PV System Grounding (top to bottom)

[t5800512]

Being confused about PV system grounding, I read a long thread on the subject. When I was done, I was still confused. No offence to those that participated in the thread, you are all very knowledgeable, and I applaud that. So I have been studying this subject and will now share what I have found. I'm going to leave out the sectional code references and attempt to put this in practical terms.

Rails, PV module frames, and in my case, micro inverter cases should all be grounded. Grounding can be accomplished with metal conduit (using appropriate grounding brushed), but it is typically accomplished by running a continuous grounding cable to all the METAL components. The grounding cable can be encased in green sheathing, but will typically be a single bare copper wire. The gage of the wire that can be as small as 10 gage, however 6 gage seems to be the norm. The actual gage will be a requirement of your local code enforcement agency. Only in rare situations will heavier gage wire be required. I will be using Wiley WEEB clips to mechanically connect the PV module frames and micro inverter cases to the rails. This is allowable in the 2011 NEC and I believe it make a much cleaner installation. I will still be connecting all my rails with the 6 gage continuous grounding cable using WEEB grounding lugs. It is very important that you do not have any splices in your grounding wire.

If you decide to connect the micro inverter cases via a grounding wire, it should also be a separate continuous wire that is routed along with the rail grounding wire, both together side by side off of the roof.

Now, what to do with that grounding wire. NEC 2011 makes provisions for an optional grounding rod for this purpose. On the roof, the ground wire should routed along with your other wires, but in my opinion, outside of the raceway conduit. I will be using PVC conduit which is fine for A/C. The grounding wire can be zip tied to the outside of the conduit. Once off the roof, the grounding wire should diverge, and run to a new grounding rod that is some distance from your house main ground. Keep in mind that this ground is for things like induced power surges from lightning or the lightning itself. This is a true grounding that is often referred to as earthing and is different from equipment grounding. This cable should be as short as possible, and should not be routed some distance around the house. It is a low resistance escape for ultra high voltages. Think again of lightning. The code also mentions protection from fallen high voltage wires, which is not a possibility at my location.

The green wire from the micro inverters (or trunk cable), etc. is the equipment ground and should be routed back the main service breaker box and bonded, connected to the grounding bar. A typical string inverter will have both types of grounds. This can often be confusing because many times both go the the inverters metal case. Read the installation manual carefully and to determine the difference.

Keep in mind that this is my interpretation of the code pertaining to a PV installation. I offer it for planning purposes. You would be well served to hire a certified PV professional. And keep in mind that not all electricians are aware of the special requirements of a solar installation. This is part of the reason I believe we need to educate ourselves. Also consider that not all cities have adopted NEC 2011 and that it may be necessary to be able to discuss this with your inspector. Mine told me that they are not on 2011 yet, but I should strive to be 2011 compliant.

I apologize for starting a new thread on this subject. This thread is more in line with what I was hoping to find on this forum. I hope it is useful. Your comments are welcome.

[thastinger]

Did you pull a permit for this installation as a "self" contractor or home owner? I'd recommend you ask these questions to your building inspector and/or hire a licensed electrician to either guide you or help you.

You'll need 2 grounding rods unless you can get less than 25 ohms with one and all systems have to be on the same ground.

[t5800512]

Thank you for your input. It is appreciated.

To clarify, the NEC has a requirement that grounding rods (and other earthing methods) should have a resistance of 25 ohms or less. As you might expect, measuring the resistance to "true earth" is going to be difficult. And because it is hard to measure, in some dry or sandy area where good earthing can be difficult to achieve, the inspectors may require two grounding rods. They should bonded together, and should be more than six feet apart. In my case one should be sufficient.

It might also be mentioned that for a number of years, the NEC has had concerns about having more than one earthing point, and has required that they all be bonded together. After all, ground is considered to be a reference point, a base that everything is measured from. Ground is important for proper GFI function along with other things. But keep in mind that the optional GEC earthing I mention is mainly for surge protection for metal on the roof. It would not be a good idea to lead a surge back to your main supply box. The NEC now makes it clear that there are two types of grounding. There is the GEC which should probably be called earthing, and a second type of grounding that can be considered as the reference ground. It is the equipment ground (EGC). This ground is going to originate at the ground busbar in the main breaker panel. This is usually a bonded dual ground, one being from an earthing source (perhaps a water pipe), and one from your service provider. This EGC runs (or should run) to every electrical outlet in the house. Note that it does not share the requirement for a continuous run of wire as the GEC ground does. The NEC has made great progress and grounding is now much less confusing.

I did not say much about D/C or string inverters. If I understand the 2011 code correctly, it is now acceptable for D/C and A/C to share the same GEC ground. Someone else may want to run with that. But for the most part a micro inverter and a regular inverter follow the same set of NEC rules. The difference is that the micro inverter is on the roof, connected to the PV panel, and there may not be provisions to ground the D/C portion of that micro inverter circuit. The NEC basically covers this by saying: follow the micro inverter installation instructions. Expect to see more code proposed to cover the "A/C PV Panel" as I believe there are still gray areas on this subject.

PV Grounding is a big subject, and I hope I'm doing it justice. I'm sure I have missed something. So feel free to jump in. I would like to see this thread become as useful as possible. Who knows, it may qualify to become a sticky thread some day.

My origional disclaimer is still in effect here. I would strongly advise you to seek a PV professional for your installation.