Single grounding electrode allowed?

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kkl

Member
Location
Canon City, CO
Please forgive yet another grounding question, but I couldn't find an answer to this (I don't think the forum's search function is working since a search for "690.47" found no results, even though I found it reading a post.) A revised section 690.47 - Grounding Electrode System in the 2011 NEC says:

(C) Systems with Alternating-Current and Direct-Current Grounding Requirements
...
The dc grounding system shall be bonded to the ac grounding system by one of the methods in (1), (2), or (3)
...
(2) Common Direct-Current and Alternating-Current Grounding Electrode. A dc grounding electrode conductor of the size specified by 250.166 shall be run from the marked dc grounding electrode connection point to the ac grounding electrode. Where an ac grounding electrode is not accessible, the dc grounding electrode conductor shall be connected to the ac grounding electrode conductor in accordance with 250.64(C)(1). This dc grounding electrode conductor shall not be used as a substitute for any required ac equipment grounding conductors.
(3) Combined Direct-Current Grounding Electrode Conductor and Alternating-Current Equipment Grounding Conductor. An unspliced, or irreversibly spliced, combined grounding conductor shall be run from the marked dc grounding electrode conductor connection point along with the ac circuit conductors to the grounding busbar in the associated ac equipment. This combined grounding conductor shall be the larger of the sizes specified by 250.122 or 250.166 and shall be installed in accordance with 250.64(E).

A comment in the Handbook says:
The two grounding systems are to be bonded together or have a common grounding electrode so that all ac and dc grounded circuit conductors and equipment grounding conductors have the same near-zero potential to earth. The combined dc equipment grounding, dc system grounding, and ac equipment grounding required by this section establishes only one grounding circuit and connection for the entire PV system from the PV array to the ac point of connection.


So...does this mean:
1. A DC-side grounding rod is not required, that both the array (dc equipment ground) and the negative conductor (dc system ground per 690.41) at the inverter connections can be wired back to the ac service entrance ground?
2. Why couldn't the the inverter's dc grounding point just be jumpered over to the ac grounding point, since the ac grounding wire is headed back to the same ac ground rod via the service panel?

I'm confused because of reading many comments previously about always having a dc ground rod and never tying the ac and dc grounds together.
 

kkl

Member
Location
Canon City, CO
I should have researched a little longer. Already found the answer to Q1:

http://ewweb.com/mag/electric_top_revisions_edition_5/

"In a somewhat surprising change, 690.47(D) was deleted. That section required that ground and pole-mounted PV arrays have a grounding electrode. This requirement was added in the 2008 edition and was intended to be optional. But the language that was used made it mandatory. By removing the rule altogether, it's still optional, but now isn't mandatory."
 

kkl

Member
Location
Canon City, CO
Further clarification of Q2:

If the inverter has a common grounding busbar, is there even any need to follow any of the options in 690.47(C)? "Photovoltaic systems having dc circuits and ac circuits with no direct connection between the dc grounded conductor and ac grounded conductor shall have a dc grounding system. The dc grounding system shall be bonded to the ac grounding system by one of the methods in (1), (2), or (3)." If there is a direct connection, it doesn't seem like any additional bonding/grounding is required other than the connections at the inverter.

My system designer is specifying a dc grounding rod and tying the dc ground at the inverter all the way back to the service entrance ground by separate wire. It doesn't look like any of that is required anymore. Is that right?
 

kkl

Member
Location
Canon City, CO
I guess I'll keep posting until someone knowledgeable jumps in. ;) Found the following in IAEA Magazine Jan-Feb 2012:
Question: Where does the inverter combined dc grounding electrode conductor/ ac equipment grounding conductor terminate under the 2008 NEC 690.47(C) or the 2011 NEC 690.47(C)(3)?

Answer: Although 690.47(C) in the 2008 is a bit murky, I believe both editions of the Code allow this combined conductor to be terminated at a grounding bus bar in the nearest ac panel that has an ac grounding electrode conductor connected to a grounding electrode that meets the requirements of the Code. Such a panel would certainly include the main service-entrance panel and also any feeder panel that has the necessary grounding...

and from 2011 NEC Changes Part 2 (Homestudy), Question 7:

Section 690.47(D), which required a separate grounding electrode for the DC side
of a PV system was deleted, and 690.47(C) was rewritten to clarify the
requirements for installing grounding electrodes and grounding electrode
conductors for PV systems.
Basically there are three ways to connect a PV system to a grounding electrode:
(1) Install separate grounding electrodes at the AC side of the system and the DC
side of the system, and bond them together. (2) Use a common grounding
electrode; the DC grounding electrode conductor can be bonded to the AC
grounding electrode. (3) Connect a DC grounding electrode conductor to the AC
equipment grounding terminal in the inverter.

The rewrite makes it much clearer that the DC grounding electrode and the AC
grounding electrode must be bonded together. Also, it clarifies that a common
grounding electrode can serve both the DC and AC side of a PV system.

I've drawn up what I think is acceptable:
wiring-diagram.JPG

So, is this right? 2011 NEC allows a single grounding conductor from the array modules (EGC) and a single AC/DC combined grounding conductor from the inverter to the service panel, with no additional bonding to the service panel ground rod required?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I guess I'll keep posting until someone knowledgeable jumps in. ;)
I thought I made a reply to your questions last night but somehow it got lost. :?

If the inverter has a common grounding busbar, is there even any need to follow any of the options in 690.47(C)? "Photovoltaic systems having dc circuits and ac circuits with no direct connection between the dc grounded conductor and ac grounded conductor shall have a dc grounding system.

You're confusing the grounded conductors (white) with the grounding (green) conductors. 690.47(C) must be followed for any solar system with an inverter.

My system designer is specifying a dc grounding rod and tying the dc ground at the inverter all the way back to the service entrance ground by separate wire. It doesn't look like any of that is required anymore. Is that right?

If your jurisdiction is on the 2011 code (or willing to go by it) you are correct. If your jurisidiction is still enforcing every requirement in the 2008 code, your designers design (or something similar) would be needed for the situations mentioned in 2008 NEC 690.47(D).

I've drawn up what I think is acceptable:

For the level of detail shown, you've got it correct. I will just add that you probably ought to read 690.47(C)(3) again, carefully. The ground from the inverter (the combined AC EGC and DC GEC) must be continuous, and must be bonded at the ends of raceways using bonding bushings, as described in 250.64(E).


So, is this right? 2011 NEC allows a single grounding conductor from the array modules (EGC) and a single AC/DC combined grounding conductor from the inverter to the service panel, with no additional bonding to the service panel ground rod required?

Yes that is what the 2011 code allows. However, all jurisdictions I have worked in expect the solar GEC to be irreversibly spliced (tapped, actually) onto the premises GEC. This seems to be a holdover from earlier codes, but no one I've worked with has yet been brave enough to risk the conversation with the inspector if they don't do it.
 
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kkl

Member
Location
Canon City, CO
I thought I made a reply to your questions last night but somehow it got lost. :?

Thanks for lending your expertise.

If your jurisdiction is on the 2011 code (or willing to go by it) you are correct. If your jurisidiction is still enforcing every requirement in the 2008 code, your designers design (or something similar) would be needed for the situations mentioned in 2008 NEC 690.47(D).

Fortunately (I think), we are on 2011 NEC in Colorado for state inspectors.

For the level of detail shown, you've got it correct. I will just add that you probably ought to read 690.47(C)(3) again, carefully. The ground from the inverter (the combined AC EGC and DC GEC) must be continuous, and must be bonded at the ends of raceways using bonding bushings, as described in 250.64(E).

I think this gets back to the semantics of 690.47(C) which seems a bit ambiguous.
(C) Systems with Alternating-Current and Direct-Current Grounding Requirements. Photovoltaic systems having dc circuits and ac circuits with no direct connection between the dc grounded conductor and ac grounded conductor shall have a dc grounding system. The dc grounding system shall be bonded to the ac grounding system by one of the methods in (1), (2), or (3).

The way I read it, and I think the way the commentators I quoted read it, is that (1), (2), or (3) only apply if there is no direct connection. They NEC Handbook makes a point of adding an Informational Note: "...In PV inverters, the terminals for the dc equipment grounding conductors and the terminals for ac equipment grounding conductors are generally connected to, or electrically in common with, a grounding busbar that has a marked dc GEC terminal."

If you don't have a "dc grounding system", because the inverter provides a direct connection, then there is nothing to be bonded to the ac grounding system.

And where (3) might apply, it appears that you have the option of using (1) or (2) instead. (2) seems easier.

Yes that is what the 2011 code allows. However, all jurisdictions I have worked in expect the solar GEC to be irreversibly spliced (tapped, actually) onto the premises GEC. This seems to be a holdover from earlier codes, but no one I've worked with has yet been brave enough to risk the conversation with the inspector if they don't do it.
Gulp.

If I'm still wrong, please set me straight. Thanks.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I think this gets back to the semantics of 690.47(C) which seems a bit ambiguous.

The way I read it, and I think the way the commentators I quoted read it, is that (1), (2), or (3) only apply if there is no direct connection.

Again, you're confusing the grounded conductors (white) with the grounding (green) conductors. Most grid-tied inverters being sold in this country have a transformer between the DC and AC side, thus there is no direct connection between the grounded conductors. Transformerless inverters have a contact that disconnects the two sides if the inverter is not operating, thus I don't think that counts as a direct connection either.

If you don't have a "dc grounding system", because the inverter provides a direct connection, then there is nothing to be bonded to the ac grounding system.

This is wrong. All PV systems require a DC grounding system, except those with AC modules which technically don't exist yet. 690.47(C) (1) (2) and (3) are three options for meeting the same requirement. Again, I believe that requirement actually applies in the case of all PV systems with a UL listed inverter, and certainly for grid-tied systems.

And where (3) might apply, it appears that you have the option of using (1) or (2) instead. (2) seems easier.

You need equipment grounding for the AC side of your inverter, and that has to run with your AC conductors, so it is usually easiest to do (3) and put one green wire in the conduit. There are situations where (1) is advisable because you want an extra electrode(s) at the array and/or inverter. I'm really not sure why anyone would do (2).
 

kkl

Member
Location
Canon City, CO
Again, you're confusing the grounded conductors (white) with the grounding (green) conductors.
Doh!:ashamed1: I finally caught on to your meaning after re-reading 690.47(C) for the 20th time and seeing "grounded" in the first sentence. I also see that you're right about (3) as long as 250.64(E) is followed. Thanks very much for your patience.
 
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