2014 NEC 705.12(D)(2) – Understanding PV Interconnections
By: JADE Learning | Aug 22, 2014
Having trouble applying the new PV interconnection rules in NEC 705.12(D)(2)? For the last few code cycles we have had just one basic rule when it comes to the interconnection of the PV system to busbars or conductors, “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”. The 2014 NEC has replaced those 31 words in 705.12(D)(2) with 477 words that are easy to read but might be difficult to apply without some real world examples.
Let’s start out with the first basic requirement in 705.12(D)(2): 125% of the inverter output circuit current must be used for the ampacity calculations for most of the interconnection methods used. Whether interconnecting the Inverter Output Circuit through a breaker at the service panel or directly to a feeder, it is important to first know the inverter output circuit current. This can be found on the nameplate of the inverter but most inspectors or installers will simply divide the wattage of the inverter by the applied voltage to find the answer. This method may be more accurate since many inverters can be used in multiple voltage configurations and the value listed on the inverter nameplate is usually based on the lowest voltage.
For interconnection to a busbar, there are 3 new options that will be commonly encountered:
- Option one, 705.12(D)(2)(3)(a) (Image 1): 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than the ampacity of the busbar. When using this option, there is no restriction on where the backfed PV breaker must be located in the panelboard.
- Option two, 705.12(D)(2)(3)(b) (Image 2): 125% of the inverter output circuit current plus the rating of the OCPD protecting the busbar cannot be greater than 120% of the ampacity of the busbar. This method can only be used where a service main breaker is at one end of the busbar and the PV breaker is at the opposite end.
- Option three, 705.12(D)(2)(3)(c) (Image 3): First, the sum of the ampere ratings of all OCPD’s on the panelboard, both load and supply devices, excluding the rating of the OCPD protecting the busbar, shall not exceed the ampacity of the busbar. In this method, we are simply adding the face value of the OCPD’s in the panelboard excluding the main. Second, the rating of the OCPD protecting the busbar shall not exceed the rating of the busbar. If this method is used then there is no restriction on where the backfed PV breaker must be located in the panelboard.
Remember, in a utility interactive system, there is no need to have an additional fastener to lock down a backfed plug on type PV breaker. Since the utility interactive inverter stops the current flow when utility power is not present, 705.12(D)(5) allows the additional fastener required by 408.36(D) to be omitted.
Don’t forget the additional labeling that must comply with 110.21(B). If using option 2 from above, then the following labeling is required at the electric panel adjacent to the backfed breaker:
WARNING:
INVERTER OUTPUT CONNECTION;
DO NOT RELOCATE THIS OVERCURRENT DEVICE.
If using option 3 from above, the following label is required at the distribution equipment:
WARNING:
THIS EQUIPMENT FED BY MULTIPLE SOURCES.
TOTAL RATING OF ALL OVERCURRENT DEVICES,
EXCLUDING MAIN OVERCURRENT DEVICE,
SHALL NOT EXCEED AMPACITY OF BUSBAR.
A new rule in 705.31 requires that if connecting the PV system on the line side of a service disconnect, then the OCPD protecting the inverter output circuit conductors must be located within 10 feet of the connection to the service (Image 4).
In the next post we will cover the remaining new interconnection rules pertaining to feeders and tap conductors.
Learn more in the 2014 NEC Changes courses at https://www.jadelearning.com.
What about feeding a PV system from a sub panel that is been feed by another sub panel???
I think you are referring to what used to be 705.12(D)(7) which was deleted in the 2014 NEC. This section had the following text: “In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors”. This text does not exist in the 2014 NEC. The new interconnection rules in 705.12(D) should address any type of connection to the premises wiring without the need to worry about panelboards upstream of the one directly related to the backfed point of interconnection.
NJ has not adopted the NEC 2014 as of yet. With the NEC 2011, if the Inverter tap is in the second subpanel and ahead of the main CB, does the conductor between the two subpanels follow 705.12(D)(2)?
2011 NEC 705.12(D)(7) states “In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors”. This means that if the second subpanel (where the PV interconnection is made) has busbars ( which it will), and is supplied by a conductors (such as a feeder), then the feeder between that panel where the PV interconnection is made and its overcurrent device are subject to the rules in 705.15(D)(2). As well, all feeders and subpanels connected in series ahead of the PV interconnection point will also be subject to 705.12(D)(2) based on the size of the first overcurrent device directly connected to the output the utility-interactive inverter.
I have a sub panel that can accommodate a 50 amp [(125 x 1.2)-100 =50amps] back feed from a PV inverter The main panel, that the sub-panel is connected to, can only accommodate 40 amps [200 x 1.2)-200=40]. Does it meet code to back feed into the sub-panel 50 amps without increasing the size of the main panel?
Dennis,
If I am understanding the situation correctly you are trying to apply Option two, 705.12(D)(2)(3)(b), but this method can only be used where a service main breaker is at one end of the busbar and the PV breaker is at the opposite end of the busbar. If this is not the case either Option one (705.12(D)(2)(3)(a) or Option three (705.12(D)(2)(3)(c) must be used. If we use Option one the sum of 125% of the inverter output circuit current plus the rating of the circuit breaker protecting the busbar cannot exceed the ampacity rating of the busbar. For example, where the busbars are rated 125 amps and are protected by a 100 amp circuit breaker the maximum rating of the PV output circuit would be 20 amps. (20 X 1.25) + 100 Amps breaker rating = 125 amps.
Im looking for info on 705.12 (D)(5) and the intention and how to use in our applications thank u
The additional fastener is not required to lock down a backfed circuit breaker if using a utility interactive system. Part of the listing process for utility interactive inverters is to ensure that when utility power is not present, the inverter cannot continue feeding current back into the grid. That being said, if one were to remove the backfed breaker while it is still connected, the inverter will sense that there is no utility power and stop producing current so that the breaker in your hand is dead. If you have further questions please feel free to give us a call at 800-443-5233.
Jeff,
In this new code section, do any of these options overrule another? For example, I have a project that complies under option 2 but will not under option 3 due to the sum of the branch circuits.
I don’t understand why the sum of the Circuit breaker ratings would be considered as apposed to the the actual accumulative load.
I have a 225 amp MSP with a 200 main with a sub panel fed by a 100 amp breaker with a 125 amp bus. I intend to install 45 amps of backfeed breaker in the sub. It falls under the 120% rule in both locations. However, b/c the sum of the branch circuit breaker exceeds the bus’s rating, I have not room for a backfeed.
Hello Jeremiah,
You have raised an incredible question.
If we are making our interconnection at a busbar then we must use Section 705.12(D)(2)(3). In that section, it states that “ONE of the methods that follows shall be used to determine the ratings of busbars in panelboards”, and then it goes on to list several methods to chose from but offers no language on one method superseding another. Also, the same code language could be intrepeted in two different ways: Do we apply the rule that most closely fits our specific installation or do we pick whichever interconnection rule that is easiest for us and results in the least amount of work?
A similar comparison can be made when deciding to use the standard method or optional method listed in Article 220 for calculating the load on a service or feeder. Often, the optional method might result in a smaller service or feeder size and is still considered code compliant.
As far as the installation that you described above, it is difficult to picture it without a line diagram but we know for sure that option 2 from the blog post, describing 705.12(D)(2)(3)(b), can only be used where a service main OCPD is at one end of the busbar and the PV breaker is at the opposite end, so we can’t apply it in the downstream subpanel. That leaves option 1, mentioned in 705.12(D)(2)(3)(a), or option 3, mentioned in 705.12(D)(2)(3)(c).
Since there is no code language in 705.12(D)(2)(3) requiring “all of the following”, we seem to be able to use “one of the following”. Hope this helps.
Hi Jeff,
Is it code compliant to use self piercing connectors on the supply side in the meter can?
fig. 1 shows the neutral (white) and ground (green) wires in common at the sub-panel, back-fed to the primary load center. is the ground (green) also connected to the pv array ground bar?
My question is, if i have a 400 amp main lug subpanel, being fed by a 200 amp breaker in the main panel. The conductors are rated for 200 amps. Am I limited? Or can i feed 80 amps into it?
would it be 400 * 1.2 = 480 – 200a main = 280amp backed possible?
My inspector has failed saying i’m exceeding 120% rule? please help!
Hello,
I was originally planning to do the 120% rule (705.12(D)(2)(3)(b)) and bring the PV supply into the main panel. Due to the fact it will be difficult to run the line into that panel, I am considering alternative options using sub panels. Here is my layout/configuration.
100A Main Breaker outside by meter
To
125A Main Service Panel inside
To
50A Breaker in middle of Main service panel
To
100A Subpanel
The sub panel has breaker’s that go across horizontally, and the line feeder comes in on both sides of the panel. So there is no real way to have the PV supply come in on the opposite end of the subpanel. Although according to 705.12(D)(2)(3), I don’t think I would need to worry about having the PV supply be on the opposite side of the line feeder for that subpanel. Because the 50A breaker supplying the subpanel and the 40A breaker from the inverter add up to 90A, which is less than that 100A rating of the bus bar of the subpanel.
So far so good, my main concern is the connection location of the 50A breaker supplying the subpanel that I’m going to connect the PV supply into. That 50A breaker in the main panel that is supplying the power to the sub panel is located in the middle of the main panel. Because the subpanel will backfeed into the main panel, I assume I will have to apply the rules to the main panel too. If I have to apply the rules to the main panel, I have to follow the 120% rule unless I upgrade that panel. When following the 120% rule, I would assume I would need to move that 50A subpanel breaker down to the bottom to protect from arcing. I’m not sure I have enough wire to move that 50A breaker to the bottom of the main panel.
What are your thoughts? Would I need to move that 50A breaker on the main panel down to the bottom in order to pass inspection?
Thanks,
Chris
Greetings,
Thank you for your comments.
Your question and the decisions you are facing with your new PV system are definitely something that you want to take to your local electrical inspections department.
“Yes,” you will likely need to move that breaker to the opposing end of the panel (away from the PV back-feed breaker.) And you are more than likely permitted to make joints in your panel-box to extend the length of the conductors to move the breaker. I have never seen an electrical-panel/panel-box/load-center that was not listed (and otherwise approved) for making joints & splices, as long as you weren’t exceeding the overall allowed percentage of space in that enclosure for splices/joints/lugs/etc, as you do have those limitations.
Often your inspections department will do a site-visit at your request, as part of the original permit fee. And they will always take your question (and set of plans) at their local office, as they would prefer to do that, versus you installing a non-compliant system. It is easier on them if you are Code-compliant with your installation the first time around, therefore they usually answer questions.
Explain your situation and ask them what they will accept.
Thanks!
Jerry Durham
I have a 100 amp main panel (100 amp buss). I also have a 125 amp subpanel. I want to connect a 7.6 kw solar system into my subpanel. This will requirte a 40 amp breaker be added to my subpanel. Is this allowed in the NEC rules. If not why? Is it electrically dangerous to do this?
Greetings,
Your question regarding back-feeding the power produced by solar PV modules into your premises wiring via the “sub-panel” is a common one, however, the answer is not so straight-forward.
First, it is worth mentioning that a “subpanel” is nothing more than a load center / panel-box that fails to contain the system’s first means of disconnect. With that, any fused or unfused disconnect (whether it is a knife-switch, a breaker, etc. and regardless of whether that same enclosure contains additional circuit-breakers for branch-circuits) located so that it interrupts the service conductors traveling between the meter base and first panel-box, is considered the “first means of disconnect.” That first means of disconnect renders any other type of panel or box located downstream, to be considered a sub-panel….even if that “panel or box” is the first enclosure to contain any circuit-breakers for the premises wiring.
So, yes – it is lawful to back-feed your PV into a sub-panel, otherwise it would be difficult to connect the PV system at all, in some scenarios.
Four big issues to consider regarding the sub-panel, among many issues that surround a PV installation:
(1) Are the correct number of conductors present in the feeder that connects the First Means of Disconnect and this Subpanel. In other words, is there a 4-wire feeder that allows for a separate equipment grounding conductor and neutral conductor?
(2) Is there a main-breaker at your sub-panel and your first means of disconnect? Are you locating the PV back-feed circuit-breaker opposite this main breaker?
(3) Are you being mindful of the 120% rule regarding the panel/subpanel busbars?
(4) Extensive labeling is going to be required at both the sub-panel and main-panel/first means of disconnect, once you have established and installed your back-feed into your sub-panel.
Be aware, Code pertaining to PV installations has changed and expanded dramatically between 2014 and 2017 NEC Code-cycles. You will benefit greatly from hiring a licensed electrician who specializes in solar PV installations, as this is a specialized industry, and it is constantly evolving.
Jerry Durham