PV Interconnection: Load-Side vs. Line-Side
The majority of US residential and commercial PV systems are grid-interactive (or grid-tied), which means that they are designed to be able to export excess power to the utility grid. Export occurs when the power generated by the solar system is greater than the power used by the loads on site. A photovoltaic system will be given permission to interact with the power grid only after a formal approval has been issued by the utility. The process through which a utility verifies a solar system’s compliance to its technical and administrative requirements is commonly referred to as interconnection process.
The interconnection process runs in parallel with the permitting process. During this process, the local Authority Having Jurisdiction (AHJ) verifies the system’s compliance to the National Electrical Code, Fire code and other local standards. While the AHJ and the utility company will generally not interact during the process, they will require and recognize each other’s approval documents.
During the interconnection process, utilities certify that a PV system meets the following general requirements:
a) The power exported to the grid is measurable and compliant with the grid’s standards in terms of voltage, frequency, power quality. that the equipment used is certified
b) The AC side of the PV system (between the inverter and the utility meter) meets the utility’s safety requirements (labeling, location of equipment, connection to electric panel)
c) The power and energy generated to meet the net metering program requirements
To gather the necessary information about the proposed system, utilities usually require that an interconnection application is submitted to their interconnection department. The application may be submitted by the account holder or, when properly designated, by the contractor that designs and installs the PV system. The interconnection application usually includes professional technical drawings, such as site plans and an electrical diagram.
In the parallel permitting process, the AHJ will complete the inspection and, when passed, will issue a signed final permit.
Once the utility has received the approved final permit, it will schedule an inspection of the installed system. During the site visit, performed by qualified utility personnel, the solar system will be tested, and, if necessary, revenue meters installed and/or replaced. When the system is approved, the utility will notify the account holder and the installer, which will then be able to turn on the system and export power to the utility grid.
Connection to the Main Panel
Grid-interactive systems have a physical connection to the existing electrical equipment, which is electrically continuous with the power grid. The technical requirements for interconnecting a customer-owned generator are listed in a technical document (usually called Electrical Service Requirements) that is published by the utility company. The physical connection to the electrical equipment can be done according to one of the following methods:
Circuit breaker connection: The AC wires from the inverter are connected to the electrical panel through a circuit breaker. This is the most common type of connection with residential systems and is always allowed by utilities. It is also used with commercial applications, whenever the main panel is able to accommodate the PV backfeed current.
The overcurrent protection devices are the main circuit breaker and the PV backfeed circuit breaker in the electrical panel.
Image courtesy of Home power Magazine
Load-side tap connection: Applied when there are no circuit breaker slots available. The wires are connected directly to the existing wires, between the electrical panel and (on the load side of) the main breaker. Some utilities do not allow this connection or do so only if it is approved by a professional electrician. The overcurrent protection device is the main breaker. Some utilities, however, may also require a fused AC disconnect between the inverter and the tap location.
Line-side tap connection: This method requires that the wires from the inverter are connected to the service wires on the line side of the circuit breaker. This connection is very rarely allowed for residential systems but is increasingly common in commercial systems. Utilities usually require that the design for this connection and the installation are approved by a professional electrician. Once the utility engineers have approved the line side tap connection, it is necessary that a transformer power shutdown is scheduled, to allow the electrical contractor to perform the connection on wires that are otherwise connected to the utility grid.
The overcurrent protection device is the PV fused AC disconnect.
Image courtesy of Home power Magazine
Line side tap can be installed without any line power shutdown by using B taps or pulling a meter.
You assert that line side connections are "very rarely allowed for residential systems." My understanding is that these are commonly used for residential, and very frequently for commercial. Can you clarify?
You are correct. Line side connections are frequently used for commercial installations. For residential systems, line side connections are typically more costly and time-consuming if the PV can successfully land on a busbar; however, they are still used.
Utilities tend to frown upon supply side connections. Most electrical equipment is not allowed to be connected on line side of existing service disconnect. PV is one of the few exceptions. I also should note that supply/line side connections are inherently less safe than load side connections (not to say that all supply side connections are unsafe). It's best practice to interconnect on load side assuming amperage allows. Note, if you're generation amperage is large relative to service amperage, load side connections are often not an option.
Safety reasons for Load side over Supply side:
1. In most cases there is no way to locally de-energize electrical equipment between utility metering cabinet and main service disconnect. This is a risk both during installation and future maintenance. Often the only way to de-energize is to call out utility and have them trip switches upstream of equipment on site. Because of the delay involved with coordination, its common to see installers and maintenance techs attempt electrical work on energized gear that is also subject to high fault current from utility.
2. By bypassing Main Service disconnect, you've also bypassed any existing overcurrent protection integrated with Main Disconnect.
3. A common form of supply side connection is via insulation piercing cable and locating a fused AC disconnect nearby. NEC 750.31 limits the length of this unprotected conductor to no more than 10'. Regardless, this small section of conductor is still unprotected from utility.
4. In California, it's common in service equipment to see main service disconnect integrated with meter or ct cabinet. In this case, in can be challenging to find a space to interconnect PV conductors. It's also likely that you will be violating equipment listing by interconnecting PV system at this point. It's common for utility to require you to re-list this equipment even if they grant you a variance.
Note, PG&E has recently started allowing meter adapters to be installed on PV sytems. This is nice option for performing supply side connections and/or avoiding main panelboard upgrades. See attached.
On a line side tap, let's say the Fused AC Disco is on the roof of a commercial building and the MPD is in the basement 100' away. Someone cuts into the conduit causing a fault. The fuses are downstream from the power source so it doesn't shut off power when they blow. The conductors are landed straight to the buss bars in the MDP so there's no OCPD there either. How is that legal per NEC???
Yes, you're correct. The installation you described is an unsafe situation. When performing a supply side connection, it is safest to locate the ocpd as close to the Point of Common Coupling as possible. The National Electric Code started requiring the unprotected conductors be limited to no more than 10' in the 2014 edition. See NEC 705.31.
Even before, it was ambiguous whether the NEC required solar conductors to comply with Feeder Tap rules. Many in the industry argued that feeder tap rules don't apply to PV because these systems are service conductors and not feeders. Regardless, it's always been best practice to locate OCPD as close as possible to connection.
That explains it! Thank you very much.
Yes, you should remove the PV conductor and tap if solar system is removed. I don't know much detail about the existing tap but I will assume insulation piercing type.
I strongly recommend against performing this work on an energized service. Ideally you can de-energize the circuit where tap was made via pulling the meter and/or an upstream disconnect. At a minimum, you can notify the utility and they will de-energize for you.
In regards to repairing the conductor, you have a few options:
1. Pull new service conductor between main panelboard and utility meter
2. Depending on length of existing conductor and where tap made, you may be able to cut off the length of conductor below tap and re-land the conductor into main panelboard. Effectively removing section of damaged cable.
3. Last, you could try repairing the insulation where tap made via either tape, heat shrink, etc. Assuming no damage to strands via the connector.
Hope this helps,
Chad Buccine, P.E.
Inspectors here insist that any connectors used for a supply side connection be service rated. These do not appear to exist. Is there any documentation I could share indicating that connectors do not have to be service rated?
I have never heard of service rated connectors either. There is a specification for service entrance panelboards and switchboards referred to as "service entrance rated". Typically this means that the neutral and ground bus is bonded together inside the equipment.
I assume you are looking for a product to tap the conductors between utility meter and main panelboard. The most common product I see used in this application is Polaris Insulated Connectors.