705.12 (D) re- visited

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david

Senior Member
Location
Pennsylvania
.

In 2008 there was language about panels in series “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 bussbars and conductors.”


Where in 705.12(D) 2014 does it address upstream towards the (service disconnect) utility supply.
I have looked very closely at the language in 2008 and 2014 codes.

Article 705 2008
(7) Inverter Output Connection. 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 or main circuit location.

 The bus or conductor rating shall be sized for the loads connected in accordance with Article 220.

I am told to do a calculation add the sum of the amp rating of the overcurrent devices supplying the panel board.

After calculating the sum of the PV source Breaker(s) supplying the panels buss and the utility source breakers overcurrent protecting the panels buss, based on the result I’m told whether the breaker can be placed at any position on the buss or must be positioned at the opposite end of the buss.

Then for series rated panels it simply tells me to do the same thing with language making it clear that the value to use for the up stream panels is the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter.

 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.


So all this section is telling me for most applications in relation to the up stream panel breakers is move the breaker to the opposite end on the buss away from the main feeder or main over current device.

Article 690 2008
 Bus or Conductor Rating. 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
 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 conductor
 In 2014 705.12 (D) other than the 120 % rule applied to the panel that the PV inverter output dedicated breaker(s) are connected to, only talks about calculations, whether it is for feeders or busses or conductorsare on the load side of the PV source connections point.

Is there anything in article 705 2014 NEC load side connections that requires, implies, or suggest that any of the calculations applied to the point of connection be applied up stream. From My reading of 705.12(D) the calculations are only required where both source currents the PV source and the Utility source currents can move in the same direction on conductors or busses.

It has been suggested that it is common sense to apply these corrections upstream. Is it to bold of a statement that the 2014 Code is moving away from concern over upstream buss ratings and feeder size source calculations? It seems to me it is more accurate that the 2014 code is only concerned where both currents move in the same direction at the same time. In other words on the load side of the PV source connection point

Article 690 2008
Bus or Conductor Rating. 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. 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.

Article 705 2008
(7) Inverter Output Connection. 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 or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. 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. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
They way the 2014 is written, all backfeed calculations use 125% of the rated inverter output, and so it doesn't matter if there there is other equipment in series midstream or upstream or downstream.

Example: A 200A service panel with a 200A breaker, contains a 100A breaker that feeds a 125A subpanel, which contains breaker for a PV inverter rated 32A max output. (Assume both panels contain load breakers in excess of the busbar ratings.)

On the subpanel, the sum of the sources is 100A+(32A*125%), which equals 140A. The 120% rule allows 125A*125%=150A if the two sources breakers are placed at the opposite end of the panel. Subpanel is okay.

On the main panel, the sum of the sources is 200A+(32A*125%), which equals 240. Note the bold part is exactly the same as the subpanel because the code simply calls for the calculation to be made using 125% of inverter output in all calculations. It does not matter that the inverter is in a subpanel or what size the subpanel breaker is or what size the inverter breaker in the subpanel is. The 120% rule allows 200A*125%=240A if the two sources breakers are placed at the opposite end of the panel, so the service panel is also okay if the subpanel breaker is placed at the opposite end.

Say there were different inverters in different locations, the calculation on the main panel would still be (200A+sum of rated inverter output*125%). It doesn't matter if one inverter is in the service panel and another is in a subpanel, or if both are in the service panel, or if both are are in subpanels. However if both breakers were in subpanels then each subpanel would only be subject to the inverter output that was installed in that subpanel.

The way the 2014 code is written implies that these calculations are to be applied to any panel that is part of an inverter output circuit, i.e. any panel in series between the inverter and service.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
.
Is it too bold of a statement that the 2014 Code is moving away from concern over upstream buss ratings and feeder size source calculations?

Yes, that's too bold a statement. The 2014 code still cares about all equipment that is connected in series between a utility and an interactive inverter.

It seems to me it is more accurate that the 2014 code is only concerned where both currents move in the same direction at the same time. In other words on the load side of the PV source connection point

I agree with your first sentence in that the rules are now written to try to address whether that is the case. I'd say that the 2014 code is organized more conceptually around how the connections are arranged and it tries to the generalize the rules. Previous codes were a bit flawed in trying to address every situation piecemeal, which is not really possible or appropriate. The 2014 code may be harder to grasp but it accords much better with electrical theory.
 

Carultch

Senior Member
Location
Massachusetts
.

In 2008 there was language about panels in series “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 bussbars and conductors.”


Where in 705.12(D) 2014 does it address upstream towards the (service disconnect) utility supply.
I have looked very closely at the language in 2008 and 2014 codes.

Article 705 2008
(7) Inverter Output Connection. 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 or main circuit location.

 The bus or conductor rating shall be sized for the loads connected in accordance with Article 220.

I am told to do a calculation add the sum of the amp rating of the overcurrent devices supplying the panel board.

After calculating the sum of the PV source Breaker(s) supplying the panels buss and the utility source breakers overcurrent protecting the panels buss, based on the result I’m told whether the breaker can be placed at any position on the buss or must be positioned at the opposite end of the buss.

Then for series rated panels it simply tells me to do the same thing with language making it clear that the value to use for the up stream panels is the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter.

 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.


So all this section is telling me for most applications in relation to the up stream panel breakers is move the breaker to the opposite end on the buss away from the main feeder or main over current device.

Article 690 2008
 Bus or Conductor Rating. 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
 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 conductor
 In 2014 705.12 (D) other than the 120 % rule applied to the panel that the PV inverter output dedicated breaker(s) are connected to, only talks about calculations, whether it is for feeders or busses or conductorsare on the load side of the PV source connections point.

Is there anything in article 705 2014 NEC load side connections that requires, implies, or suggest that any of the calculations applied to the point of connection be applied up stream. From My reading of 705.12(D) the calculations are only required where both source currents the PV source and the Utility source currents can move in the same direction on conductors or busses.

It has been suggested that it is common sense to apply these corrections upstream. Is it to bold of a statement that the 2014 Code is moving away from concern over upstream buss ratings and feeder size source calculations? It seems to me it is more accurate that the 2014 code is only concerned where both currents move in the same direction at the same time. In other words on the load side of the PV source connection point

Article 690 2008
Bus or Conductor Rating. 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. 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.

Article 705 2008
(7) Inverter Output Connection. 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 or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. 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. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:


I'm having a lot of difficulty finding your actual questions in this long list of quoted text. Can you organize a bullet list of questions?
 

david

Senior Member
Location
Pennsylvania
I am not questioning sound design practices, I am approaching this from a non design manual application. Strictly from a min safety standard an enforcement approach.
.
For normal source distribution you build a service with service over current protection including load calculations in the case of a main lug service panel.
You add to that feeder over current protection a feeder to a sub- panel with branch breakers and branch loads. Current moving from the normal source to supply loads through normal distribution

In 705.12 2014 the NEC is inviting PV sources to be added to a normal source distribution system.

705.12 invites PV source currents onto the normal distribution then dictates what happens at the PV source connection point or points.

From a min safety standard and shall be requirements it seems the NEC is moving away from calculations in regards to up stream distribution

Looking at the feeder tap rules it is evident that the PV source calculations only apply to the portion of the feeder on the load side of the PV source connection point

It is clear in that section by load side they are referring the direction the Normal source current would travel to the load on the distribution system
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
...

From a min safety standard and shall be requirements it seems the NEC is moving away from calculations in regards to up stream distribution

As a general statement I don't agree with that. Some restrictions have been relaxed where appropriate but others still exist where appropriate. Conceptually it does not matter whether the distribution is upstream or downstream of some other distribution. Rather what matters is the type of distribution and the locations of each connection. In my opinion each connection in series must follow the rules for that specific connection. In my opinion this is implied in the 2014 code although it is not explicit. Maybe it should be made explicit. A lot of AHJs seem to struggle with it.

Looking at the feeder tap rules it is evident that the PV source calculations only apply to the portion of the feeder on the load side of the PV source connection point

That is correct, but the scope of your statement only includes the feeder. It does not extend, for example, to an upstream panelboard. An upstream panelboard would still have to follow the rules for busbars.

It is clear in that section by load side they are referring the direction the Normal source current would travel to the load on the distribution system

That is correct. (But again, we're only talking about the feeder.)
 

david

Senior Member
Location
Pennsylvania
In my opinion each connection in series must follow the rules for that specific connection. In my opinion this is implied in the 2014 code although it is not explicit. Maybe it should be made explicit. A lot of AHJs seem to struggle with it

Since the feeder tap rule introduces normal distribution panels (up stream) and load side (down stream) in parallel with the PV source connection point.

And we now need to know how to calculate the PV source for panels in parallel down stream of the connection point which is different than the panels in parallel up stream of the connection point

as well as panels connected in series with the PC source connection point the enforcement community does need it to be said how the code wants you to calculate what is up stream of the PV source connection point

It seems to me the 2014 code concentrated on panels in parallel with the PV source connection point and accidentally pulled out the language that addressed panels connected in series with the PV source connection point
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It seems to me it is more accurate that the 2014 code is only concerned where both currents move in the same direction at the same time.
Current can move only in one direction at a time. You cannot simultaneously have different currents running in opposite directions in the same conductor. Kirchoff.

I don't know why you brought this over from the PV forum. If you are feeding 100A into a sub panel it should be obvious that if there are no loads in the sub or if all that are there are turned off, you are also feeding that 100A into the MDP through the connection to the sub and the 125% rule applies to the MDP as well as the sub.
 
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Carultch

Senior Member
Location
Massachusetts
Current can move only in one direction at a time. You cannot simultaneously have different currents running in opposite directions in the same conductor. Kirchoff.

What this means is that current on one side of the bus moves one direction, and current on the other side of the bus moves in the other direction. Somewhere in the middle, it will diminish to zero, being consumed by loads on both sides of the zero point.

Of course, we are really talking about power flow direction, rather than current flow, because current flow has the AC and multiple phases and it is difficult to associate it with just one direction at any point along the length of the busbar assembly.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What this means is that current on one side of the bus moves one direction, and current on the other side of the bus moves in the other direction. Somewhere in the middle, it will diminish to zero.

Not necessarily; it depends on the loads. You can have a net nonzero flow out of the MDP into the service and no zero point anywhere on the bus.
 

Carultch

Senior Member
Location
Massachusetts
Not necessarily; it depends on the loads. You can easily have a net nonzero flow into or out of the MDP and no zero point anywhere on the bus.


True. But in the example of a backfed panelboard, fed opposite the main supply by a secondary source, current will diminish to zero if it flows inward from the bottom, and inward to the top, at the same time. It does depend on the mix of loads and sources operating at any given instant, but for the purposes of understanding what is meant by currents moving in opposite directions at the same time, we are considering a case where the panelboard is consuming a net load from the main supply.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
True. But in the example of a backfed panelboard, fed opposite the main supply by a secondary source, current will diminish to zero if it flows inward from the bottom, and inward to the top, at the same time. It does depend on the mix of loads and sources operating at any given instant, but for the purposes of this language, we are considering a case where the panelboard is consuming a net load from the main supply.
Of course, but 125% of the inverter maximum output contributes to the 120% rule in every panel between the PV and the service, don't you agree? From the MDP's perspective it makes no difference if the PV is coming in directly through a dedicated breaker or through a subpanel feed breaker.

My goal is to design and build safe and compliant systems. I do not look for loopholes in the code that would enable me to build an unsafe system. Even if someone's interpretation would somehow allow me to load 100A of PV into a subpanel fed by a 125A MDP with a 125A main breaker, I'm not going to do it.
 
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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It seems to me the 2014 code concentrated on panels in parallel with the PV source connection point and accidentally pulled out the language that addressed panels connected in series with the PV source connection point

The language wasn't accidentally removed. It was removed very much on purpose, because it referred to breaker sizes for the PV source, and those breakers are no longer used in any calculations. So it would have been contradictory and redundant to leave that language in.

With that said, I am somewhat inclined to agree with you that there should be a sentence at the beginning of 705.12 that would address series connections. It could say that where connections are made in series, each connection shall comply with the rules that are applicable to its location. In my opinion that's already implied but I'm inclined to agree that it should be explicit.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
The language wasn't accidentally removed. It was removed very much on purpose, because it referred to breaker sizes for the PV source, and those breakers are no longer used in any calculations. So it would have been contradictory and redundant to leave that language in.

With that said, I am somewhat inclined to agree with you that there should be a sentence at the beginning of 705.12 that would address series connections. It could say that where connections are made in series, each connection shall comply with the rules that are applicable to its location. In my opinion that's already implied but I'm inclined to agree that it should be explicit.

It is my opinion that 705.12(D)(2)(3)(b) is explicit, if not totally clear. "Where two sources, one a utility and the other an inverter are located at the opposite ends of a busbar..." does not differentiate between an inverter directly connected to the busbar through a dedicated breaker and one connected to the busbar through a subpanel. It's still a connected source. The 125% rule applies.

You can pick the language apart syntactically looking for a loophole (After it all, it says "inverter", singular; what if there are more than one? Does the rule not apply?) that will get you around it all you want, but it won't make your design safer and your AHJ will almost certainly fail you on your inspection.
 
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david

Senior Member
Location
Pennsylvania
It’s not about finding a loop hole or picking the code apart it actually just the opposite.

If someone builds it wrong I have to be able to point to a section in the code that says it is wrong.

In the NEC style 705.12 Makes a controlling statement

705.12 Point of Connection.
The output of an interconnected electric power source shall be connected as specified in 705.12(A), (B), (C), or (D).

So everything under 705.12 is only about the connection point unless in its subparts it directs you to do something in relationship to that connection point

(7) Inverter Output Connection. 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 or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. 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. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:

“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.”

“shall be used in the calculations for all busbars and conductors.”

The statement above is not just directing you to a value (the rating of the first overcurrent device directly connected to the output of a utility –interactive inverters).

"Shall be used in calculations for all busbars and conductors."

The language wasn’t perfect but I could point to it and say you are directed to do a calculation for panels in series.

Now to illustrate this lets change the language.

“In systems with panelboards connected in series the sum of 125% output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.”
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It’s not about finding a loop hole or picking the code apart it actually just the opposite.

If someone builds it wrong I have to be able to point to a section in the code that says it is wrong.

In the NEC style 705.12 Makes a controlling statement

705.12 Point of Connection.
The output of an interconnected electric power source shall be connected as specified in 705.12(A), (B), (C), or (D).

So everything under 705.12 is only about the connection point unless in its subparts it directs you to do something in relationship to that connection point

(7) Inverter Output Connection. 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 or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. 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. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:

“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.”

“shall be used in the calculations for all busbars and conductors.”

The statement above is not just directing you to a value (the rating of the first overcurrent device directly connected to the output of a utility –interactive inverters).

"Shall be used in calculations for all busbars and conductors."

The language wasn’t perfect but I could point to it and say you are directed to do a calculation for panels in series.

Now to illustrate this lets change the language.

“In systems with panelboards connected in series the sum of 125% output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.”
Good luck with that windmill, Mr. Quixote. :D

The language as it exists is clear enough for me. I understand what it says and I understand why it says it.

YMMV.
 

david

Senior Member
Location
Pennsylvania
Good luck with that windmill, Mr. Quixote. :D

The language as it exists is clear enough for me. YMMV.

That may be so but we have homeowners that are starting to install PV systems on there own property. Also I been doing electrical inspections since 1990 and I know a little bit about what I need to have before I can say it is a violation. As far as I know most inspectors here just hope the system is safe relying on the installer to get it right. I never approached this job that way and I'm not about to now

I'll have to Google this to find out what that means Mr. Quixote. :D
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That may be so but we have homeowners that are starting to install PV systems on there own property. Also I been doing electrical inspections since 1990 and I know a little bit about what I need to have before I can say it is a violation. As far as I know most inspectors here just hope the system is safe relying on the installer to get it right. I never approached this job that way and I'm not about to now

I'll have to Google this to find out what that means Mr. Quixote. :D
Nevertheless, the language is clear to me. If it's not clear to you, then you are certainly as free as is anyone else to try to change it, hence the Don Quixote reference. He didn't do so well against that windmill, though.

Here is a scenario: Say you have a service with a 200A MDP with a 200A main breaker. Inside that MDP about the middle of the bus you have a 100A breaker that feeds a 100A subpanel and there are load breakers in the MDP in significant excess of 200A. In the subpanel you have a 100A breaker that is fed by an 80A inverter and there are no other breakers in the panel. The subpanel is compliant under 705.12(D)(2)(3)(c).

If you were the inspector, would you pass it? No inspector around here would.
 

david

Senior Member
Location
Pennsylvania
Nevertheless, the language is clear to me. If it's not clear to you, then you are certainly as free as is anyone else to try to change it, hence the Don Quixote reference. He didn't do so well against that windmill, though.

Here is a scenario: Say you have a service with a 200A MDP with a 200A main breaker. Inside that MDP about the middle of the bus you have a 100A breaker that feeds a 100A subpanel and there are load breakers in the MDP in significant excess of 200A. In the subpanel you have a 100A breaker that is fed by an 80A inverter and there are no other breakers in the panel. The subpanel is compliant under 705.12(D)(2)(3)(c).

If you were the inspector, would you pass it? No inspector around here would.

First off your building a normal distribution to a sub panel at the load end of a feeder and going strictly by what the code says . The first thing to ask is how many slots does the 100 amp sub panel have.
Is it capable of supplying branch circuits and feeders. The installer says it s just to combine the PV inverters outputs. Ok what side of the PV meter is it on. The inspector needs to make a decision.

Now the main service panel I know what previous codes said. So I say you need to do a source calculation on the main MDP. The installer says on both points where does it say that in the code.
 
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