannot occur, since power is fed from two different directions. As we will see, a similar scenario can exist when tapping into feeders with a PV system.
Figure 1. PV load side connection at breaker complaint and non-compliant breaker locations
Tapping into Feeders
Figure 2 shows conductors from a PV inverter connected to the system by tapping into feeder conductors coming off a breaker in the service panel. The figure shows a service that has very few breaker slots, only enough for feeders to a subpanel and several others for loads such as AC and range. On such a panel, if all slots are taken, the installer cannot add a breaker to connect the PV. The connection could be made with a breaker at the subpanel (if all rules are followed), but routing conduit to a subpanel inside the house is often difficult. A line side connection could be made at
this service panel, but this entails installation of an additional service disconnect and could violate the terms of listing of the meter enclosure, which was designed to handle only utility power. A separate subpanel could be installed adjacent to the service, but this is expensive. Tapping directly into the feeders is easy, but is it safe and compliant?
Figure 2. Load side PV tap overloaded feeders and busbars
Is This a Tap?
NEC 240.2 defines a tap conductor 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.” In figure 2, the overcurrent protection for the feeders exceeds the value permitted for the PV conductors spliced onto the feeders, but one could argue that those feeders are not the point of supply for the PV conductors, so that these conductors are not “taps” at all. However, in the event of fault current between the splice and inverter, the feeders will supply fault current to the spliced PV conductors, and the only overcurrent protection to clear this fault is the feeder OCPD. For this reason the PV conductors qualify as tap conductors, even though in normal operation they will put current onto the feeders rather than pull current off of them. If such an installation is allowed by the AHJ, the tap rules of 240.21(B) must be followed.
Can a Tap Connect PV?
Section 705.12(D)(1) states that the connection from an inverter must be made “at a dedicated circuit breaker or fusible disconnecting means.” Figure 2 shows the tap conductors originating from a 60-amp fused disconnect. Is the connection of the PV system to the other system made at the tap splice itself, or is it made at the 60-amp disconnect? Opinions seem to vary on this point. Some consider that the tap conductors from the splice to the disconnect are part of the other system, so that the two systems are connected at the disconnect. Others consider that the tap conductors are part of the PV system, so that the two systems are connected at the splice. Taking the latter position, the connection is not compliant because it is not made at a breaker or fusible disconnect, but rather at a splice. Taking the former, it is compliant since the connection is at a fused disconnect. Since the code is silent on this issue, each AHJ must decide this point for himself. The remainder of this article presumes that the tap connection is judged compliant if it leads to a breaker or fusible disconnect as in figure 2.
Overloading the Feeders
Figure 2 shows the PV system spliced into the feeder conductors somewhere along their length. The feeders are protected by a 100-amp breaker. Let us say for sake of discussion that the feeders themselves and the busbars on the subpanel are rated at 100 amps each, and that the PV system is capable of putting 30 amps onto the feeders in sunlight. When few or no loads are being pulled off the subpanel during daylight, the current from the PV system will backfeed toward the utility. When more than 30 amps are pulled from the subpanel in daylight, the 30 amps from the PV will go to the
subpanel and the utility will make up the difference. If the building users connect enough equipment during daylight, they can conceivably pull a total of 130 amps through the subpanel before the 100-amp feeder breaker trips or the 30-amp PV breaker trips.
Let’s say the users are pulling 130 amps and no breaker trips. The section of the feeders from the 100-amp feeder breaker to the splice is carrying 100 amps, its maximum rating. The PV tap conductors from the 30-amp disconnect to the splice are carrying 30 amps, for which they are rated. But the section of feeders from the PV splice to the subpanel busbars is carrying 130 amps. As well, the tops of the busbars themselves, before any breakers take loads off the busbars, are carrying 130 amps. In this case, both the feeders from the splice and the busbars are well beyond their ratings and can overheat, distort, burn insulation, etc., with no breaker tripping to protect the circuit.
If the PV system were smaller and its breakers were at 20 amps, the feeders from the splice and the busbar tops would carry 120 amps, which is 120 percent of their rating, and therefore compliant with 705.12(D)(2), the 120 percent rule. However, this installation still violates the intent, if not exactly the wording, of 705.12(D)(7) which requires that the connection in a panelboard be made at the opposite end of the busbars from the primary power source. But in this case, we are dealing with a connection at feeders as well as at a panelboard. Since the provision regards only a connection at a panelboard, it is debatable whether it applies to feeders. But still the intent of the provision is clear — the connections must be made in a way that avoids overloading any given section of busbar or conductor. In both the 30-amp and 20-amp PV scenarios described above, such serious overloading is possible because the tap does not connect at the opposite end from the primary feed. It is added to the primary feed, so that the sum of amperages enters the feeders mid-way, and the panel from only one end, in these cases overloading both feeders and busbars.
Size Down the Feeder Breaker
Let us say that the installer suggests sizing down the breaker for the feeders from 100 amps to 70 amps (in the 30-amp PV scenario). In this case, it will no longer be possible to pull more than 100 amps through any section of conductor or busbar. However, this assumes that 30 amps are always available from the PV system which, of course, they are not when sunlight is not on the PV array panels. At night the maximum that can be pulled through the subpanel is the 70 amps. If oad calculations for the building show that 70 amps is enough, this is a quick fix for an otherwise non-compliant situation. If load calculations are over 70 amps, there is a risk of nuisance tripping at the feeder breaker; but the absence of a working calculation should prevent the installation at all.
Oversized Feeders and Busbars
Now let’s consider the situation if both the feeders and the subpanel busbars are oversized to begin with. In figure 3, the feeders are rated for 140 amps and the subpanel busbars are rated at 200 amps, but the feeder breaker is 100 amps and the calculated load is below 100 amps. Now the installer taps into the feeders with 30 amps, and the users connect enough equipment to pull 130 amps through the subpanel during daylight. Current in the feeders is below their rating, and the panel is well below its capacity; but this still seems to violate the provision in 705.12(D)(7) which tells us to locate the PV connection at the opposite end. Or does it?
The text reads, “Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location… .” In this case, the sum of the ampere ratings of the breakers is less than the ratings of the subpanel busbars or the feeder conductors, so no possibility of overload exists. This code provision is written to cover scenarios where busbars are not oversized to begin with, where the addition of another power source presents a possible overload problem. But the provision allows for situations where equipment is oversized to begin with, which avoids the possibility of overload so long as the total amperage fed into the busbars is below the capacity of the equipment. The same principle applies to the feeder conductors. Section 705.12(D)(7) is in place to prevent overheated equipment. It is not necessary to locate the PV connection at the opposite end of the busbars in this scenario because connecting it ahead or anywhere else cannot cause an overload.
Figure 3. Load side PV tap oversized feeders and subpanel busbars
Tapped Feeders in a Gutter
Figure 4 shows an installation where three subpanels are fed off of tap conductors in a gutter. Two possible locations are shown for tapping a PV system onto the main feeders along with the taps for the subpanels. The same principles apply here as in the above scenarios. If the PV system tie in occurs upstream of the taps for the subpanels (PV tap location 1), the section of feeder between the PV tie in and the first subpanel tap splice can theoretically have an amperage running through it that is equal to the sum of the main disconnect breaker and the amperage supplied by the PV system. If this total amperage is greater than 120 percent of the rating of the feeder, 705.12(D)(2) is violated. If it is within 120 percent, it is still necessary to locate the tap at the opposite (load) end of the feeder, just as you would do with locating a breaker on a busbar, to comply with 705.12(D)(7). PV tap location 2 shown in the figure is located where its amperage cannot be added to the amperage coming from the main disconnect on any given length of the feeder, avoiding overloading that section. But if the main feeder is oversized to begin with, locating the PV tap upstream of the subpanel taps is compliant so long as no section of conductor (or busbar) is overloaded.
Figure 4. Load side PV tap in a gutter with feeder taps
Most residential service load centers are not designed to accept secondary sources of power such as PV systems. Connecting to these load centers can be easily done with a backfed circuit breaker, within certain parameters. But installers are faced with connecting to a wide range of load center configurations, where sometimes such a breaker cannot be installed. Sometimes a load-side tap is the option of choice. Such a connection raises various code issues which inspectors and installers must be aware of. The basic principles of installation laid down in the code for use in connecting with circuit breakers also apply when installing a load-side tap. Thorough understanding of the principles behind the provisions is essential to ensure safe, effective installations of PV connections with load-side taps.
John Wiles articles at New Mexico State University
can be found at:
Solar Ready Residential Service Panel
The photo shows a Siemens “solar ready” service panel which is produced as a response to the need for easier connections with PV systems.
This panel design constitutes a line-side tap. A separate set of busbars, rated at 60 amps, is supplied for connecting to an inverter with a set of breakers. That set of busbars is connected directly to the load side of the meter socket before the main disconnect, bypassing the main busbars altogether. This also bypasses all the difficulties involved with load-side connections.
Since the line-side tap connection is integral with the equipment and a part of its listing, it avoids the problem of violating the listing of a panel by connecting a line-side tap where the equipment is not approved for such use. But it will be a long time before significant numbers of such panels are installed; and, meanwhile, the majority of PV installations will be done on older service panels that were not designed for the use.
With effective application of existing code provisions, those panels can be safely utilized with PV while we make the decades-long transition to equipment designed for the purpose. But 20 years from now, you’ll still see someone trying to do a line-side tap into a rusty Zinsco 100-amp service at the meter socket, because there is no room for more breakers and the owners would rather spend their cash on granite countertops than a nice shiny new service panel.