Solar PV earthing - cable size?

[Sunking]

I answered your question the way I did because to me, grounding describes a connection to the earth,

I understand and as I said you are making a common mistake. I am not trying to be critical of you, far from it. It is for that very reason that the last 2 NEC code changed a lot of the terms. Today there is only one cable known as Ground That would be the GEC. That cable originates at the Service connects where Neutral and Ground Buss. Usually at or near the Main Breaker Panel Box. The other end goes to the facility Ground Electrode System aka GES. All other conductor names do not have the word Ground. I gave one example EGC was changed to EBC. About everything thing else is just called a Bonding Jumpers.

Point here is Ground is a loosely used word. But if you were to have a test question to define Ground as it applies to electrical code means: Purposely electrically connected to Earth, or something else in place with Earth. Like I said the chassis of a car, hull of a ship, airplane frames, or even a Ham Sandwich

Perhaps this might help, think of ground as a Reference Point for Voltage. It is the ZERO Reference Point.

When you asked the question "Why ground?", I thought you meant "Why connect to (earth) ground?" Sorry if I misunderstood. Can I change my answer to "f"?

The right answer is F. The question did not have any specific sub system like a Bonding Jumper, it is all inclusive.

[sdold]

I don't mind if you're critical, it helps me to remember. Thanks for taking the time to talk about this. I'm having a lot of fun watching Mike Holt's videos and reading his papers about this.

[cebury]

Awesome job Sunking

[Mike90250]

I see, thank-you for that explanation.
........
I thought the main reason for earthing PV metal frames, is to prevent lightning strikes.

HA. Grounded metal will attract lightning like umbrellas at a golf course !
But the ground wire and rod direct the current around the outside of the house (if installed correctly) and you don't have a fire.
All your electronics and electrical gear will likely be fried, that's what insurance is for, Ground wires prevent fires.

[emartin00]

I've seen a many websites say otherwise. some also say Lightning strikes some distance away can damage your equipment and this is a reason to earth the PV frame.

I hate to break it to you, but those many websites are incorrect. I'm an engineer and I work for a very large company that designs and manufactures electrical connectors, so it's my job to know these things.
Lightning protection is and entirely different thing, and requires MUCH bigger connectors and conductors.

[Sunking]

I don't mind if you're critical, it helps me to remember. Thanks for taking the time to talk about this. I'm having a lot of fun watching Mike Holt's videos and reading his papers about this.

Steve try to remember this and it may help. In Low Voltage applications (600 volts or less) NEC forbids earth (dirt and rock) to be used as a Conductor for the simple fact earth is a poor conductor despite what your 4th grade science teacher told you. Stop and think about that for a minute. You go out and drive a couple of rods into the ground, and you are lucky if it test 50 ohms. Now ask yourself this. What would the fault current be on a 120 volt circuit with a 50 Ohm fault return path.?

Did you come up with roughly 2 amps in your head? What is 2 amps on a 20, or 15 amp breaker? A small 250 watt load circuit right? It is not going to operate any 15 or 20 amp device now is it? 2-amps is load current, not fault current. Ideally we want Fault Current to be 6 times the OCPD rating so it operates any OCPD real dang fast.

Now when we get into high voltages Earth is used as a conductor and utilities use earth as a conductor. All the rules change when you go to high voltage. But that is another subject.

[sdold]

Thanks Dereck! I'm sorry if I implied that the ground rod plays a part in fault currents, I was trying to say the opposite, that it has nothing to do with them and that it's the job of the EBC to CAUSE a large fault current (back to the source) to open the OCPD. I thought you were trying to make the same point by asking "Why connect to earth ground" but I see now that wasn't what you meant. Sorry about that. There are at least a couple of Mike Holt videos describing what you talked about, with the ground rod resistance you mentioned. In one of them, he actually connects 120V to the rod and measures voltage at the rod (which is still 120V) and at various points outward from the rod.

[somawheels]

In my case, I have no nearby utility lines and have a small PV (1.5kw) on a hillside.
It sounds like I am better off not grounding the solar panel frames at-all.
I have a Flexmax charge controller, and have grounded it because the manual seems to suggest I do this whatever my circumstances.

[bcroe]

I'm not arguing with the experienced. But it certainly is true, the safety ground is a different situation than lightning protection. My ground
array is out in the open, some distance from any building. It contains 48 concrete post grounds and 10 ground rods, well distributed to
tie the framework to ground. The resistance of each might be 25 ohms, but collectively they will at least reduce lightning damage.

Those are not a safety ground, which is an entirely different set of conductors with a very low resistance connection to the distribution box.
Bruce Roe