Off Grid Solar and Grounding questions

JYanke

Hello, 
Im in the process of relocating components of my small off-grid solar system in my cabin. While Im at it, I want to consider grounding some of the components.
The system will consist of the following:
Power Supply 1: Suresine 300W Inverter with isolated/floating AC output connected to a transfer switch on one side. 

Power Supply 2: A 9000 watt gas generator with a mfg. installed neutral/ground bond at frame connected to the other side of switch. 

The transfer switch then feeds to a small breaker panel with isolated neutral and ground lugs.(NO Neutral/Ground bond) From the panel is 4 main cabin 15a circuits consisting of 14/2 romex with all bare ground wires terminated on the ground bus bar which will be connected to an earth ground rod outside the cabin. 

Question 1. Is it ok to have the isolated/floating output from inverter feeding a panel that will have standard grounded circuits without neutral/ground bond in panel? I understand the suresine manual instructs installing a jumper on the neutral output to the ground lug on the DC side, but was planning to omit that based on other discussions here on the forum. 

Question 2: With generator power, since it is neutral/ground bonded will there be any issues here? Especially since the switch inputs and outputs only accept my designated hot and neutral wires (no bare wire) and ultimately will feed a non neutral/ground bonded panel. Im thinking this will be ok but not sure about ground loop issues. 

Question 3: Since I will have a designated earth ground bus in the AC panel, I am considering grounding other solar components to that same bar. Such as the negative side of the PV array (per manual), the DC ground lug on inverter (per manual), negative side of battery bank. See attached sketch. 

Look forward to your expertise! 

BB.

From an AC wiring point of view, having the 300 Watt Suresine inverter powering your 120 VAC lines as "floating" is not going to be a fire hazard. You are using 14 AWG or heavier wiring, so there is no chance of having over current (>15 amps) on the neutral/white/return line(s).

If you are worried about shock hazard, you can use GFI outlets where needed (near sinks, outside outside outlets, etc.

With the 9 kWatt AC genset, you really need the Neutral+Ground bonding. This ensures that "Neutral" is never above Zero Volts, so there is no chance of a short between Neutral and Ground elsewhere in the system. "True floating" AC (and DC) outputs should have breakers/fuses on both "Hot and Return" to prevent overheating of wiring in some short circuit cases.

You have several options here... You need to connect a green wire ground from your AC ground bus bar to the genset frame (or through the 120/240 VAC genset cord (assuming N+G bond inside genset). This N+G bonding when genset is running, and N floating with AC inverter running...

Then the transfer switch... Some transfer switches only transfer H+N (or H+H+N) and others will also switch the ground (i.e., floating ground when set to Genset, and N+G bonding when connected to AC inverter).

OR--Lift the N+G in the genset and connect N+G bond inside the main panel. That is the traditional method. The only downsides are a) genset should have N+G connection made when moved genset is moved elsewhere/stand alone operation.And b) not all genset have an obvious/easy access to N+G bond connection.

The details matter here... Is the genset 120 VAC only or 120/240 VAC? Most 9 kWatt gensets (guessing) are 120/240 VAC. Some have switchable AC output (i.e., 120/240 VAC at 120/240 @ 40 amps or 120 VAC only @ 80 amps).

A 9 kWatt genset uses a lot of fuel--And is pretty fuel inefficient below ~50% (to 0%) of rated load (i.e., if using 300 Watt load, 9 kWatt genset will use almost as much fuel gallons/hour as running a 4.5 kWatt load). It would seem like using a much smaller genset (1-2 kWatt) would be more fuel efficient unless you plan on a major upgrade to the cabin AC power (larger AC inverter) in the future.

If 120/240 VAC split phase power--What are you doing at the panel? Everything on 120 VAC circuits (use 1/2 of main panel breaker slots, or wire Red+Black 120/240 panel to 120 single phase all breakers)?

The transfer switch, how many switching contacts (1, 2, 3, 4)?

Depending on the specific transfer switch, you can have the transfer switch doing the switching (N+G bonding for inverter, let genset N+G bonding running on genset).

Don't connect the Green wire from system ground to the negative solar panel lead... That gives you parallel current path back to battery through the negative solar wiring and the green wire wiring--It is a redundant connection, and we "don't want" normal load currents flowing through the safety ground systems. Plus there are a (very?) few solar charge controllers where wiring negative panel wire "around' the charge controller can cause issues (those few solar charge controllers measure current in the negative solar lead).

If you have lightning in the area, it would be better to use good quality surge protection on wiring that enters the cabin (solar panels, genset, AC mains--if ever):

https://www.solar-electric.com/search/?q=surge+protection

Or for the AC power side, install a surge protector in the main breaker panel.

Sorry for the complicated answer (and confusing, I am sure). This is not a simple yes/no question.

-Bill

JYanke

Thanks Bill. Exactly what I was looking for. Heres the answers to your questions:

1. I was intending to replace the outlets in the bathroom,kitchen,outside with GFCI for awhile now but wasn’t sure how they would function on a floating AC?

I like the idea of connecting the ground wire coming from genset cable to the ground bus bar in breaker panel. Making Genset power N/G bond and Inverter power floating. The manual for my generator says its a neutral/ground bond at frame and suggests running a ground wire from frame to an earth ground rod if using at a permanent location. 

The 9000 watt dual fuel (LP/Gasoline) generator was purchased to meet my sometimes high power demands such as Tablesaw, Miter saw, sometimes a window AC unit, etc. Otherwise I do have smaller generators as well. The 9000 watt unit has multiple power options: 

120v, 20A Duplex GFCI
120v, 30A Twist Lock
120v/240v, 30A Twist Lock
120v, 50A 

I only intend to use the 120v, 20A GFCI when connecting to the cabin. Which in theory makes the whole supply GFCI protected? 

2. The Transfer Switch is a 6A-63A, 2P/2 way, 110v 60Hz. It accepts a hot and neutral wire on the inverter side and a hot and neutral on the generator side. Then,a hot and neutral output going to the breaker panel with an automatic or manual switching function.  

3. In the panel, I just have all the breakers (only 4) wired to single phase 120v. I will also look into the surge protector idea that u suggested. 

4. As for fusing the AC and DC hots and returns: I currently have a 100A fuse on the DC positive side between battery bank and Inverter. Should I also put a 100A fuse on the negative?  

The Suresine manual says a 3a fuse on the designated hot output on AC side. But have concerns considering the inverter has a peak wattage of 600 and worried that might cause nuisance trips? What are your thoughts on fusing the hot and return on the AC side of the Suresine? 

I updated the grounds on my sketch removed the negative ground on panel and brought ground from genset to AC panel ground bus. (See attached) 

Thanks again! 

BB.

1. GFI outlets will work fine on "floating AC" power... They measure the "sum of the current' on H+N is less than 0.010 amps. It does not "care about" grounding and such... Only the Sum of the Current between the two power lines is near zero.

You could run into issues with MSW (modified sine/square wave inverters) and other possibly "noisy" AC inverters causing false GFI trips.

Where you can run into GFI issues is the example of a genset with GFI... N+G bonding inside genset connected to your AC panel with N+G bonding... You now have a path for current flow in the Green Wire and the H+N is >0.010 amps (rest is flowing through the green wire in parallel). That is why N+G in only one location is important with GFI protection.

Connecting to the cabin with the twist lock / non-GFI generator output is fine... Use GFI inside the cabin where needed...

I suggest not using GFI to cabin power connection... Use the 30 Amp 120 VAC twist lock (my suggestions). Don't genset GFI if N+G in generator and cabin. And personally, I don't want to "kill" AC power to the whole cabin... A GFI trip (10 milliamps is not much current) that trips the genset GFI cuts AC power to the whole cabin--And kills your lighting too--Don't want that to happen at night.

GFI where needed (sinks, outside outlet, wet areas) only trip limits the outage to where it happens--And you don't have to hunt around the whole cabin to find the the fault location (or do that in the dark).

Note that your green wire/ground rod should also be connected to cold/hot metal water pipes, and even to metal gas/propane lines (for example, s short from 120 VAC Hot to sink finds its way back to green wire safety ground and trips a breaker/fuse/inverter. If sink is not grounded, then could become "120 VAC hot" and somebody could get a shock from sink/plumbing to something out--Such as wet ground, grounded power tool, etc.).

2. The transfer switch "works". If you are only using 120 VAC single phase with N+G bonding in AC main panel (and lift/disconnect bonding for N and G in genset)--Then you only need to switch the one 120 VAC "Hot" (black or red) wire. The neutral does not need to be switched.

That N+G bonding in panel, and lifted/floating genset is the simplest method with transfer switch (one or two wire switching, don't really care).

Floating your 9 kWatt works. And if you have 3 kW or smaller other gensets--They generally "float" their AC output anyway (always check manual and/or with an Ohm meter to be sure if in doubt).

3. 4 circuit with single phase 120 VAC works. N+G bonding would be "to plan".

4. The Suresine is 300 Watts continuous, and 600 Watts for something like 15 minutes (as I recall)... The DC current would be:

• 600 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage = 67 Amps worst case DC current to inverter

You don't say what AWG your DC wiring to inverter is... But remember fusing/breakers are their to protect the wiring (not the device directly). From NEC table (simplified):

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

4 AWG cable is a nice fit for inverter power.

Note that DC cable to inverter should be kept short (lower voltage drop). If cable is longer than 10 feet (one way run), then may need to revisit and look at heavier cable (to keep voltage drop to 0.5 volt max @ 67 amps).

The continuous current derating (this is really worst case worst case):

• 67 amps * 1.25 NEC derating = ~84 amp rated branch circuit (80 or 85 or 90 amp rated fuse/breaker).

And using a circuit breaker vs fuse... Makes it easy kill all power to the inverter before heading home from the weekend/for winter. Instead of fuse+main battery switch.

In the main panel, suggest a jumper wire from Neutral bus bar to Ground bus bar (N+G bonding). Some panels, the Neutral bus bar has a "Ground Bonding Screw" integrated into the bar.

-Bill

Photowhit

JYanke said:

1. I was intending to replace the outlets in the bathroom,kitchen,outside with GFCI for awhile now 

You should only need 1 GFCI outlet per circuit or a GFCI breaker.

Bill likely cover this, but I didn't see it skiming over...

JYanke

Thanks for the great info. This helps a lot.

So if I understand correctly, Say for instance I left the genset N+G bonded with the ground wire tied to the ground bus in the NON N+G bonded breaker panel. And the Hot and Neutral wires from the Genset tied to the transfer switch that ultimately  feeds the breaker panel with the H+N from genset

Would the problem arise when I switched the transfer switch to floating AC Inverter power, since the switch I have does not switch the ground wire from generator? Leaving that ground wire going back to genset as another path to ground from the breaker panel while Im switched to Inverter power? 

I understand the idea of N+G bonding the AC panel and removing the N+G bond at Genset but have concerns about access to where that actually is on my generator (based on some quick google searches) and that I would need to reconnect that bond if and when Im using the genset stand alone outside this application. 

BB.

The "problem" with two N+G grounds is when there is a GFI outlet involved. N+G bond at geset--Power through GFI to cabin, then N+G at main panel. This will trip the GFI (parallel current path through white neutral and green wire safety ground).tr

If you do not use the GFI--You still have a parallel current path through white neutral and green ground cables. This does is not allowed by code (NEC--National Electric Code) and while everything will work--There is a secondary issue where bringing in AC current through two different paths into a metal breaker box can overheat the sheet metal of the box when heavy current flow is present).

As Photowhit says--With GFI outlets, they do have an AC power "IN" and "OUT" set of connections. For example, the AC wire to several outlets (say two outlets near sink inside, and an outside outlet on the wall behind the sink). You can wire the AC power to GFI IN at the sink 1, and wire GFI OUT to the Sink2 and outside outlet--All down stream outlets on the GFI OUT are also ground fault protected.

Another grounding option is to tie Neutral + Ground together AT THE INVERTER"s AC output--I.e., do the N+G bonding at the inverter--Just like it is done at the genset. This way when your two contact transfer switch is switched--Your AC power will be ground referenced in either case.

Note that the Suresine has a "floating" AC output... There are no Hot/Neutral AC terminals as far as the inverter is concerned (both AC outputs are identical). There is only a Hot & Neutral when you connect one terminal to safety ground.

And you will be fine.

You do have the "issue" of no Cabin AC power N+G bonding if the genset is disconnected and the transfer switch is set to GENERATOR--But it should not be a problem as there is no AC power present anyway.

I forgot your other question... Fusing the AC output of the inverter with a 3 or 6 amp fuse... Technically, the AC inverter's internal electronics are faster than any fuse/breaker would be (inverter should shutdown if over current on output--Or inverter gets hot/overheats). More or less, in my world, fuses and breakers are their to protect your wiring. Since you will be using 14 (or 12) AWG cable--the 3-6 amp output of the inverter is less than the 15 Amp output of the inverter--So no output current protection is needed.

Lastly--You may want to wire the Remote AC On/Off of the AC inverter to a handy wall switch inside the cabin. This be a handy way to save power without using the Main On/Off switch in the DC wiring (wall switch easier to reach--Don't "wear out" the main switch or circuit breaker with multiple cycles per day.

When you leave the cabin, turning off the DC power is still a good idea--The inverter still draws a little power in Remote Off mode.

There is also Search Mode for the inverter... More than 6 Watts of AC loads, inverter turns on. This may not work with small AC loads (like USB p;hone charger or smaller LED lights) unless other loads are turned on too.,

-Bill

JYanke

Okay. 

So my plan is as follows. 

SureSine Inverter: Safety Ground lug terminates on ground bus bar inside service panel. Make N+G bond with Neutral and Safety Ground lug and tie the H+N to the 2P switch. 

Genset: N+G bond is already made at unit/frame. Bring power from Genset on the 30A Twistlock outlet (Not GFCI) and terminate H+N on the other side of the 2P switch. Bring the green/bare ground wire from Genset and terminate on ground bus bar inside service panel. 

2PSwitch: Bring H+N into service panel as “normal”. 

Service Panel:  The N+G bond is removed from service panel. 4 main circuits take power throughout cabin with 14/2 Romex. Ground bus accepts all bare ground wires from main circuits. Ground bus also connects to an earth ground rod which ties to inverter safety ground, battery bank negative side, and Genset ground (heres where Im confused at having two potential earth grounds?)

GFCI: Realistically Ill update the bathroom outlet to GFCI and one outside outlet I have on the porch. 

Remote AC On/Off:  I do have a switch installed for this currently on the SureSine and its very handy

 

So with this set up above, I should be good switching the 2P switch between power sources and still maintain a safe ground in both scenarios?   

See anything I missed or have wrong? See updated sketch. 

Thanks again 

BB.

Looks good... A "semantic" argument would be to make the "ground rod" (part above ground) as your "Ground Window" where all AC and DC grounds meet.

You can meet the AC Ground on the negative battery bus--But it is "code" is to connect grounds at the ground rod. That way, you do not "lose" the AC safety ground if the DC bus is taken apart (say to service/replace the battery). One ground wire to battery negative terminal/bus. A second ground wire from rod to AC panel ground bus bar.

The code intention is that the Safety Ground (at ground rod, in appliances, etc.) need the service person to explicitly remove the safety ground...

And not remove the safety ground "by accident" when working on something such as taking apart a metal case, working on battery/bus terminals, etc. --- Cannot use "an assembly screw" as a ground point too. If we have a stud that has multiple connections... The "safety ground" is first, then nut, then local "grounds" are stacked and a second nut.

Make sense?

-Bill

JYanke

Makes sense. Thanks for all the help. I really appreciate it. Ill let you know if I run into anymore questions. 

JYanke

I thought of something else we haven’t discussed yet. I know earlier you mentioned not connecting the negative from PV to ground which I will not. 

I have the Midnite Solar MNPV3 combiner box and only utilize one 15A~150VDC breaker between my PV Array and my charge controller. Inside the combiner box there is the PV positive + bus and PV negative- bus and also a separate bus for equipment ground (which appears to be isolated from the PV negative.)

Would it be okay to also take a green ground wire from the equipment ground bus in combiner box to the ground bus in my AC panel? (which is in close proximity of eachother) and the AC panel ground will be tied to earth. 

The MNPV3 manual also shows tying all the solar panel frame grounds to the equipment ground inside the combiner box. See attached pic from midnite solar manual. Thanks!

BB.

I have the Midnite Solar MNPV3 combiner box and only utilize one 15A~150VDC breaker between my PV Array and my charge controller. Inside the combiner box there is the PV positive + bus and PV negative- bus and also a separate bus for equipment ground (which appears to be isolated from the PV negative.)

That is correct... The PV- bus is separate from safety/equipment ground.

Would it be okay to also take a green ground wire from the equipment ground bus in combiner box to the ground bus in my AC panel? (which is in close proximity of each other) and the AC panel ground will be tied to earth. 

I suggest that the green wire ground from the solar panels be physically separated from all other wiring (DC and AC) from solar panel/mounting grounds to the ground rod (outside of foundation next to array/exterior wall) if there is any reasonable chance of getting a lightning strike in your area.

You don't want to bring lightning energy inside of your home. If you connect the array ground to your AC inside your AC panel--Who knows which path(s) the lightning energy will find to ground rod/earth.

Because you will be connecting all three grounds (array, AC, and DC) grounds to the ground rod (or multiple ground rods tied all together with 6 AWG cable--The 6 AWG stitching all ground rods together ensures a path for AC/DC short circuits--To provide electrical path to trip breakers and fuses in case of any "hot to ground" shorts.

The MNPV3 manual also shows tying all the solar panel frame grounds to the equipment ground inside the combiner box. See attached pic from midnite solar manual. Thanks!

In your case the Combiner Box is mounted on the side of the house closer to your DC/AC power systems than the solar panels (I think). I would suggest not to "Bring" the array grounding wires to the DC combiner box--You are bringing the lightning energy next to your DC circuits after they have been separated at the array.

Generally you will combine all panel ground wires (either a single wire "stitching" panel frames together, or separate grounds to each panel) and bring one wire from the array down the outside of the building directly to the ground rod.

Obviously, separating DC/AC from lightning ground to ground rod by a few feet does not matter to a bolt of lightning that just traveled several miles from sky to earth--But avoiding co-mingling ground and electrical wiring at your home is something I would suggest avoiding.

The above is just based on my personal understanding of lightning. For example, routing lightning cabling should have "soft rounded" corners vs a sharp "right angle" turn because the increased impedance of the "sharp corner" can cause lighting to "leave the wire" and find a different path to ground.

Bringing such energy inside a "box" on the wall of a home with DC (and/or AC) wiring--Is just something I would avoid.

-Bill

JYanke

Thank you for the info. 

A side question not related to grounding:

The new location for batteries and all other components requires a few changes in wire runs.

The distance from pv array to combiner box & charge controller has increased about 10-12 additional feet one way with mc4/4mm2 cables. Combiner box has 15a breaker. 

Im going to try to keep the one way wire run under 10ft from 15a Sunsaver charge controller to batteries and still use 12awg wire. 

Now, on my current set up with very short runs between everything I had 12awg from controller to batteries, and for some reason I used a post mounted 25a fuse on the positive side where controller connects to battery. I think this should be either a 15a or 20a fuse between battery and controller for my 12awg wire? 

1. Is it okay to continue using 12awg and the 11awg for runs from panel->charge controller->battery bank? Panels to charge control is roughly 20ft one way. Charge control to battery is <10 ft one way. 

2. I think I need to use a 15a fuse between battery and controller assuming I stay with 12awg?

JYanke

Hi Bill, I have a question about inverter cable sizing
we discussed earlier the following for the SureSine 300W inverter:

• 600 Watts * 1/0.85 AC inverter eff * 1/10.5 battery cutoff voltage = 67 Amps worst case DC current to inverter

• 67 amps * 1.25 NEC derating = ~84 amp rated branch circuit (80 or 85 or 90 amp rated fuse/breaker).

With that said, my new layout has my distance from inverter to 12v battery bank at roughly 8 foot run (one way)
Using this calculator: 
https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=7&necconduit=steel&necpf=0.85&material=copper&wiresize=0.1608&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=8&distanceunit=feet&amperes=67&x=46&y=30&ctype=nec
 Im thinking I need to use at least 2awg instead of the 4awg I previously used on my shorter run in order to stay close to a 2% voltage loss. I understand it would make more sense to keep the inverter as close to batteries as possible but Id like to install the inverter in the new section of cabin where most other components are going and that puts me closer to 8ft away. 

Appreciate your thoughts on this

BB.

Some thoughts...

For the SureSine inverter, typically we call it a 300 Watt inverter with 600 Watt surge--This inverter is rated (as I recall) to run 600 Watts for ~15 minutes(?). What are your plans? >300 Watt surge (few seconds/minute)... Or are you looking for 15 minutes?

Nominally, the math is something like this:

• Lead Acid battery down to 11.5 volts under load for ~50% SoC. 10.5 volts being the battery cutoff voltage for inverter.
• I do 0.5 volt @ 300 Watts--That allows you to pull (for a bit) 600 Watts @ 1.0 volt drop (technically surge).
• If you want to do 600 Watts @ 0.5 volt drop--That is fine, need ~2x thicker copper to accomplish this

To do 8 feet @ 0.5 volt drop @ 67 Amps with the voltage drop calculator... A few "adjustments" are to assume DC, 1.0 PF, PVC (or air) for wiring:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=4&necconduit=pvc&necpf=1&material=copper&wiresize=0.1608&resistance=1.2&resistanceunit=okm&voltage=12&phase=dc&noofconductor=1&distance=8&distanceunit=feet&amperes=67&x=51&y=22&ctype=nec

6 AWG cable @ 75C or better insulation copper cable from NEC cable:

Result

Voltage drop: 0.53
Voltage drop percentage: 4.38%
Voltage at the end: 11.47

This looks "doable" if you want to pull 600 Watts @ higher current & conservative voltage drop.

There is the other question... Inverter needs to be in good ventilation (not under shelf, in small cabinet, etc.). And if you can shorten wire by 1/2 to 4 feet--That will reduce voltage drop by 1/2 too.

Doing the 1.25x NEC derating for "surge current" is not normally done. The NEC derating is intended for circuits that run "continuously" at "operating current" such as Gym Lighting circuits--And one could argue Water heaters, well pump, etc.

The 1.25x does keep wire cooler (I have seen 50 year old apartment NEC standard 14 AWG in conduit to a water heater drawing 19 amps--And no sign of overheating...).

Did have issues of 50 year old thermal breaker false tripping every few weeks until I replaced (and I did the conservative thing of 1.25*19amps=23.75amp derating--Or 30 amp branch circuit). Old apartment, difficult to find new (or "recycled" circuit breakers, etc.).

Fuses/breakers in North America are typically rated to "not trip" at 80% or less rated current. And will trip (minutes-hours) at 100% or greater of rated current....

It is the old "fix it once" or possibly "fight the issues" years down the road.

-Bill

JYanke

Thank you for the great info as always. 

You are correct the SureSine manual states peak surge is 600watts ~ 15 minutes. 

Realistically even if I were to power on everything in my cabin at once Im close to needing >300 watts continuous. But that is never the case. I have a lot of lamps and light fixtures throughout the rooms which are all at about 8 watts each using LED bulbs but never all on at once anyway. 

My biggest power draw moments come from a Shurflo rv pump rated at 104 watts (but on demand and only run for short intervals ie sink toilet etc.) Also a small television and dvd player. 

Other than the pump or tv it’s mostly lights and a small radio or charging cell phones. So the 600 watt isn’t really something I need or am concerned with at the moment. 

As for the location I wish to put the inverter, its a nice well ventilated roomy location in a dedicated area near the main AC service panel. I ran some 1” pvc conduit for the inverter cables down through the wall to below the cabin where the batteries will be moved to my ventilated battery box. However its really about 8’ even with building a shelf up higher to bring the batteries closer to the floor. 

Ive had the inverter running fine for almost 10 years using 4awg, but very short run from inverter to battery box. And its never seen 600 watts. 

I have no problem going up to the 2awg wires if that’s necessary but I do have some 10’ lengths of 4awg and 6awg that would save me from buying more wire.