Off-Grid Grounding

gengwall · October 2016

I've read a number of posts here related to grounding, and done a bunch of other research. One post here recommended looking at the Outback wiring diagrams, which I have done, as well as just googling "solar wiring diagram". I'm confused about grounding, especially on the DC side. In all the diagrams that I've looked at that have grounds shown, it does not look like the DC side of the inverter ever goes to ground. The grounds are tied together on a bus bar, but they don't show any connection to the actual earth. The only earth connection is out of the AC mains panel. I've read in these forums that both sides need to tie at a single point to the earth (in my case, a grounding rod), but I can't actually see it being done that way in the diagrams. Just want to make sure I get things right. Thoughts?

FYI - I will have a small off-grid solar system with a generator backup. There will never be utility power to the property as we few property owners aren't willing to pay the $100 a foot to bring it in 2 miles. So please keep that in mind.

Here is an example of a diagram I've looked at: http://www.homepower.com/sites/default/files/articles/ajax/docs/7_HP136_pg78_Munro-5.jpg
As mentioned, I have also looked at the Outback off-grid diagrams.

Thanks in advance for any advice. I will have one more follow-up peculiarity about my set up but I want to get straight on this first.

BB. · October 2016

I don't have much time right now--But a comment or two...

The drawing you have is almost electrically correct. It is missing a ~80 circuit breaker from the Battery+ to the charge controller Vbatt+ connection. Also, from the Battery+ to the charge controller+ should be heavy gauge wire (gauge not listed, but thinner line suggests smaller awg wire--Circuit Breakers are there to protect THE WIRING against short circuits. You should keep the wiring from the Battery+ to the circuit breaker short and well supported/insulated (using smaller awg is "allowed" for ~16" runs or less--i.e., very little chance that length will get shorted somehow).

Next--For most systems (larger/fixed installation say larger than ~1,500-3,000 Watt AC power/inverter/genset), the Battery- connection is generally connected to the common ground for the installation.

Another issue--AC and DC wiring, you can run ground wires and a single ground rod just about any way you like (within reason)... However, if you have a good chance for lightning strikes in/near to your home... Lightning has "different" rules. RF energy wants to take the "straight line" from strike to ground... If you add 20' of 6 AWG ground wire, it may take a hop off the cable and go elsewhere.

For Lightning, multi-point grounds (ground rod at based of solar array, ground rod at electric box+surge suppressors entering the home), etc.

And, while there is a lot of arguments both ways--My suggestion is that each ground rod be connected by 6 AWG cable (direct bury is nice--dirt connections help) between the main ground of the home/power system, and radiating out to other "local ground rods" at the base of the solar array, outside electrical box entrance to home, etc. The interconnect 6 AWG prevents the (for example) panel mount from becoming electrically "hot" with >40 VDC or VAC (i.e., a 120 VAC wire shorts to the frame, the fault current runs back to local ground rod, back through 6 AWG to home ground rod, then back to system AC+DC central ground, then back to the AC breaker (and it gets trip from short).

So, tell us a bit more about your system... Amount of loads (WH per day, AH @ xx volts per day), what the loads are (LED lighting, 1.5 hp deep well pump, etc.). Where the array will be mounted relative to the charge controller/battery bank, etc...

-Bill

gengwall · October 2016

Thank you for the quick reply. Our system is really very small.

We have a 400 sq ft cabin which literally only has a few LED lights for night lighting. No high watt appliances: fridges, ovens, etc. The only other need for power is in charging battery packs for tools, as all power tools on site are Dewalt 20v max and 60v flexvolt tools. (That's a separate question to deal with another time - best way to charge the battery packs).

So, I have:
Four Canadian Solar CS6P-265P panels (with two back ups) arranged 2 series x 2 strings,
A Midnite Kid 30A charge controller,
A 6x6v trojan battery bank all in series for a 36v battery system
A Tripp Lite 3600w 36v inverter
A DuroMax XP4850EH dual fuel generator

Assume for this discussion I have all the appropriate combiners, breakers, disconnects, transfer swtiches, etc.

We are in the central Minnesota deep woods so plenty of thunderstorms but very low frequency of ground strike lighting (virtually never in the woods). I can definitely run a separate ground for lightning if necessary and I understand the straight line rule. BTW I have a steel roof. Any thoughts there?

But back to the electrical grounding. Just trying to make sure I get it right. My point on the diagram was that there is no earth connection (that I can see) on the DC side.

mcgivor · October 2016

Without going into any details, here is Morningstar's take on things.

t00ls · October 2016

I have never seen an earth ground on the battery - post.....new to me ...I dont see the advantage of it

Dave Angelini · October 2016

Well, that is because you have an XW system and in this application the ground is provided by the fuse in the charge controller or controllers.
It is a protection to your system and to ground fault electrocution if bus full of kids parked at your solar array and there was a defect.....

BB. · October 2016

A typical off grid battery based system will work fine without DC grounding. Most smaller system simply float the DC. Reasons for grounding:

DC Safety--If there is a is a short circuit to metal (cold water pipe, sink, ac ground, etc.)--Having a DC ground (and AC ground, if TSW type inverter) gives a return current path to blow fuses/breakers back to the Battery bank (or AC panel, if used). If you do not ground the DC battery negative terminal--It is possible to have a DC negative ground (for example in a light fixture short circuit), then later a positive to ground short somewhere else (say AC inverter + input). Now the fault current goes through the DC fixture ground/negative wire and starts a fire (without tripping the DC breaker for the inverter). Floating power systems really are supposed to have a two pole breaker on both the + and - leads from the battery bank for safety (if there is a short in the + or -, both poles are tripped). But, this does double the costs of breakers (and the mechanical installation).
Lightning--If you have a hit on the solar panels or else where in the DC side (and helpful for an AC wiring hit)--It gives a safe path for lighting to go to ground--Rather than finding at escape to ground elsewhere.
Static Charges--Solar panels on a roof can build up a static charge (especially under a thunder cell)--DC grounding bleeds off static buildup to earth ground.

-Bill

Dave Angelini · October 2016

And grounding at the array to a local ground system at the array, as well as by a code required connection to the home
is just as safe for lightning, DC safety, and Static charges.

gengwall · October 2016

Hey - great info everyone. Thanks. I think the ultimate answer is that floating the DC ground is acceptable but there is no reason why I can't tie it in at the ground rod if I want that extra assurance.

On a related note - the peculiarity I mentioned in the opening post is this. My charging station will be distant and physically disconnected from my cabin. The battery pack(s) will be trailered and brought to the cabin and connected to the inverter at that point. So, at the charging station, where there is no AC side of things, can I still float the ground for all the DC and the PV panel frames? Again, the Outback diagrams and even the MornignStart diagram Mcgivor referenced seem to imply this would be fine. On the other hand, the charging station is much more out in the open (i.e. not under a bunch of 60 foot tall trees) so maybe lightning could be a greater concern. I mean, it isn't like in a 100 acre field or anything like that. More like what I would call a clearing. But still, it is open ground.

t00ls · October 2016

ahhhh......I just had ah ha moment...yup it all makes sense now

BB. · October 2016

If there is a metal trailer frame--that would be the "Chassis" ground.

If you don't want, or cannot do, a ground rod... Look at a ground plate that can be buried not to deep--To give you a lightning path.

http://www.tlpinc.com/products/grounding/ground-plates.html

Is the AC inverter in the trailer? I am sure I see a DC run to an inverter in the cabin (lots of voltage drop).

And, you probably would want surge suppressors and ground rod/plate with the AC output in the trailer. From one, now retired, NAWS owner, most lightning damage seemed to take out the AC inverter's AC output.

-Bill

gengwall · October 2016

Metal trailer - check

No, the inverter is in the cabin. Going to bring the trailer to the cabin and "plug in". Very short run between batteries and inverter - 4ft max.

I can definitely do ground rods. Very sandy soil down to 10 ft deep, then hits water table (on the edge of a swamp). Plus I got a big old nasty SDS rotary hammer to help me pound it in.

To summarize -

Inverter is in the cabin and will be ground to earth on the AC side through the main panel. Will have cord coming out of outside disconnect box to connect to battery box in the trailer. Trailer will be backed right up to the building so very short cable run from batteries to inverter.
Cabin is 1 story + loft with metal roof, but surrounded by 40-60 ft tall pines. No evidence of ever having lightning ground strikes in the area or even any tree hits. I know, never say never. Lightning causes most wild fires. But buildings in central MN rarely if ever get hit by lightning even in the open (or so I assume because I've never heard of one being hit).
Charging site is apx. 450 ft away (line of sight - so let's say 500 ft) through the woods in a clearing at basically the same elevation as the cabin.
PV array will be ground mounted.
The batteries, when on the charging side, will be connected to charging station the same way - cord from charge controller (via a disconnect) connects to battery box (i.e. same battery box receptacle used for in and out - problems?). Again, trailer is backed right up to array so very short run between CC and batteries.
All expected breakers, disconnects and combiners will be used throughout.
Generator is also at the cabin. Have a transfer switch and was planning on using the inverter AC as the "utility" side of the switch (I have a question about making it 120/240 to use both hot lines to the mains, but that's for another day). On the other end, was planning on using a main panel breaker to feed the inverter AC input for charging when I use the generator. The genset has enough output to feed the cabin circuits and charge the batteries.

At this point you all are probably asking "why not just run 500ft of underground cable and bring the AC from the charging station to the cabin". Several reasons.

Forest is pretty thick and trenching around all those trees and through all those roots is impractical. Going overhead is equally impractical.
We eventually plan to have 3 of these cabins scattered around the 40 acre site and won't necessarily use the same one all the time. Bringing AC from the charging site to 3 buildings compounds #1.
We eventually plan to have several battery trailers to support #2, and want the ability to be both stand-alone charging and using stored power simultaneously - i..e swapping out trailers as needed.
We also have various sheds and out buildings on the property that we might want to bring power to, but don't necessarily need permanent power.
Generator hookup gets real complicated if I'm using the generator for both cabin power and charging. Basically have to either have generator and transfer switch remote at the charging station or make two 500ft AC runs.
And...the first "trailer" is actually a golf cart, so...just saying it is going to be mobile anyway.

Cost isn't the issue - I know in this scheme I am going to need multiple inverters at a much higher cost than a few hundred feet of wire and a trencher rental. Flexibility and keeping things small - at a 30A level - is the objective.

BB. · October 2016

What size inverter and voltage/amphour battery bank?

Charge during day and discharge at night?

-Bill

gengwall · October 2016

Bank is 6 x 6v 225ah in series for 36v. Inverter is Tripp Lite 36v 3600/7200W. Panels are 4 Canadian Solar 365W connected 2x2. Controller is Midnite Kid.

Yes, charge day discharge night right now, until I get a second trailer. Then I can charge trailer A during day while simultaneously using trailer B, or Charge either as necessary during day and use both at night, as energy needs dictate. And once a third trailer comes - well, you get the idea. We anticipate that under minimal use, the bank would power the LED lights for several, maybe even many nights without getting discharged, so once a second trailer is in place, it would be able to be charged over several days. And, of course, we also have the generator to charge the batteries or provide power if needed.

Normal energy consumption is only a few led lights and charging 20v power tool batteries and cell phones. Occasional energy consumption would be from maybe a laptop. Rare energy consumption might be an induction hot plate for short periods. Also, maybe a couple of small exhaust fans or water pumps - but none of that exists today and we really don't think we will need them. But time will tell. We just got the cabin enclosed this fall so we haven't experienced summer heat yet to know if we need to exhaust the loft (and we may go with solar chimney or other non-electric air movement, or use stand-alone small solar fans). We know the main floor stays comfortable all day with a breeze and the shade of those pines even in 90 degree temps.

It really is a sweet set-up and we anticipate our energy needs will be very light, but need some flexibility because of our future development plans.

BB. · October 2016

Some suggestions...

First, I would suggest thinking twice about the 36 volt battery bank--That is pretty much an orphan these days for solar/off grid power. You will not find many AC inverters and DC loads that will run at that voltage anymore... I would suggest staying with 12/24/48 volts. Yes, you can get solar charge controllers that can be programmed for a 36 volts (many of the major brands will support 36 volts)--It is the balance of the system that is a bit dicey (including finding a DC Genset or AC to DC battery charger--Although, fork lift chargers are available).

Lead acid batteries really like to be cycled and not on a steady drain for more than a couple of days (they start to sulfate). So, you may need to look at something else (LeFePO4, NiCad, possibly AGM) if you want to (for example) go for a week between charges.

I believe that playing the trailer dance is going to get old pretty soon. If you discharge a battery bank to (for example 50%), it is going to take 2-3 days to fully recharge it (summer maybe less, but winter in Minnesota--That will probably need a genset).

Also--There is a pretty big difference in power/battery bank/inverter/wiring etc. between running a laptop, led lighting, and a few DC fans vs an induction hot plate. If you are going to have propane for a refrigerator, I would sticking with propane for cooking/hot water/etc.

If you have any possibility of stringing wire (above ground or buried)--I would highly suggest that. Also, I might suggest wiring up a "high voltage" solar array (upwards of 150 to 400 VDC Vmp-array) and using a high voltage array solar charge controller. They are much more expensive, but will allow you to keep the long distance wiring much smaller awg (less to string/bury).

https://www.solar-electric.com/misoclli250m.html 250 Voc input
https://www.solar-electric.com/morningstar-ts-mppt-600v-48-solar-charge-controller.html 600 Voc input
https://www.solar-electric.com/xaxwmp80amp6.html 600 Voc input

For security--Now you can lock up all of your equipment in the cabin(s) vs having it spread around (array, trailers, cabins).

I am going to try very hard to diswade you from 36 volts and the battery trailer.

Also--Need to define your loads better. Until you have a good handle on your loads, it makes it a real shot in the dark to design the rest of the system...

Just to give you some ideas:

500 WH per day -- Minimum LED lighting, cell phone charging, a bit of laptop use, 12 vdc pump (micro system)
1,000 WH per day -- Nice size for lighting, 12 vdc pump, laptop charging, efficient fan, etc. (small, nice for weekend cabin)
3,300 WH per day -- above + refrigerator, well pump, washing machine, TV, etc. (full time off grid very energy efficient home)

Anything larger--That is a pretty big system.

-Bill

gengwall · October 2016

Ironically, sunny days in winter in MN with snow on the ground can be much "sunnier" than summer. Not warmer, but plenty bright.

Yes, I have propane and have considered a propane fridge and cook top, although we aren't to that point yet. Right now, just being there on weekends, a big cooler with a 20lb bag of ice fulfills our refrigeration needs. And in winter...well...refrigeration is the least of our worries. Cooking right now is either on the grill or on a camp stove, so no induction burner. Just thinking about possibilities in the future. If it doesn't work to use induction cooking, no great loss.

I agree on 36v, but that's what i'm kind of stuck with, or better put, inherited - i.e. the golf cart - for right now. Of course maybe the best course of action would have been to convert the golf cart to 48v. LOL. Hindsight...

Yes - I will have to get a better handle on my loads, which is one reason why I'm not going whole hog into the charging side yet. The cabin isn't even wired yet, so everything is a guess.

If I can get by with what I've proposed, and I'm into it for less than $2k, then I will be a happy camper. If I can't, and have to build out a bigger system along the lines you've described, I am only out $2k, but not even that because some can be reused, re-purposed, or incorporated into the expansion. So it is what it is right now.

Our plans may change over time but I'm dealing with what I have on hand first, then will evaluate. At any rate, my original question about grounding has been answered. I've started another post on my next issue, so we can pick it up over there. Thanks for all the input.

BB. · October 2016

You can continue in this discussion too. The information about your goals is very helpful.

-Bill

BB. · October 2016

For a starting point... Lets look at 1,000 WH per day rule of thumb design (nominal choices). Assuming 2 days storage and 50% maximum discharge (longer battery life):

1,000 WH * 1/0.85 inverter eff * 1/12 volt battery bank * 2 days storage * 1/0.50 discharge = 392 AH @ 12 volt battery bank

2x 200 AH @ 6 volt golf cart batteries * 2 parallel strings (4x batteries total) will give you a 400 AH @ 12 volt battery bank

The maximum AC inverter I would suggest would be a 1,000 Watt inverter (based on 400 AH @ 12 volt per 1,000 Watt inverter or max solar panels).

The actual inverter I would suggest would be a 300 Watt 12 volt TSW inverter from MorningStar. It has remote on/off and a "search mode" to reduce standby current. 300 Watt continuous (600 Watt peak) will supply a cabin very nicely (LED lighting, laptop/LED TV, cell phone charger, 12 volt RV pump for water pressure).

To charge such a flooded cell battery bank, I would suggest 5% to 13% rate of charge--For a full time off grid system, 10%+ rate of charge suggested:

400 AH * 14.5 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 753 Watt array "nominal" for full time off grid (good battery charging current)

Then there is sizing the array based on your load and hours of sun... Not sure which is closest to your place, but here is a fixed array tilted to best year round performance::
http://www.solarelectricityhandbook.com/solar-irradiance.html

Grand Forks
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 42° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.32	3.46	4.21	4.94	5.22	5.19
Jul	Aug	Sep	Oct	Nov	Dec
5.40	5.24	4.42	3.53	2.62	2.10

This one may have more choices (enter in your city/state):
http://pvwatts.nrel.gov/pvwatts.php

If you toss the bottom three months (have to use a genset when there), February "break even" month:

1,000 WH per day * 1/0.52 off grid AC system eff * 1/3.46 hours of sun (Feb) = 556 Watt array (Feb)

A 753 Watt array will "break even" down to:

1,000 WH per day * 1/0.52 system eff * 1/753 Watt array = 2.55 hours of (noon time equivalent sun) per day (pretty deep winter).

Get a roughly 40 Amp AC battery charger and a Honda eu2000i (1,600 watt) genset--And you would be set to go.

Also, to better estimate your AC loads, you can get a kill-a-watt type meter--Also very handy for conservation measurements of your plug in appliances at home.

-Bill

softdown · October 2016

I'll admit to being a tad lost on some of the computations here. Just wanted to say that I would recommend getting away from 36 volt and into 48 volt as soon as possible. I recently searched for a 36 volt charge controller and found exactly one that was still available. A month later, it too was seemingly gone.

With all of your plans for future expansions, I see almost nothing but problems with trying to use a rare 36 volt system. 48 volt systems double the carrying capacity of wiring.....vs 24 volt. Saving money and increasing efficiency. Mostly saying this for the benefit of others.

As for grounding and steel roofs.....I am preparing to run either 4awg bare copper or 4/0 aluminum (have a bunch) from one end of the roof to the other. With 8000 square feet of steel roof, it could act as a lightning rod. I use grounding rods in three places:
(1) solar panel array
(2) inverter
(3) AC panel
The strand across the steel roof will be the fourth ground.

***I am not an expert. BB may be our most knowledgeable frequent poster.

BB. · October 2016

I am not an expert in the sense that I can pick/design "your system". I look at more of an educational discussion that helps you to design a system that will reliably meet your needs.

Regarding metal roofs/walls--Forgot to answer that one. In general, metal roofs do not supply any special/better lightning protection (from damage) than any other construction. If you are in a lightning area--Then you should do the lightning rods/cabling just as you would on any structure. A metal roof may keep you a bit safer--But unless the roof has lightning grounding bonding cables attached and taking the energy to ground (vs a steel framed building where the frame does help)--You will still be subject to damage as the energy finds "some paths" to ground (in my humble, not an expert, opinion).

Using a well designed lightning rod+cable+ground system helps reduce the chances of the lightning getting near you and your stuff inside the home/building.

-Bill

softdown · October 2016

Just about used up what I thought was a large roll of 4awg bare copper. 90' one way, 20' to connect to the copper grounding wire on the solar panels, and ~45' to grounding rod. Not true bonding cables but not bad one would hope.

This building does have steel siding. Perhaps I likely wasted my time? Can a person overdue lightning protection? I am mostly trying to draw lightning away from more sensitive areas.

BB. · October 2016

I am not sure you can over do lightning protection, but you can do it wrong.

Read up about how to do it and what to avoid.

-Bill

Vic · October 2016

softdown said:

I'll admit to being a tad lost on some of the computations here. Just wanted to say that I would recommend getting away from 36 volt and into 48 volt as soon as possible. I recently searched for a 36 volt charge controller and found exactly one that was still available. A month later, it too was seemingly gone.

With all of your plans for future expansions, I see almost nothing but problems with trying to use a rare 36 volt system. 48 volt systems double the carrying capacity of wiring.....vs 24 volt. Saving money and increasing efficiency. Mostly saying this for the benefit of others.

As for grounding and steel roofs.....I am preparing to run either 4awg bare copper or 4/0 aluminum (have a bunch) from one end of the roof to the other. With 8000 square feet of steel roof, it could act as a lightning rod. I use grounding rods in three places:
(1) solar panel array
(2) inverter
(3) AC panel
The strand across the steel roof will be the fourth ground.

***I am not an expert. BB may be our most knowledgeable frequent poster.

The Specs for the OB FM CCs state that they will support 36 V battery charging.

The MidNite Classic and KID CCs note that these CCs also support charging 36 V batteries.

I have not used any of these on a 36 V battery, FWIW, Vic

softdown · October 2016

Vic said:

softdown said:

I'll admit to being a tad lost on some of the computations here. Just wanted to say that I would recommend getting away from 36 volt and into 48 volt as soon as possible. I recently searched for a 36 volt charge controller and found exactly one that was still available. A month later, it too was seemingly gone.

With all of your plans for future expansions, I see almost nothing but problems with trying to use a rare 36 volt system. 48 volt systems double the carrying capacity of wiring.....vs 24 volt. Saving money and increasing efficiency. Mostly saying this for the benefit of others.

As for grounding and steel roofs.....I am preparing to run either 4awg bare copper or 4/0 aluminum (have a bunch) from one end of the roof to the other. With 8000 square feet of steel roof, it could act as a lightning rod. I use grounding rods in three places:
(1) solar panel array
(2) inverter
(3) AC panel
The strand across the steel roof will be the fourth ground.

***I am not an expert. BB may be our most knowledgeable frequent poster.

The Specs for the OB FM CCs state that they will support 36 V battery charging.
The MidNite Classic and KID CCs note that these CCs also support charging 36 V batteries.
I have not used any of these on a 36 V battery, FWIW, Vic

Fair enough....but what about the inverter? My memory is a bit rusty on this one.

BB. · October 2016

Probably a computer/19" rack based ups.

There are many that don't fall into our 12/24/48 VDC realm.

-Bill

Off-Grid Grounding

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Grand Forks
Average Solar Insolation figures

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Off-Grid Grounding

Comments

Grand ForksAverage Solar Insolation figures

Categories

Grand Forks
Average Solar Insolation figures