Grounding system for off-grid

HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
Greetings,

Looking for some advice please. I'm trying to design a system for grounding to use as little wire runs as possible. My system is 810 watts (17.7V @ 7.6A per panel) six 135-watt panels total and six 225 Trojan 6V golf cart batteries wired for 675 Ah @ 12V.

The following will need ground: Inverter, CC, all six panels, 6V batteries, and combiner box.

Note: I'm not planning on using DC GFP breaker system.

Should I:
1) Run a ground wire from each component to a grounding rod (in the ground)?
or
2) Install a third busbar to attach all grounds, then wire that to a grounding rod (in the ground)?
or
3) Attach all grounds to the "negative" busbar and then wire that to a grounding rod (in the ground)?

In DC, it's my understanding that the ground can be connected to the negative, right? So why would I buy a third copper busbar just for ground? What items should, if any, go directly to the grounding rod and not the bus bar? Do I need "bare" copper wire to go from ground busbar to the grounding rod?

Also, can I simply run a ground wire from the main negative battery lead to the grounding rod?

Can someone please clarify the "why's and how's" for me to set up a safe reliable grounding system, like when do I use bare copper wire compared to insulated wire for grounding purposes?:confused:



(Thanks again to this forum for getting me within spittin' distance of my install)

Hairfarm

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding system for off-grid

    My opinion: run the ground wire point-to-point from CC to inverter to battery negative to ground rod.
    Ground the PV frames/mounts with a separate wire run outside the building to reduce the chance of introducing high Voltage from lightning to the other components.

    AC grounding kind of depends on the inverter involved. MSW can give you trouble with neutral-ground bond on AC. In that case you can still run grounding, just skip the bond. But if something goes wrong on the circuit the inverter may go "poof".

    The principal is that the ground wiring carries no current under normal conditions. If a short occurs, it should pull Voltage to near zero and increase current to the point where circuit protection is triggered and power is thus disconnected. It is useless without CP. If the negative is used as ground everywhere you haven't really got ground; you've got negative. You're trying to create an alternate current path that will be a better option (lower resistance) for the electricity than through someone's body.

    There are a lot of different opinions on proper grounding. The rules/regs and reasons have changed over the years. If you want to start an argument, get two engineers and ask them "how do I ground this properly".

    Oh wait; you just did. :p
  • mike95490mike95490 Solar Expert Posts: 9,359 ✭✭✭✭✭
    Re: Grounding system for off-grid
    .....If you want to start an argument, get two engineers and ask them "how do I ground this properly".

    And don't forget to invite the 4 local inspectors to the party - each will likely want it done a different way, and they send the next one out for your re-inspect, and he wants something else done !
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid
    If a short occurs, it should pull Voltage to near zero and increase current to the point where circuit protection is triggered and power is thus disconnected. It is useless without CP.

    Is "CP" short for current protection?

    I'm confused (big surprise). On some installs I've seen on this forum, all of the negative connections are wired to a common point (say in a DC breaker box busbar) then the breaker box itself is grounded.

    Wouldn't that basically be negative connected to ground and not separate?

    Btw, my inverter is a true sine wave.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    Is "CP" short for current protection?

    Well, "circuit protection" but same difference. :D
    I'm confused (big surprise). On some installs I've seen on this forum, all of the negative connections are wired to a common point (say in a DC breaker box busbar) then the breaker box itself is grounded.

    One of us is misunderstanding something. DC negative runs are separate from grounding circuitry. Grounding a metal casing/box of anything is not the same as tying (-) to ground, which is usually only one at the battery negative. Note I said "usually". And if you're looking at diagrams with DC GFCI that becomes completely different as negative is kept floating in order to detect current difference between the (+) and (-) wiring so the breaker will trip if this occurs.
    Wouldn't that basically be negative connected to ground and not separate?

    One thing you have to think about with grounding is the difference between current path when everything is working as it should (zero current on ground conductors) and what the path is when something goes wrong. If you 'randomly' ground the negative, part of the circuit may still conduct along that wire instead of the grounding wire and so the current may not be high enough to trip the CP. Yes, it is complicated.
    Btw, my inverter is a true sine wave.

    That's good! Makes the AC side of things much simpler and safer.
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid

    Thanks for the input Cariboocoot,

    If I understand you correctly, you said to daisy-chain a green ground connection from the CC to inverter to battery negative to ground rod, all in one run, correct? Or I could just wire all of the grounds directly to a ground-only busbar. I already have the three busbars I need for + - and ground. According to another moderator on this board, the DC Ground to Earth Ground can be made on the battery negative bus connection too?

    One final note, a solar installer that I called in my area last night told me that grounding my battery bank could lead to drainage of the batteries.



    And all this time I thought grounding would be the easiest concept to understand!:confused:
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Grounding system for off-grid

    Now you are getting to where an Epanel starts to make sense, it simplifies all the grounding for one thing...
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid
    Now you are getting to where an Epanel starts to make sense, it simplifies all the grounding for one thing...
    I'd rather not have to spend the exra $$ on an epanel for a Xantrex 12v inverter if I can find a workaround. Those look like pretty pricey items and thanks for the suggestion too.

    According to another post by a moderator on this board (BB)
    ,Typically, the DC Ground to Earth Ground is made on the battery negative bus connection.
    http://forum.solar-electric.com/showthread.php?10979-Battery-ground
    I understand this to mean that the negative Busbar can be grounded to earth(?)

    Apparently it's also acceptable to daisy-chain a ground connection from the CC to inverter to battery negative to ground rod, all in one run according to Cariboocoot if I understand correctly. Or can I just wire all of the grounds directly to a ground-only busbar. I already have the three busbars I need for + - and ground.

    I hate/love options.:D


    Light at the end of the long tunnel...

    :)
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding system for off-grid

    The difference between grounding from the negative bus bar (not always used) and the negative battery terminal is negligible. It's just a matter of the wire from the battery to the bar being an "extension" of the battery post. Obviously if you have multiply batteries (or strings of) connected to a bus bar the bar becomes the easy point to ground everything because it's where it all comes together.

    But is not necessary to run a separate ground wire from each unit to a ground bus and then to the earthing rod. This does not mean you can't or shouldn't do it, just that it's not essential to do so.

    There is no reason why grounding one terminal of a battery should drain it. Anyone foolish enough to ground both posts can watch as their battery energizes the ground and drives the mice away as it discharges into nothingness. :p
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Grounding system for off-grid

    The single, unbroken run, is from the ground rod to the ground bus bar (i.e., neutral/ground bus in main panel). All the rest of the AC green wire grounds can go to that one ground connection (and, I don't think there is requirement that the AC Green Wire cannot be wire nutted as it goes to the various locations of use, along with the Black/Red/White wires).

    You can use thermally fused ground connections too (permanent bonding of ground wires together) instead of one unbroken wire run).

    The DC ground can be done similarily... If you ground the DC Bus (or - battery post, etc.) to a ground rod/cold water pipe. Then the DC Bus can also be your DC Green Wire ground.

    The NEC requirement to run a single wire from the ground rod to each of the solar panels is a bit excessive--But they don't want somebody unbolting a connection (or removing a wire nut), then leaving the system ungrounded.

    In reality--I prefer direct runs from the panel(s) to the earth rod--In the case of Lightning, those "S" bends back and forth across the array are not helpful.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Grounding system for off-grid
    BB. wrote: »
    You can use thermally fused ground connections too (permanent bonding of ground wires together) instead of one unbroken wire run).

    Or mechanically crimped, which is also irreversible. You cannot use split-bolts or other threaded connectors. I don't think soft-soldered meets the requirement either.
    SMA SB 3000, old BP panels.
  • PhotowhitPhotowhit Solar Expert Posts: 5,653 ✭✭✭✭✭
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    ... for a Xantrex 12v inverter ...

    If you have one of the inexpesive prowatt or pro series Xantrex, they might be made for mobile use and may have a negative bond internally. I'd check the manual. I think even some of the ProSine and Exeltechs made for mobile use have bonds but they are at the connection and can be user removed easily. Worth looking into, or maybe post the model here and someone will check for you...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid
    The DC ground can be done similarily... If you ground the DC Bus (or - batter post, etc.) to a ground rod/cold water pipe. Then the DC Bus can also be your DC Green Wire ground.

    BB, By "DC Bus" do you mean my "DC NEGATIVE BUS" or "DC GROUND BUS"?

    Btw, what size gauge wire (for grounding purposes) do you recommend going from the battery negative to the grounding busbar? And what size fuse should I put in that line, if any? I'll have a 12V system @ 674 Ah.

    Also, I plan to have my panel array ground go directly to the ground rod.
    If you have one of the inexpesive prowatt or pro series Xantrex, they might be made for mobile use and may have a negative bond internally. I'd check the manual. I think even some of the ProSine and Exeltechs made for mobile use have bonds but they are at the connection and can be user removed easily. Worth looking into, or maybe post the model here and someone will check for you...

    My Prosine 2.0 specifies that it has a movable screw that will auto-bond between shore power and the AC load panel when sensed. It automatically unbonds when connected to shorepower, assuming that the shorepower source is grounded. But my generator will not be grounded, so I will have to move that screw setting to the unbonded hole. In this mode, the bond to neutral can be made manually at the AC Load panel and will stay bonded to neutral at all times, even with non-grounded generator, such as mine.

    thanks for the suggestions...
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    BB, By "DC Bus" do you mean my "DC NEGATIVE BUS" or "DC GROUND BUS"?

    If you ground reference the battery bank, then the point where the ground connection is made is both the DC Negative Bus and DC Ground bus (bus being just a common point to attach all the various wires and cables for the system).

    If you do not ground reference your battery bank, then the two are electrically separate. There is the DC Negative Bus connection point, and a separate Ground Bus connection point (for a fixed installation, tying the DC Ground Bus to the Earth Ground rod/metal cold water pipe/etc. is my preference).
    Btw, what size gauge wire (for grounding purposes) do you recommend going from the battery negative to the grounding busbar? And what size fuse should I put in that line, if any? I'll have a 12V system @ 674 Ah.

    The wire from the Ground Bus to the Ground Rod only needs to be around 6 awg (some code may allow thinner). There is really not much current to a ground rod, and 6 awg is usually the minimum size cable for direct burial.

    Now, the "green wire ground" (inverter case grounding, etc.)--Should be about the diameter of the DC +/- wires going to the device. I.e., if you have a 2 awg wire carrying 100 amps to the inverter, then the "nominal" green wire should be 2 awg to handle the short circuit current needed to blow the fuse/breaker if there is a short inside the inverter's case.

    Practically speaking, the green wire does not have to be as large as the supply wires--The green wire only needs to supply sufficient current to pop the breaker/fuse in a fraction of a second or so--So, having a green wire ground (device to battery negative/ground bus common point) a somewhat smaller diameter than the supply cable is OK (I believe NEC should have a chart on grounding wire sizes). There are other issues (i.e., length of cable run, fusing current rating, etc.) that make using smaller awg green wire cable a bit more difficult to determine (i.e., if the cable run is long and the resistance of the smaller gauge wire is too high--The green wire may not blow the fuse/breaker in time before overheating).
    Also, I plan to have my panel array ground go directly to the ground rod.

    That is fine. If the ground wire is buried, then 6 awg is the smallest diameter recommended.

    If the ground rod is "far" from the solar array (say more than 10-20 feet), would drive another ground rod at the base of the solar array (especially if you have lightning issues in your area). And continue the ground wire back to the "main panel" ground rod. This second ground wire connection is "optional" but is safer (keeps all grounds physically connected so you cannot get an "elevated ground" if there is a short to ground somewhere in the system--the extra wire carries the current and pops the breaker).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid

    Ok. that is really helpful to me.
    Also, I plan to have my panel array ground go directly to the ground rod.
    That is fine. If the ground wire is buried, then 6 awg is the smallest diameter recommended.

    And that wire should be bare, right? Non insulated?
    If the ground rod is "far" from the solar array (say more than 10-20 feet), would drive another ground rod at the base of the solar array (especially if you have lightning issues in your area). And continue the ground wire back to the "main panel" ground rod. This second ground wire connection is "optional" but is safer (keeps all grounds physically connected so you cannot get an "elevated ground" if there is a short to ground somewhere in the system--the extra wire carries the current and pops the breaker).

    My array is less than 10 feet from the main panel ground rod so I won't be needing an extra ground rod. I know that AC ground and DC ground should be on the same rod.
    If you ground reference the battery bank, then the point where the ground connection is made is both the DC Negative Bus and DC Ground bus (bus being just a common point to attach all the various wires and cables for the system).

    Do some people do it as a way of measuring electrical voltage where the ground pressure is zero? By tying the negative to ground?

    From an electrical forum I found this: The voltage at a given point in a circuit may vary depending upon how components are adjusted. Using a reference ground always provides zero pressure on one side of a component and the voltage drop across it and current through it may be measured (or calculated?).

    Is that what you meant by "ground reference"? I won't be using this method, but I was just curious.

    Thanks again BB,
  • BB.BB. Super Moderators, Administrators Posts: 32,007 admin
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    And that wire should be bare, right? Non insulated?

    Either is fine... In theory, the buried wire without insulation will provide a slightly better earth ground.
    Do some people do it as a way of measuring electrical voltage where the ground pressure is zero? By tying the negative to ground?

    Earth ground is just defined as Zero Volts for that location. We use it so that the utility neutral is at zero volts relative to the plumbing in the house, and for excess static charges/lightning strike energy to go too.

    Is is not a "copper plate". Some grounds are better than others (rocky ground, dry ground, tundra, etc. are very poor electrical grounds). In some cases (radio transmitters, radar installations, etc.) have a "good earth ground" can help direct the signal propagation in the "optimal" direction.

    There are very few devices that "need" earth ground to function. A few of them may include some tube type electrical fixtures, spark type ignitors with flame sensors for stoves, etc.
    From an electrical forum I found this: The voltage at a given point in a circuit may vary depending upon how components are adjusted. Using a reference ground always provides zero pressure on one side of a component and the voltage drop across it and current through it may be measured (or calculated?).

    Hmmm... It is more of a "reference. Say you want to measure how tall a bunch of people are. If they are all standing on a flat surface, then all you have to do is measure to the top of their heads with reference to the floor. However, if the people are in a building, on a hill, in a depression, etc., you cannot use the one "reference", but must measure everything relative to their "feet".

    If you were measuring water level in a dam (the higher the level, the more pressure available)--Then you have to ask is it the ocean's sea level "zero", or is it the water level at the base of the dam more relevant for a "zero" reference. Obviously each "zero reference" has its uses.

    For our case, local earth ground (ground rod, cold water pipe buried in earth, etc.) is a handy thing to call "zero volts". However, you cannot assume that is always the case.

    I was working around a large aquarium/theme park with lots of large pumps and lots of salt water (and located on SF Bay Fill). I put a ground rod near a tank and a 100' away I had another ground rod. I ran a ground wire from the remote rod to the local ground rod--Touched the wire and rod and got a nice shock... There was about 60 VAC between two ground rods ~100 feet apart. Most likely, because of some stray electrical current due to the large electrical pumps and lightning used around the park.

    Grounding is a very complex subject, and I always ask "why are you asking about grounding" first before giving an answer.
    Is that what you meant by "ground reference"? I won't be using this method, but I was just curious.

    You know what curiosity did to the cat, right? :p

    Curiosity killed the cat


    There is no, one good answer, about grounding and how best to approach the subject. For most of our electrical work here, we have two reasons for ground bonding.

    The first is that if there is ever a short between "hot" (DC or AC) and a piece of large metal (electrical panel, kitchen sink, etc.), the grounding of the sink+plumbing and the ground referencing of the AC neutral will mean that a short will pop the fuse/breaker in the "hot" wire/cable.

    There are "floating" or isolated ground systems, but those should use two pole breakers (like we do for 240 VAC circuits in North America, because both Red and Black are hot with respect to "ground" or neutral). By tying AC Neutral (or DC return) to "earth ground", then we don't need to have a fuse/breaker in the return lead for electrical safety.

    The other major issue is grounding of static electricity and lightning current... To prevent shock and reduce the chance of fires/damage from lightning. You could grab a large metal structure (radio mast, wind turbine tower, etc.) and if it was not grounded, get a shock of hundreds to thousands of volts just from the earth's normal electrical field (roughly 100-300 VDC per meter, much higher under a thunder cloud).

    There are other issues such as grounding for cathodic protection... Depending on how you have two objects (say a natural gas main) set relative to each other (positive or negative bias)--You could have electrolosys very quickly corrode the steel in a gas pipeline. So the utility will setup cathodic protection devices to "charge" the natural gas pipeline to reduce corrosion (the pipeline is insulated with plastic wrap too).

    Cathodic protection
    Cathodic Protection 101
    Cathodic Protection | Office of Underground Storage Tanks

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid
    Is is not a "copper plate". Some grounds are better than others (rocky ground, dry ground, tundra, etc. are very poor electrical grounds). In some cases (radio transmitters, radar installations, etc.) have a "good earth ground" can help direct the signal propagation in the "optimal" direction.

    That's very true in my case. I live in the high desert next to Joshua Tree National Park and our soil is sandy and dry. The electrical code up here requires two ground rods to be installed together (as one) to compensate for the dry sandy soil. Now I know why according to your info.
    Hmmm... It is more of a "reference. Say you want to measure how tall a bunch of people are. If they are all standing on a flat surface, then all you have to do is measure to the top of their heads with reference to the floor. However, if the people are in a building, on a hill, in a depression, etc., you cannot use the one "reference", but must measure everything relative to their "feet".

    If you were measuring water level in a dam (the higher the level, the more pressure available)--Then you have to ask is it the ocean's sea level "zero", or is it the water level at the base of the dam more relevant for a "zero" reference. Obviously each "zero reference" has its uses.

    These types of analogies are extremely useful for a person like me with zero electrical background. Although I have struggled with many of the hard, complex concepts of AC/DC electricity over the last year, I have managed to understand most of it by an information-saturation process. I'm realizing how my brain functions. Usually a couple of days after struggling with an electrical concept something will "click", then, satisfyingly, a new door of perception will open up for me. This has been a trying process for me and yet a satisfying one. I'm not well versed in the solar vernacular, so often times I have to "decipher" the code of this forum before I can even begin to reflect on it. I guess it would've been better to simply pay a pro to install the system for me but I need to know how it functions so that I can fix it myself when problems arise. I've already amassed 15 full pages of copied and pasted notes from this forum, specific to my setup, which will prove extremely valuable during the installation. I've learned that the more I cram into my gray-matter the more the fear is neutralized. Thanks to you and others on this forum, I now have the confidence to "go it alone" and am ready to pull the trigger.
    There are other issues such as grounding for cathodic protection... Depending on how you have two objects (say a natural gas main) set relative to each other (positive or negative bias)--You could have electrolosys very quickly corrode the steel in a gas pipeline. So the utility will setup cathodic protection devices to "charge" the natural gas pipeline to reduce corrosion (the pipeline is insulated with plastic wrap too).

    Oddly, I was just watching a video on cathodic protection two days ago on Youtube. One of those situations where a member of this forum (Cariboocoot) had referred to "CP" on one of my threads. I didn't now what CP was so I Googled it and discovered the wonderful new world of cathodic protection. He was actually referring to circuit protection (CP). See, there's that vernacular thing again biting me on the a**.

    You know what curiosity did to the cat, right?

    Well thank god for my 7 lives….don't ask:roll:
  • PhotowhitPhotowhit Solar Expert Posts: 5,653 ✭✭✭✭✭
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    ...My Prosine 2.0 specifies that it has a movable screw that will auto-bond between shore power and the AC load panel when sensed...

    Sorry, it sounded like you were trying to get off cheap, didn't figure you went with the most expensive of the Xantrex inverters. I've been happy with my Prosine!
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • HairfarmHairfarm Solar Expert Posts: 225 ✭✭✭
    Re: Grounding system for off-grid
    Now, the "green wire ground" (inverter case grounding, etc.)--Should be about the diameter of the DC +/- wires going to the device. I.e., if you have a 2 awg wire carrying 100 amps to the inverter, then the "nominal" green wire should be 2 awg to handle the short circuit current needed to blow the fuse/breaker if there is a short inside the inverter's case.

    The wire going into my inverter is 4/0. So am I supposed to use another 4/0 wire for ground too? I guess I should just ask what is the minimum I can get away with for grounding that 2000 watt inverter?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Grounding system for off-grid
    Hairfarm wrote: »
    The wire going into my inverter is 4/0. So am I supposed to use another 4/0 wire for ground too? I guess I should just ask what is the minimum I can get away with for grounding that 2000 watt inverter?

    Actually ground wires do not have to be as heavy because they aren't meant to take continuous current; just enough long enough to trip circuit protection. Usually ground wire is no heavier than 6 AWG.
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