PV Grounding Question

[Jennie1220]

I am going to install a PV system on a roof of a small office, but the power is going to be run down the side of the building and trenched underground to a small free standing 100amp panel. The inverter is going to be mounted on the outside of the small office. My question is all about grounding! The small 100amp panel has a ground rod and I was planning on installing a ground rod at the inverter. Can the PV system be tied into the build steel that is up in the crawl space or does it have to be run all the way down to the ground rod? There is also going to be a combiner box up in the attic. Does the ground from the frame of the pv panels go thru this combiner box? I had planned on running #10 (black, white and green) from the combiner box to the inverter and the same from the inverter back to the 100amp panel. Grounding these PV systems is extremely hard for me to figure out. Thanks for the help

 

[jaggedben]

Can the PV system be tied into the build steel that is up in the crawl space or does it have to be run all the way down to the ground rod?

There must be an equipment grounding conductor run to the inverter. It is not acceptable to use the building steel as a grounding conductor or a grounding electrode. But if the roof is metal, or if any other parts of the building steel are near the open air PV conductors, you will need to ground those metal parts. (If a metal roof, some attachment clamps for standing seams are listed to provide a grounding connection.)

As for the ground rod, the grounding electrode conductor (GEC) that you run to it must be a #8 (or larger, if not run in conduit, see 250.64). What would probably be best is to run that GEC from the grounding electrode terminal on the inverter to the rod, and then tap another #8 to it to run to the AC ground rod at the 100 amp panel, since you must also connect the inverter to the AC grounding electrode system and bond them together.

There is also going to be a combiner box up in the attic. Does the ground from the frame of the pv panels go thru this combiner box?

Yes. And the box must be grounded itself as well.

I had planned on running #10 (black, white and green) from the combiner box to the inverter and the same from the inverter back to the 100amp panel.

Between the inverter and the 100amp panel, the green must be #8, and it must be unspliced or irreversibly spliced. It can be connected (tapped) at one end to the grounding electrode conductor going to the ground rod at the inverter. At the other end it is connected to the grounding electrode conductor going to the rod at the 100 amp panel. It is best to use a c-tap listed for grounding. I'm assuming the 100 amp panel is a main service panel. If it is not, then you will need to go further back to ground to the AC grounding electrode system.

Grounding these PV systems is extremely hard for me to figure out.

I agree. The way I have described is not necessarily the only way to do it.

 

[Jennie1220]

The roof is metal seam roof and I am planning on using s-5 clamps with the pv clamping system which is UL listed to bond the panels together. I was planning on running a #8 bare ground from one panel back to the combiner box (outside of my dc power conduit). Then switch from bare to insulated #8 ground and run the ground inside the conduit to the inverter (with the dc circuit). The inverter will be tied to the ground rod using #8. Now can I take a #8 ground and run it over to the 100amp panel and use c-taps at both the inverter and the panel to bond that wire to the wires that are going down to the ground grounds? I am planning on using a sunny boy 3000hf inverter. Do you know if that inverter needs a neutral or just two hots and ground going back to the 100amp panel? Just trying to get and idea of my conduit fill. Thanks so much for the help!!!!

 

[Jennie1220]

Instead of c-taps can I just use split bolts?

 

[jaggedben]

I believe the S-5 clamps are listed for bonding the metal seams of the roof, so if you use their system you need only bond the panels as their instructions say and run this conductor to the inverter through the combiner as you described.

If the whole building is metal, that should also be bonded to earth in some way, such as directly to the ground rod you are installing at the inverter. See 250.106(C).

I believe that it's okay to bond together the whole system as you described, which is the same as what I described. There might be some differences of interpretation and variation according to which code cycle you are on. For example, in the 2008 NEC your ground rod at the inverter is a requirement. In the 2011 NEC this requirement was removed.

Note that all the green wires should be bonded to each end of every conduit as specified in 250.64(E). That is, with bonding bushings or the like where there aren't threaded hubs.

I would assume your inverter calls for an AC neutral, but if you need to be sure you can look up the installation manual at sma-america.com.

Split bolts are technically not up to code according to the 2011 NEC. Not totally sure about 2008. Ultimately your inspector would decide whether to accept them or not.

 

[Marvin_Hamon]

Grounding and bonding are quite difficult to get down correctly, as is evidenced by many incorrect installations out there. Jaggedben has done a great job and I would only add that it is easier to wrap one's mind around the subject if you break it up a bit. There are three things you are dealing with grounding electrodes, bonding, and grounding.
Here is how I would design this.

Grounding electrodes; I'm going to assume that the grounding rod at the 100A panel is existing and connected to the existing AC grounding electrode system making it the AC grounding electrode. Step one would be to add your new grounding electrode at the inverter into the existing AC grounding electrode system. You can do this by running an AC bonding jumper from the new electrode to the electrode at the 100A panel, using the trench you are digging. The bonding jumper should be the same size as the AC GEC in the main AC service entrance panel. Now your new grounding electrode is considered part of the AC grounding electrode system.

Bonding; bond your array with the #8 EGC down to the DC combiner and then to the inverter in the conduit with the other conductors. If your DC conductors go into conduit from the array to the DC combiner then the #8 has to go with them. Run a EGC from the inverter to the 100A panel, without knowing the rating of your AC breaker I don't know what the size should be. Watch out with the bare #8 routing on the metal roof, the copper can leave discoloration on the roof over time and most clients find this to be not so attractive.

Grounding; from the inverter DC ground run a #6 (NEC 240.166(C)) DC GEC to the grounding electrode at the inverter. Since you tied this into the existing AC grounding electrode system it is as far as you have to go.

There are several other way that the grounding and bonding could be done that are equally valid.