Is it a PV Disconnect or a PV Service Disconnect?

[ggunn]

I think I'm going to have to eat my words from my last post. I don't think what you are proposing is compliant.

First of all, the EGC you mention from the PV disco. through the tap box to the MDP ground bar does not comply. This would be akin to having 2 service disconnects while only one has the neutral bonded and the second disco does not have the neutral bonded but instead has an EGC run to the first disco. In your scenario, if there was a short to the enclosure of the PV disco of the conductor(s) from the supply side tap, the fault current supplied from the service would flow over an EGC to get to the service neutral in another panel.

250.24(C) requires neutral bonding at EACH disconnect that is connected to service conductors. In addition, unless the GES system is connected at a common point ahead of the original disconnect, the PV disconnect would also have to have a GEC from it to the GES.

Setting the NEC aside for just a moment... What electrical difference does it make if the ground and neutral are bonded in the PV system AC disco or not when they are bonded a few feet away in the MDP and there are no switches, breaks, or splices in either conductor between the two points? I know of at least 12 systems 150 to 400kW designed and installed the way I describe which were vetted by three different AHJ's and passed all electrical and PV inspections. Something I have not mentioned before, if it makes any difference, is that there also are supplemental ground rods in these systems at the inverters which are tied to the MDP GEC through another conductor.

Beyond what we have discussed here, bonding in both places would put current on the ECG between the two points if there were any on the neutral, which is, I thought, what we are not supposed to do. Are you saying that the ECG from the inverter should terminate in the disco and not be carried back to the MDP?

EDIT: If anything I have written comes across as combative, I do not mean it to be. I have a vested interest in designing the safest and most compliant systems possible. Grounding sometimes resembles gris gris (Cajun black magic) to me; maybe I should bring a dead cat and some burnt chicken bones to inspections.

 

[Smart $]

... Something I have not mentioned before, if it makes any difference, is that there also are supplemental ground rods in these systems at the inverters which are tied to the MDP GEC through another conductor.

Beyond what we have discussed here, bonding in both places would put current on the ECG between the two points if there were any on the neutral, which is, I thought, what we are not supposed to do. ...

This is why I am a proponent to a combo GEC/EGC, even concerning SDS transformers.

 

[ggunn]

This is why I am a proponent to a combo GEC/EGC, even concerning SDS transformers.

What would that look like?

 

[Smart $]

What would that look like?

It would look like a larger EGC, typically... and no external GEC between "supplemental ground rods in these systems at the inverters which are tied to the MDP GEC".

 

[ggunn]

It would look like a larger EGC, typically... and no external GEC between "supplemental ground rods in these systems at the inverters which are tied to the MDP GEC".

So in that case would the EGC for the PV system and the EGC for the building never touch? If so, that would seem to be problematic if there were a neutral bonded to the PV EGC in the disco and also bonded to the building ECG in the MDP.

 

[texie]

Setting the NEC aside for just a moment... What electrical difference does it make if the ground and neutral are bonded in the PV system AC disco or not when they are bonded a few feet away in the MDP and there are no switches, breaks, or splices in either conductor between the two points? I know of at least 12 systems 150 to 400kW designed and installed the way I describe which were vetted by three different AHJ's and passed all electrical and PV inspections. Something I have not mentioned before, if it makes any difference, is that there also are supplemental ground rods in these systems at the inverters which are tied to the MDP GEC through another conductor.

Beyond what we have discussed here, bonding in both places would put current on the ECG between the two points if there were any on the neutral, which is, I thought, what we are not supposed to do. Are you saying that the ECG from the inverter should terminate in the disco and not be carried back to the MDP?

EDIT: If anything I have written comes across as combative, I do not mean it to be. I have a vested interest in designing the safest and most compliant systems possible. Grounding sometimes resembles gris gris (Cajun black magic) to me; maybe I should bring a dead cat and some burnt chicken bones to inspections.

No worries, I'm not taking this as combative. This is a complex area.
I think the Mike Holt graphic that smart$ posted sums my position up well except that the commentary should not say "should" but rather "shall".
As I said in previous post, I don't believe what you describe is compliant. The fault current would not be flowing along the same path back to the POCO source which would raise the impedance and it would be flowing over an EGC. In my view the rules of Art. 250 are pretty clear on this.
And to make things even more muddy, how are you sizing this "EGC" to the MDP? On the inverter OCPD or per the size of your supply side tap conductors and 250.66?

PS I know this is not a "tap", just using the term for convenience.

 

[Smart $]

So in that case would the EGC for the PV system and the EGC for the building never touch? If so, that would seem to be problematic if there were a neutral bonded to the PV EGC in the disco and also bonded to the building ECG in the MDP.

All grounding is still bonded together... just one less conductor.

 

[Smart $]

All grounding is still bonded together... just one less conductor.

Well... not just one less conductor.

An EGC/GEC is either inside a raceway or cable jacket, and insulated... so not exposed to touch by commoners. Objectionable current on metallic raceways can be isolated with non-metallic raceway either in part or complete.

 

[ggunn]

Stepping back from the problem at hand, a general comment. The NEC is written, in the main, for situations where current flows from a service ultimately out to loads. Sometimes I kinda have to stand on my head to try to figure out how it applies to PV systems feeding current the other direction.

 

[texie]

Smart$...The way the code is written, the so called PV Disconnect is not subject to any of the important safety rules that Service Disconnects are cover by.

Question 1... Without Service requirements applied to the PV Disconnect in Mikes diagram, the PV Disconnect would not have to be located (grouped) next the Meter and MDP. There could be a supply side connection in a JB by the Meter (or in the Meter) with Service Entrance Conductors running into the building to the PV Disconnect at the opposite side of the building to a location of the inverter without Overcurrent protection until it lands at the PV Disconnect. This would be allowed now right?

Question 2... Without Service requirements applied to the PV Disconnect in Mikes Diagram, if the PV system didn't require a Grounded Conductor, one would not need to be installed? (And if the PV system did require a Grounded Conductor...705.95(B) says it only has to be sized to the equipment grounding conductor size.)

Question 3... Without Service requirements applied to the PV Disconnect in Mikes Diagram, a Grounding Electrode Conductor would not be required at the PV Disconnect right?

So now we could have a Supply Side Connection with say 100amp Service Entrance Conductors running through a building in PVC (or any allowed wiring method under 230.43) to a PV Disconnect without overcurrent protection near the point of tap...without a Grounded Conductor or Main Bonding Jumper...and without a large enough grounding path back to the point of supply.

I'm really trying to view your logic from a safety aspect, but I can't see it. I'm not trying to fight over this either...just like the constructive open dialog on the issue. This is what the forum is for.

Good points.

 

[Smart $]

... Sometimes I kinda have to stand on my head to try to figure out how it applies to PV systems feeding current the other direction.

I resemble that remark at times myself.

 

[ggunn]

Question 2... Without Service requirements applied to the PV Disconnect in Mikes Diagram, if the PV system didn't require a Grounded Conductor, one would not need to be installed? (And if the PV system did require a Grounded Conductor...705.95(B) says it only has to be sized to the equipment grounding conductor size.

A couple of points about that. If a three phase system is built using multiple single phase inverters, there would have to be a grounded conductor and it would have to be the same size as the ungrounded ones. In the case where the grounded conductor is needed by the inverter(s) but only as a voltage reference, you are correct that it only need be as large as the grounding conductor.

But Austin Energy (my local AHJ) requires that a full sized neutral be pulled from the service all the way through the PV meter irrespective of any other considerations. They are OK with transitioning to a smaller wire (or eliminating it where appropriate) somewhere beyond the meter.

 

[jaggedben]

This might be of interest regarding neutral bonding for supply side PV connections: http://solarprofessional.com/articl...ec-section-70512-and-utility-interconnections

And then there's this article, by Marvin Hamon, essentially taking the other side.
http://solarprofessional.com/articl...-for-commercial-pv-applications-2012/page/0/8
(see Diagrams 3-5).

So it's not just people on this forum who see it differently.

 

[Smart $]

... In my view the rules of Art. 250 are pretty clear on this.
And to make things even more muddy, how are you sizing this "EGC" to the MDP? On the inverter OCPD or per the size of your supply side tap conductors and 250.66?

PS I know this is not a "tap", just using the term for convenience.

Actually, the service-entrance conductors to the PV System Disconnect are service tap conductors, that is, if they are of an ampacity rating less than the service rating.

Again, it is not an "EGC" [from the PV System Disconnect] to the MDP (Service Disconnect). It is a bonding jumper. If you read 250.102(C)(1) you will discover it is sized per 250.66.

Between PV System Disconnect and inverter(s), it can be either an EGC sized per 250.122 (which does not preclude a separate GEC) or an EGC/GEC combo sized per 250.66.

 

[shortcircuit2]

1) See 690.14(C) and 690.14(C)(1).

Didn't we determine that 690.14 applied to the DC side of a PV system?

If we debated that too...well the 2014 NEC has written that it applies to DC conductors of the PV system.

 

[shortcircuit2]

Actually, the service-entrance conductors to the PV System Disconnect are service tap conductors, that is, if they are of an ampacity rating less than the service rating.

I don't see any reference in the NEC to service-entrance conductors being called "Tap Conductors"

 

[Smart $]

Didn't we determine that 690.14 applied to the DC side of a PV system?

If we debated that too...well the 2014 NEC has written that it applies to DC conductors of the PV system.

"We" didn't determine anything of the sort. Someone mentioned that in this thread (I don't recall who that was at present).

If 2014 code has specified 690.14 only applies to the DC portion of the PV system, then there will be a lack of correlation between 230, 250, and 690... unless other sections of 230, 250, and/or 690 were modified to handle such. I don't have the 2014 yet, so cannot comment further at this time.

 

[jaggedben]

1) See 690.14(C) and 690.14(C)(1).

This section seems to have been written with DC conductors in mind. (For example, it makes no sense to apply the exception to AC conductors.) Did the 2014 final language not eliminate 690.14 and combine the language into 690.13, making this explicit? Even if it didn't, applying this section in any way to service entrance conductors for a PV system seems like a bad road to travel down. Sure, the 2011 wording doesn't tell you not to do so, but it's not a good idea.

 

[Smart $]

I don't see any reference in the NEC to service-entrance conductors being called "Tap Conductors"

You won't, other than...

230.33 Spliced Conductors. Service conductors shall be
permitted to be spliced or tapped in accordance with
110.14, 300.5(E), 300.13, and 300.15.

 

[texie]

And then there's this article, by Marvin Hamon, essentially taking the other side.
http://solarprofessional.com/articl...-for-commercial-pv-applications-2012/page/0/8
(see Diagrams 3-5).

So it's not just people on this forum who see it differently.

Well, I take exception to these drawings. Diagram 4 and 5 says it is a 4 wire service yet no neutral is shown but rather an EGC. There should be no EGC on the line side, only a bonded neutral. There is no such thing as a service with an EGC on the line side. The diagrams imply a 4 wire Y service with a grounded neutral. That green wire should be a white or gray neutral sized per 250.66.
It is worth noting, however that they do show a bond.