Residential Supply-Side Interconnection

A supply-side connection offers some real advantages to the designer of a grid-connected photovoltaic system. In particular, the rated output current of a PV system using a supply-side connection may be as large as the ampacity of the service entrance conductors, which is usually considerably more than the permitted output current of a load-side connection. But this increased flexibility in system size often comes hand in hand with added complexity and cost because the installation requires adding a new service disconnect and service entrance conductors.

Many homes today are built with an all-in-one service entrance device that combines the utility meter, the main circuit breaker—which acts as the service disconnecting means—and a distribution panelboard or load center. Making a supply-side connection can be impossible in this case because tapping the internal conductors or busbars may void both the device’s listing and the manufacturer’s warranty.

With older construction, however, you occasionally encounter an all-inone service entrance device in which a single main breaker does not protect the busbars between the utility meter and the circuit breakers. Backfeeding a PV breaker in this type of panel looks like a load-side connection, but it is actually a supply-side connection. Knowing the difference helps you design a safe Code compliant PV system, and it just might save you time and money during the installation.

Background
Article 690.64(A) of the National Electrical Code allows the point of connection to be made before the building’s service disconnecting means. Typically this means tapping the service entrance conductors between the utility meter and the main circuit breaker, adding a fused disconnect and then connecting the output of the PV system to the new disconnect. Article 230 covers many of the requirements for these changes.

In the simplest case, there are only two service disconnecting means after the PV installation: the original main breaker or switch and the new fused disconnect for the PV system. Article 230.71 allows up to six disconnects per service. Frequently this is referred to as the six-handle rule and simply means that it should take no more than six movements of your hand to completely shut off all electrical service to the home. The six handles may be a combination of disconnect switches and circuit breakers, and they should all be grouped together with each clearly labeled as a service disconnect.

Other considerations may be important. Because the service entrance conductors do not have overcurrent protection at the point where they receive their supply, in a fault condition they would be subjected to the full brunt of the fault current available from the utility transformer. Wiring should follow established best practices, and it never hurts to check with the local AHJ to determine if there are any special requirements for the conductors.

A Simpler Supply Side
The photograph in Figure 1 shows a residential service entrance panel, and Figure 2 shows the same panel with the front cover open. At a glance it looks like a typical all-inone device. The utility’s service lateral comes underground through the sealed portion of the enclosure at the lower left, passes through the meter portion of the box at the lower right, then feeds the distribution panel at the top.

Looking more closely at the top section, you can see indications that there might be something different about this particular distribution panel. First, there are only two breakers: a 100 A breaker feeding a subpanel in the garage on the bottom and a 40 A breaker for the air conditioning above it. If the 100 A breaker were the main breaker, it would be unusual for there to be only one additional two-pole breaker in the entire panel. Second, to the left of the four blank breaker slots the words Service Disconnect are stamped into the steel dead front panel cover. This indicates that breakers in these slots will be connected directly to the service entrance conductors.

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