Solar Photovoltaic System Interconnection Best Practices Header

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. The utility will only permit the photovoltaic system to interact with the power grid after issuing a formal approval. The process through which a utility verifies a solar system's compliance with its technical and administrative requirements is commonly referred to as the interconnection process.

PV Interconnection

The interconnection process runs parallel with the permitting process. During this process, the local Authority Having Jurisdiction (AHJ) verifies the system's compliance with 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 regarding voltage, frequency, and power quality.

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 meet net metering program requirements.

Utility companies usually require an interconnection application to be submitted to their interconnection department to gather the necessary information about the proposed system. The account holder or the contractor who designs and installs the PV system may submit the application. It 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 a site visit, qualified utility personnel will test the solar system and, if necessary, replace or install revenue meters. When they approve the system, the utility will notify the account holder and the installer. The installer 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 the utility company publishes. 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 connect 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 can accommodate the PV backfeed current. The overcurrent protection devices are the main circuit breaker and the electrical panel's PV back feed circuit breaker.

Load-side tap connection: This is applied when no circuit breaker slots are 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 a professional electrician approves it. The overcurrent protection device is the main breaker. Some utilities 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 connect to the service wires on the line side of the circuit breaker. This connection is rarely allowed for residential systems but is increasingly common in commercial systems. Utilities usually require that a professional electrician approve the design and installation of this connection. Once the utility engineers approve the line-side tap connection, a transformer power shutdown must be scheduled to allow the electrical contractor to connect the wires otherwise attached to the utility grid.

The overcurrent protection device is the PV-fused AC disconnect.

Image courtesy of Home Power Magazine.

Do you still have questions regarding the interconnection process? Reach out to Greentech Renewables Design Services to learn how we can support your team, expedite your permitting process, and help you make the most of available solar design tools and techniques.

Comments

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?

Thanks.

Pete,

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.

Chad Buccine commented 6 years 2 months ago

Hello Dale,

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.

File Attachment(s)

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???

Hello Montana,

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. 

Montana Busch commented 6 years 2 months ago

In reply to by Chad Buccine

Hi Chad, 

That explains it! Thank you very much.

Chad Buccine commented 5 years 4 months ago

Hello James,

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.

Burke O'Neal commented 4 years 9 months ago

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?

Hello Burke,

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.  

 

 

 

Pedro Roman commented 4 years 1 month ago

Can you tell me the issue of the Home Power magazine where the images of this article were taken from? I want to read the full Home Power article.

Hello. Good question. The employee who wrote this article in 2015 is no longer with the company so we don't know the exact issue but if you go to their site, you can download and scan through all the old issues of Home Power Magazine. Thanks again for the question. Sorry I can't find it, If I do I'll post the link.

Nyekah Washington commented 4 years 1 month ago

How big can you make a line-side tap? How small?
The protection at the disconnect needs to be assigned to the inverter. What would that be?

Thanks for the question. Line-side taps for solar can be sized up the service size, ie a 200A service can technically be tapped for 200A providing appropriately sized equipment without a minimum.  The protection of an inverter should be sized at 125% of the rated maximum output current.

Jesse commented 3 years 6 months ago

What code is this under in NEC

Happy New Year and thanks for your question. NEC 2017 has reference to Load vs. Line Side in section 705.12, with (A) describing Supply Side(Line Side) and (B) describing Load Side.

proxybox commented 3 years 4 months ago

Just had the Siemens MC2442S1200FC 200 amp load center installed here in Los Angeles to my single family home without any issues and passed inspection. This combo panel allows line side taps within the box and serves a dedicated breaker for the solar inverter up to 100 amps. If you're upgrading your panel, take a look at this one. My electrician didn't even know it existed and it took a week to order it. It's nice because it allows plenty of room for expansion in the future.

File Attachment(s)

 Thank you for sharing this. The move to retrofit homes to these types of electrical panels is becoming more prevalent as other interconnection methods are just not feasible for a number of reasons. They offer a very clean and safe installation and should be considered for solar-ready new home construction. 

Wendy Angellot… commented 3 years ago

In PV systems, where do you interconnect when you have an LG battery? The battery manufacturer won't allow a meter tap. What is a load side protected tap, and where does that get installed? Do you have any diagrams?

Thank you for your comment. I’m assuming you mean the LG Chem battery.  In that case, the means of interconnection will depend on the overall Energy Storage Solution selected.  As far as the difference between line side tap and load side tap I would refer them to a different article https://www.greentechrenewables.com/article/pv-interconnection-load-side-vs-line-side.  From there a protected load side tap is just a tap between two load side breakers, most commonly done as between a meter main breaker and a main breaker in a main service panel.  Hope this gets to your comment. Thanks again.

Ramya Tipparaju commented 2 years 10 months ago

Can the line side tap happen with a sub meter panel or does it have to be at the main meter panel?

Thank you for your question.  

This is a very uncommon situation, but in general, the answer is maybe.  It depends on how the wiring configuration is done from the “Main Meter”.

Typically this wouldn’t be considered a line side tap since to sub-meter you’ll likely have a load breaker feeding off the “Main Meter” to the “Sub Meter”, but if there aren’t breakers between the meters and more or less just pass through lugs this could be considered a line side tap situation.  If there are breakers then a load side tap is typically still permitted. 

It should also be noted that there are many configurations of how this system could be installed and each situation could call for different interconnection methods/meanings. Be sure to get professional guidance for each situation to ensure a proper installation. 

 

Caleb commented 2 years 5 months ago

I've got a question about the line side vs load side. In the case that the existing meter combo has a lug to lug breaker, no busbar, can you not just do a work around on the system? Adding a sub panel and pulling the house breaker off the sub panel but also using that subpanel as the interconnection point of the solar breaker? It was explained to me that this would essentially make this a line side connection for all intents and purposes.

Sounds like a workable solution. Why not just take out the existing breaker and land your new sub panel on the existing meter lugs. Size the new sub-panel for the solar.  Make sure this is ok with local codes and that the existing feeders and lugs are big enough for whatever you are adding. Thanks for you question!

Joseph DiMatteo commented 2 years 5 months ago

Does it explicitly say in the NEC 2017, or any earlier version, that the supply side connection has to be done through a "PV fused AC disconnect" per your article? The fused disconnect is required by many utilities but I have not been able to find anything in code that it has to be fused.

Adam commented 2 years 4 months ago

Hi guys great site! I have a question. Are we still allowed to use a junction box below the meter and tap in that box instead of doing a tap in the main panel? I can’t find anything in the 2020 code on it. Thanks in advance!

Thank you for you comment. Yes, it is still allowed to perform taps in enclosures below the utility meter and upstream of the main service panel. The enclosure needs to be rated for the environment it is being installed in. Typically, these types of interconnections are done outside so a NEMA 3R enclosure will suffice. See 2020 NEC 705.11 for more details.

Stephan Zechmair commented 2 years 4 months ago

Hello All,
Thanks for all the good information.
I just finished 2 installations here in OC with a GMA adapter. Both have been 125A panels and the PV system required a 40AMP breaker which is not allowed on a 125A main. I opted for GMA (Generation Meter Adapter) with SCE and got super frustrated and disappointed. Southern Califonia Edison is just terrible. They have given me the runaround and delayed both project for a long time. I spend over 7 hours on phone (mostly on hold with SCE) and finally got one GMA installed after I filed a complaint with the public utility commissioner.
Is there a better way? I know panel upgrade (3k-3.5k or derating the main breaker).
I hope there is something better. Any suggestion? Thanks

Thank you for your comment. There is additional information that we will need to and answer your question. Are you working with a Greentech Renewables location in Southern California? If so let's have them connect you with engineering team to support you. Can you complete the contact us form and we can get the process started. Thanks again.

Devlyn commented 2 years 1 month ago

Does the line side tap backfeed current to the main house panel at all? From my understanding AC current is not mono-directional.

The main panel is connected to the grid. If there are loads on the main panel, then some or all of that PV source current will flow to the loads. If there are no loads, the current will flow towards the loads on the grid.

 Electricity flows to where loads require it, so for a line side tap the generated power(Voltage X Current) will go to the panel if the loads can use it locally, otherwise the power will flow back into the grid as other loads further down the line might use it. 

 

- Thanks again for your question.

Chase Jensen commented 2 years 1 month ago

When doing a load side tap what would be the back feed capacity per say. If we where doing a breaker connection the breaker would be 100 amps but the panel can't take that. We also can't do a line side tap so one of our thoughts was to do a load side tap. The main panel has a 200 amp main breaker feeding a bus bar with Feed through lugs the interior sub panels has a main as well that is 200 amps can we do a tap between those two panels even though it would be 100 amps if it was a breaker connection.

BASANT KUMAR commented 1 year 7 months ago

sir
Ever when we do Line Side Tap interconnection, we bond neutral and grounding in fusible AC Disconnect. Why we bond neutral-Grounding instead of carry out till MSP like other Connections?

When doing a line side connection, the PV system fused AC disconnect can now be considered a service disconnect since there are no other disconnects upstream (between the PV system fused disconnect and utility meter). Due to this, the fused disconnect for the PV system now needs to follow requirements for a service disconnect, meaning a main bonding jumper is required.  Hope this is helpful. Thank you again for your question.

Jonathan Fishman commented 1 year 3 months ago

Line side tap is the only solution for integrating photovoltaic systems with whole house generator backup. This is a common setup in our area, which is prone to frequent electrical shutoffs. If the inverter connection is on the load side, it will kick on when the generator kicks on, but without the capacity to take the energy produced, causing a voltage surge. A connection on the supply side will keep the inverter off when the utility supply is off.

Kevin b commented 1 year ago

Here’s what I know for Texas ,we are now going off 2020 nec at least.
Inspectors are wanting an exterior service disconnect by meter.
Not just a fusible disconnect,.
If there is an emergency at the house ,fire wants to be able to go to one place and shut it all down.
So if your mpu is outside by meter you’re good,you add a fusible disconnect to mpu and you compliant.
Now if you only have a meter outside you’ve got issues,the easiest is to just add a main. Breaker enclosure and size it to your panel inside and call for a shutdown. Adding a fused disco for solar is not gonna fly.