GROUNDING FOR UTILITY INTERACTIVE INVERTER?

[mta145]

just installed a 100kw system with a utility interactive inverter. it is remotely located so at inverter we put 2 ground rods creating a DC grounding electrode system. Ran #6 cu to rods connected to the inverter DC GEC terminal. Inverter has integrated AC and DC disconnects. 690.47(c) says that i now have to bond the AC and DC elctrode systems together. So my question is since my AC grounding electrode system is at the main panel can i use my equipment ground in my inverter output circuit to bond the to systems together and if so what size does it have to be??

Thank you

 

[jaggedben]

On a system this size, it's pertinent to ask a couple questions right off the bat:
-Did you submit plans to your AHJ? Did they approve the plans?
-Have you asked your AHJ this question?

Your AHJ's opinion matters a lot more than that of anyone here.

With that out of the way...

I can give you two answers, based on what the 2011 code says, and what I've actually seen done. It's also pertinent to mention that 690.47 (almost the whole thing) was completely revised between the 2008 and 2011 code, so it may matter a lot which code cycle you're on.

First answer:
2011 version of 690.47(C)(1) states in part:

"... The size of any bonding jumper(s) between the ac and dc systems shall be based on the larger size of the existing ac GEC or the size of the dc GEC specified by 250.166. The dc grounding electrode system conductor(s) or the bonding jumpers to the ac GES shall not be used as substitute for any required ac EGCs."

So first there's your size requirement. What does that second part mean? It means at the very least that if you have one conductor for both EGC and bonding jumper, it (or a conductor tapped off it) must terminate in the same location as an EGC would. And a more conservative reading would say that you simply can't have a combined EGC and bonding jumper.

Second answer:

I've seen at least one system similar to the one you describe where a single grounding conductor was used from the inverter and DC GES back to the AC distribution equipment. At the inverter end it was basically treated as a bus, with various pigtails for all DC and AC grounding requirements tapped to it, including the DC GEC. At the AC end, it was terminated in the grounding busbar, which also contained the terminations for the AC GEC and other AC EGCs. I believe this system passed inspection under the 2008 code (and not without some debate peripherally related to these issues).

 

[mta145]

Thank you for your reply..
Doesn't 690.47(c)3 say that i can combine the DC GEC and the AC EGC? as long at it meets 250.166 for sizing? so if i am going to run 3/0al for my phases that is the size of the combined conductor?

 

[jaggedben]

Thank you for your reply..
Doesn't 690.47(c)3 say that i can combine the DC GEC and the AC EGC? as long at it meets 250.166 for sizing?

Yes you can combine those, but that's not what you're talking about doing. You are talking about putting in a bonding jumper according to 690.47(c)(2) in the 2011 code. (Your DC GEC is at the array and goes to the two ground rods you mentioned.) The code is less clear on whether you can combine an AC EGC with a bonding jumper, but the 2011 pretty much implies that you can't. 2008 may not imply that in the same way.

I think the intent of the code is to ensure that a bonding jumper run separately from the AC conductors is not used as the EGC. But if you run the bonding jumper with the AC conductors, as you are proposing, then I think it might be okay to combine the functions into one conductor, if terminated properly. Then again, I'm not an EE or a code maker.

As far as 690.47(C)(3) goes, if you are on the 2011 code and not the 2008 code, the option would be to remove the ground rods at the array and run your combined GEC/EGC all the way back to the AC distribution equipment. The requirement to have a grounding electrode at the array is only in 2008, it was removed again in 2011. On the other hand, there may be good reasons to keep the rods at the array to help with lightning protection.

It would be helpful to know which code cycle you're required to follow.

so if i am going to run 3/0al for my phases that is the size of the combined conductor?

Again, the bonding jumper requirement is the greater of the AC or DC GEC. The EGC requirement in 250.122 is based on the overcurrent protection rating. I suppose if combined you'd take the largest requirement, but again, I think it's a stretch to assume the code even allows that.

If you're worried about doing it twice, ask your AHJ.