Grounding

Chris11

Happy Thanksgiving All,

I'm confused about grounding.  My 1 KW Array with all appropriate electronics and batteries are going to be together out in my field.  Then I planned on running the AC from there into a small shed, mostly for lighting.  The array/electronics are 25 feet from the shed's sub panel.

I was planning on putting a ground rod into the ground right at the array.  

Questions:  

My soil is very rocky.  Can I use my small backhoe and lay the rod horizontal a couple of feet under the surface since I don't think I can drive an 8 foot rod into the ground?

Can I also use a separate ground rod 25 feet away by the shed's sub panel?  Or will this create that "hard for me to understand" ground loop?

Thank you.    Chris.

  

zoneblue

Theres lots of alternatives to rods. You can use any trench you happen to have (maybe to a ground mount array) and lay bare solid copper wire. You can tie in to the reinforcing steel of a concrete slab. You can even use bodys of water.

If you use multiple rods, then the best practice is to tie them together with copper wire, to prevent the possibility  of differential voltages appearing during lightning type events.

Its quite educational to the watch the powerco build an HV transformer earth. The last time i saw this they used a vibrating hammer machine to whack in several very long rods about 6 feet apart, and they were all joined underground by copper strap. The long rods were installed in 8 foot sections and they were joined somehow and whacked until they would go no further, about 3 sections per rod. Im not sure how they joined them except in the first instance they screwed together. Whether they were subsequently welded i didnt notice.

Chris11

Thanks.  I didn't know code allowed alternatives.  

Can you answer another question please.  Is it possible to safely have another one, now I know I can put a horizontal one at the array, beside the shed?  Same configuration....horizontal 25 feet away from the first one.

Thanks,  Chris

BB.

Are you planning on using the same Midnite controller for both arrays? Or are you planning on installing a second charge controller?

In general, you can parallel two (or more) charge controllers to one battery bank.

If the arrays are similar (same mounting angles, similar mounting conditions), you can put both arrays into one MPPT charge controller. However, if the arrays are "different" (one facing south east, another facing south west, or one on the roof and the second on the ground), using the same MPPT charge controller can reduce your output current/power. If the arrays have different cell temperatures, then they have different Vmp-array voltages. Then the MPPT controller cannot get an accurate/most efficient harvest Vmp voltage.

Assuming the two arrays are still less than the controller's maximum input ratings--You can certainly try both arrays going to one MPPT controller and see if you get close to predicted output (within 90% or so of expected output) or not.

-Bill

zoneblue

As i said multiple stakes should be bonded together. If your aim in having mutiple stakes is to avoid running ground wire, then that doesnt really pan out. It used to be that a seperate stake at the array was acceptable, however that tightened up and that stake must now be tied to the main stake.  Its a bit of a technicality, because the kind of scenario that they are looking out for is something like using a (non double insulated) powertool on a long lead from the house out to the array... during a lightening storm. You can get dangerous ground differentials between one ground stake and the other.

As for laying the std 8ft rods on their side, youd need to check the NEC for the details. I vaguely recall specifcations for horozontal grounds. Certain length, guage, depth etc.

Chris11

Sorry for not making myself clear.

I plan on putting a ground right beside the array and all electronics for that array.  Then running the AC output from the inverter via a buried cable in ridgid conduit to the shed.  Then up into a subpanel.  I'd like to put a ground rod right there beside the shed connected to the AC subpanel and it's 3 circuits.  I probably won't be inspected but for safety's sake I'd like to be as code compliant as possible.  

There are parts of my system not code compliant...I'm mainly trying to make it safe compliant.  

Bill...there is only one array,  one charge controller, one combiner, one inverter and necessary fuses, one bat bank consisting of 4 six volt batteries.

So in a nutshell....one horizontal ground wire/rod for the DC electronics and one horizontal ground wire/rod for the AC circuits 25 feet apart.  Is that safe?  Or just one ground wire/rod with both DC and AC "stuff" connected to it?

Thank you, Chris

vtmaps

Chris11 said:

So in a nutshell....one horizontal ground wire/rod for the DC electronics and one horizontal ground wire/rod for the AC circuits 25 feet apart.  Is that safe?  Or just one ground wire/rod with both DC and AC "stuff" connected to it?

One ground wire with everything. 

As per zoneblue's advice, you should bond the two ground rods.  This is important!  By code, the cable bonding the two ground rods must be in conduit, but a solid, # 4 or #6 bare wire in the trench between the two rods is better electrically.  The NEC wants it in conduit so that the wire is not damaged by corrosion or rocks.  If you decide to put bare copper in a trench, it should be solid (not stranded) because solid will hold up to corrosion better than stranded.

--vtMaps

zoneblue

So lets get this straight you have CC, battery and inverter next to an array that is some distance from the house. All that runs from the array to the house is AC.

 In this scenario, you have to factor lightning. One option is a single rod near the building, then running a ground wire back to the array. However that isnt good from a lightening perspective, as any lightening hitting the array has no nice direct path to ground. Thus, if your ground is rocky, i would do as vt said, use 4 guage bare solid copper, buried nice and deep at the bottom of the conduit trench , well compacted and tie into that from both ends. You might also consider some recycled half inch copper water pipe maybe? Then just to be on the safe side run 3 core AC cable in your conduit. In my view in this situation parallel ground paths are OK because of the failsafe redundancy it creates.

Ethan Brush

Chris11 said:

Happy Thanksgiving All,

I'm confused about grounding.  My 1 KW Array with all appropriate electronics and batteries are going to be together out in my field.  Then I planned on running the AC from there into a small shed, mostly for lighting.  The array/electronics are 25 feet from the shed's sub panel.

I was planning on putting a ground rod into the ground right at the array.  

Questions:  

My soil is very rocky.  Can I use my small backhoe and lay the rod horizontal a couple of feet under the surface since I don't think I can drive an 8 foot rod into the ground?

Can I also use a separate ground rod 25 feet away by the shed's sub panel?  Or will this create that "hard for me to understand" ground loop?

Thank you.    Chris.

  

What is your array structure made of?  You can likely use that as your array electrode and save 10 bucks on a ground rod.  If its steel and concrete that will be a way better electrode than a ground rod.  Note that depending on what code cycle you are on, you may not even need an electrode at the array.  I wouldnt even bother if its not required, its not going to do a thing.  If lightning hits the array its toast anyway, a ground rod isnt going to help at all.  You do need an EGC run from the shed to the array to bond the panels and racking.  The shed will need a grounding electrode system there.

Chris11

Thank you Mr. ZoneBlue and VTmaps.

Lightening isn't a big factor in my area...but possible.  I like that idea VT....heavy copper wire in the trench and attached at both ends...DC array one end and AC shed ground at the other.  

I appreciate everyone taking the time to reply with their thoughts and advice.

Chris

vtmaps

Chris11 said:

I like that idea VT....heavy copper wire in the trench  

That's what I did in a 100 ft trench between my panels and electronics... But remember: copper is soft and rocks are hard.  There is a reason (mechanical protection) why the code requires the ground to be in conduit.  I back filled my trench with a few inches of sand, laid the conduit and ground wire, more sand, and then the rest of the backfill.

I would suggest you take Ethan Brush's advice... do you know what a 'ufer' ground is?

--vtMaps

Hill_Country

Hi Chris,

Check out this thread; it's similar, but not identical:  http://forum.solar-electric.com/discussion/22466/solar-array-and-grounding#latest

Hopefully that helps a bit as I went through a very similar decision.  However, remember, code-wise it depends on whether you have to have an electrical permit for where you're at, and, if so, what the AHJ (Authority Having Jurisdiction) finds acceptable or not.  That sometimes controls what you can/cannot do.

Thanks!  Let me know if you have any questions...I'm happy to help!

Dave Angelini

Having seen arrays that were not grounded subjected to nearby lightning strikes, I would always ground an array. Always!
I would bond the array grounding to the house with the minimum size wire that is required by code. Always!

zoneblue

Weve been here three years and never had any lightning even remotely close to us. The mountain acts like a barrier. Then one (relatively fine) day out of nowhere, we get a direct hit on the house. It hit the tallest thing which was our wireless internet mast. Had that mast not been there, the next tallest thing is the array. Grounding the array frame is good. As are SPDs. Oh and the mast...

And for the actual fixtures use something that wont corrode like these: http://www.solar-electric.com/ironridge-grounding-lug.html

Chris11

I'm sorry Ethan Brush .... I never saw your reply.  I can only think we were typing/posting at the same time.  I am getting older and comprehension isn't what it used to be but missing a complete reply is strange.

The array is made out of wood.  Wooden posts into the ground, wooden framework to hold the panels, and framework hinged at the post tops so I can change the angle throughout the year.  The bottom of the framework has adjustable pipes anchored in concrete to allow the bottom to go up and down.  All painted or treated. The electronics and batteries will be housed in a vented waterproof  "cupboard" bolted right on the posts.  

So the only purpose of a grounding at the array is lightening?  I thought it might have something to do with safely for both me and the electronics.  I still think I'll run a ground wire from the each electronic box to a ground wire/rod/pipe....something in the ground.  I want to keep those pesky electrons which escape their captivity with somewhere to go.

The chance of an inspector happening onto my non compliant system is remote, however if that happens and he shuts me down I'll just run 24 volt light bulbs.  I don't believe they have jurisdiction over low voltage.

Thank you for your help...and again, I'm sorry for missing your reply.

And a general thank you to all who take the time to help if I don't mention you by name.

Chris

Chris11

Hill Country....
I've started to read that thread you suggested.  It'll take this evening to finish and digest. Thank you.

Thomas

I used a 10 foot copper grounding rod inside of a 1/2 in. pvc cut 8 feet for 2 exposed feet of copper 8 feet down.  Insulated ground. Should not interfere with other groundings.

zoneblue

Chris, grounding is a complicated subject. From a code perspective it depends on your array Voc. If youre running short strings of panels (depending on the jurisdiction sub 50-120V) then grounding is entirely optional or unnecessary. The point of the grounding is "help" a current carrying conductor to chasis short deliven before it shocks someone who touchs it. Ie you need a shock-able array voltage like 3S 60 cells or such before this makes any sense. But its much more complicated than that. In general household size systems as an industry norm ground the negative side of the DC, even with 2S arrays. Its a tradition, but there are real advantages to it, like only having to fuse the negative side. If you go this route, which for most people, you probably should, (to go against tradition means haivng to think harder!), then you do have to get to grips with the nuances of RE grounding. Some of which i go into here:
http://zoneblue.org/cms/page.php?view=system-earthing and http://zoneblue.org/cms/page.php?view=ac-distribution

You want:
- a SINGLE bond between DC neg and ground bus.
- a SINGLE bond between AC netrual and ground bus.
- all metal system components tied to the ground bus.
- as few connections in the ground system as possible, avoid daisy chains.
- adequate ground conductor sizes for the worst expected load and physical wear
- a decent earth stake

Dave Angelini

Chris11 said:

So the only purpose of a grounding at the array is lightening?  I thought it might have something to do with safely for both me and the electronics.  I still think I'll run a ground wire from the each electronic box to a ground wire/rod/pipe....something in the ground.  I want to keep those pesky electrons which escape their captivity with somewhere to go.

The chance of an inspector happening onto my non compliant system is remote, however if that happens and he shuts me down I'll just run 24 volt light bulbs.  I don't believe they have jurisdiction over low voltage.

If you lived near me I would want you grounded because there is always a small chance that your solar could fail and start a wild fire.
We already have plenty of humans doing that and they do not need your help. At a bare minimum, all the frames should be bonded well and earth grounded at the array.

Agree it is rare offgrid that an inspector will do much. Do keep in mind that they can do just about anything if someone gets them upset.
Revoking a certificate of occupancy is just the start.  Do things right and you will be fine. In my county the last thing they want is DC wiring in a house except near the battery.

WaterWheel

There are several different ideas about the best way to ground a system.      My panels are ground mounted away from the house.      Since I'm running fairly high string voltages I wanted a separate ground for the panels while minimizing the  potential of bringing the lightening into the house if the panels were zapped.

Since my sometimes grid tied inverter instructions are adamant that only one ground be used to prevent the GIF from tripping the entire system out to the combiner box is grounded with the 6ga house ground rod.       I used a wood spacer to electrically insulate the combiner box from the panel support rack.      Previuosly the combiner box had screws attaching it to the panel rack upright.      I then drove a 8' ground rod into the dirt and ran a 6ga wire through grounding clamps attached to the panels.

There are pros and cons with this method of grounding but if an electrical short occurs between the panel frames and the wires inside the panels having 2 separate grounds tripping the GIF will be the least of my troubles.

Dave Angelini

More cons than pros. They should be connected with the smallest wire size in the array. Probably a #10 or #12. In a near strike you now have 2 different potentials. A good way to fry the CC or more.

You are also mixing up the GFI. It is the DC (-) bond to ground that is only in one place not the ground. In the XW cc's this is the fuse to ground and if blown, the XW system can escalate other faults and shut down the inverter to hopefully protect it.  No guarantees but I have seen this happen in 3 different strikes.

A remote array should have all the panels grounded well. Use multiple ground bars, have at least one more than 6 feet away all bonded with number 6 or better. It is the ground bond from this array point to the house that should be the minimum size to avoid bringing in high voltage. It is bonded but hopefully will open if you get hit and arrest at the array. This is bare minimum and if you are prone to the lightning nasties,  there are quite few other things to be doing also.