Grounding question

pogonip

I am installing a two strings of eleven 225 watt panels for a grid tie system.

Does each string have to have a separate ground or can I run one ground wire for both strings?

What size grounding wire would be recommended?

Thanks

Dan

ggunn

Re: Grounding question

pogonip wrote: »

I am installing a two strings of eleven 225 watt panels for a grid tie system.

Does each string have to have a separate ground or can I run one ground wire for both strings?

What size grounding wire would be recommended?

Thanks

Dan

You can run a single equipment grounding conductor serpentine through lugs on all the modules and rails. #8 bare copper is commonly used.

niel

Re: Grounding question

each string should have its own ground lead going to a single common ground rod even if code would otherwise allow. if separate rods are used due to distances between the strings being too great then bury a bare copper wire of at least #6 and tie the rods together and thus making them appear as one larger ground point. i would not recommend zig-zagging a single continuous ground lead between separate strings as ground leads should not have many bends in them and zig-zagging would not only add many bends, but would also add more distance and resistance to reaching the ground rod.
as was said, #8 is fine to use to ground the pv strings with, but it can be a larger sized wire if you would like. be sure the rod is at least an 8ft copper ground rod. it's usually copper plated steel so the rod has some strength to it to drive through some hard grounds or rocks that would easily bend a straight copper pipe being driven. i'm not sure how well that works with permafrost though.

ggunn

Re: Grounding question

niel wrote: »

each string should have its own ground lead going to a single common ground rod even if code would otherwise allow. if separate rods are used due to distances between the strings being too great then bury a bare copper wire of at least #6 and tie the rods together and thus making them appear as one larger ground point. i would not recommend zig-zagging a single continuous ground lead between separate strings as ground leads should not have many bends in them and zig-zagging would not only add many bends, but would also add more distance and resistance to reaching the ground rod.
as was said, #8 is fine to use to ground the pv strings with, but it can be a larger sized wire if you would like. be sure the rod is at least an 8ft copper ground rod. it's usually copper plated steel so the rod has some strength to it to drive through some hard grounds or rocks that would easily bend a straight copper pipe being driven. i'm not sure how well that works with permafrost though.

YMMV, but every rooftop multistring system I have worked on has used a single continuous bare copper EGC connecting all module frames and racking components. Ground mounted units usually have a separate ground rod for each subarray. String architecture and frame grounding have no electrical relationship to each other.

niel

Re: Grounding question

the singularity is negated by the zig-zagging between strings and is adding to the resistance in getting to ground. each string can have its own continuous unbroken run to ground as long as it is to the same common ground rod with the exception i noted before.

pogonip

Re: Grounding question

Thanks for the help. The two strings are on the side of a building with the main service also on the building. There is a 5/8 inch 8 ft ground rod bellow the meter. Would it be ok to run #8 bare copper in a loop and attach both ends to that rod?

niel

Re: Grounding question

pogonip,
i'm not sure exactly what it is you are describing with a loop. now if it is one string above the other on the side of the building and you start with a straight continuous ground for the top string and then continuously go down to and through the bottom string and finally to the ground rod then i see no problem with that. start with the highest string, or farthest string, and work your way back for the run to the ground rod as it should always go from top to bottom or farthest to closest to the rod. it would be wrong to start at the lower string or continuously zig-zag between pvs on both strings. just keep the ground wire from sharp turns or bends as any turns should be somewhat gradual and smooth. something more like this ) rather than this ] or this >.

i do acknowledge that somebody starting at the bottom string and then going up might argue it to be ok to do in some cases as the bottom string would thus be shielded somewhat from the top string if the top string is directly above the bottom one and will protect the bottom one due to the effects of a faraday cage providing the wire run is of the same overall length or less. this can get tricky with what-ifs and a bit of common sense in visualizing what the purpose is is in order. that purpose is a low resistance path to ground for lightning or the emp effects of it secondarily as the nec does not worry of emp like we do.;) the problem with that particular cage is the bottom string being ungrounded except through going up to the higher string could create a secondary path for the lightning to travel as lightning can follow multiple paths meaning the cage (loosely using the term) isn't text book. i stand on using the top to bottom method rather than bottom to top for the reason of the lightning potentially jumping off of the lower string as lightning does not like to travel in the opposite direction.

does this make any sense to you?

pogonip

Re: Grounding question

I think I understand you reply. I was thinking of starting at the ground rod and go across the top string and back along the bottom string and connecting it back to the ground rod. (loop)

Dan

mr.radon

Re: Grounding question

By code and my utility a single ground wire is all that is needed. I zig-zagged a single ground wire (6AWG) per array for each rail and each micro inverter then entered the house through the drop down to a junction box. (I have five arrays)
I would not run a bare copper wire between separated arrays (trip hazard). I might just run two separate wires and then splice them at the first chance.

The custom J boxes for the drop downs turned out nice. These ground bars work from 16AWG to 4AWG. Only cost about $2.90 at Lowes.


At the junction box the 6AWG was grounded to the 10-3 ground wire for that string. Then in the new service panel the (3) 10-3 grounds were collected and sent to the 200A service's ground bar. (mind you in 10-3 the ground wire is like 14AWG I think)
You do not need to hammer in a second ground rod. (at least not with micro inverters)
For my sister we added a separate ground wire down the side of the house to a new ground rod for each array, this was for lightning protection. CO has far more lightning strikes then WA.

Here you can see the three 10-3 ground wires going to the ground bar and a single 6AWG jumper to the 200A main ground bar.


In truth the ground wire is way overkill. In Europe (way more PV systems then the US) you can run 1000VDC off the roof without a DC disconnect switch and no ground wire.
Funny that roofs in Germany don't seem to catch fire and the people don't seem to get shocked. but then again they only let qualified people work on electrical systems.

niel

Re: Grounding question

pogonip,
essentially you are running 2 ground wire runs, but the connection from the top string to the bottom is not needed. it is sufficient to have the continuous ground wire go from each string to ground as if the other did not exist making the interconnection between them to make a loop unnecessary and i strongly advise against a loop arrangement as it can create a problem.

mr.radon,
that is fine that you did that as it is not against the law to zig-zag the wire, but i am telling you from a lightning and emp standpoint that zig-zagging is a very bad thing to do and will negate some of the protections a ground wire outside is supposed to accomplish. very high voltages with harmonic content act very differently on the ground wire than inner household grounds and most inspectors haven't much of a clue of lightning or emp grounding techniques. why do you think it is against the rules to connect one wire to another and to another to continue to ground for pvs rather than the continuous rule and they don't require the continuous run inside a home? connections can be made, but given the numbers of problems that has arisen from those unfamiliar with the problems that a bad ground connection has when employed outside to ground large areas of metal and not small electrical boxes. zig-zagging between 2 or more strings just to still use 1 wire was not the intention of the nec as this negates a low resistance path that is as short as possible to ground. even i see the difficulty they have in legislating some things, but i don't think they foresaw the results of taking their rules literally in all cases and is one of the reasons they have to keep modifying or adding to their own rules.

let's start this understanding of some of what i am conveying here with an example install. let's say it has 3 pvs in a series arrangement on the left side of a 45ft ranch home's roof and for simplicity it is an off grid battery to inverter setup, but due to shading they needed to put an expansion to the system with 3 series pvs on the right side of the home's roof. how would you ground it?

mr.radon

Re: Grounding question

niel,

might be good to post what the ground wire is for. Its not for an emp pulse nor a lightning strike.... emp pulse, never in my life have I seen that outside a university lab. The grounds wire is just a redundant path to ground so a person touching the equipment will not get shocked. It will also ensure the circuit protection device lifts and prevents fires. Its not for lightning. Here in Seattle after 20+ years have I yet to see or hear of a ground strike.

For emp pulse the Al frames do a great job for EMI/EMC shielding and shorting out any emp pulse. Heck I had to go to great lengths to get TV reception after my installation as I had a nice Faraday cage over my head.

Lets all take a wistful look back to 96'prior to most 240V appliances having a ground wire. Thankfully, we now have four wire 240 plugs standard in new houses. Solar systems aren't any different, the ground wire is just insurance on insurance.

And if he had two arrays, its okay to splice into the existing ground wire, run it to the new array and protect it from tripping, or drop it down to a interior junction block and splice it there.
The only reason the ground wires are 8AWG or bigger is mechanical strength, nothing more. Once protected they can be down sized to 14AWG for a 60A circuit (i believe, NEC's are at work)

If you are worried about lightning, install lightning rods.

pogonip

Re: Grounding question

Thanks for the help. I think I will just ground each string separate since that is the easiest. When running the ground from the panels to the Sunny Boy inverter should I just go from the closet grounding lug to the inverter. It is about 4 ft.

Dan