small cabin inverter/backup generator grounding

mxsolar

I have a small solar powered off grid cabin that I am about to change from DC only to AC using a 700 W Samlex inverter (model SK700) and a Honda Eu2000i backup generator.

I am going to use an IOTA auto transfer switch to make the switch between solar and generator power. The output of the transfer switch will be connected to a small AC load center that feeds the cabin. At first I was going to use the AC load center as my neutral to ground bond but I then realized my inverter's neutral and ground are internally connected so I decided to use the inverter as the bonding point.

Unlike my inverter, my generator does not have an internal connection between neutral and ground.

My question is the following:

How do I properly make this neutral to ground connection when I am running my generator instead of the inverter?

Any suggestions will be greatly appreciated.

Thanks.

icarus

Re: small cabin inverter/backup generator grounding

You have nearly the exact same set up that I use. (Does your inverter permit grounded neutral? if not disregard this advice). (I am away from home,so this is from memory!)

The way I have mine wired is this. The genny connects to the building via a male plug in a weather proof box, like an RV. This feeds the generator side of the Iota transfer (the side that requires energy to keep the relay closed) The hots tie through the switch, ( as do the neutrals if I recall, it being a double pole switch. The inverter ties in the same way but on the other side of the relay. (I carry one circuit through so that it is only energized with the genny, so it runs the Xantrex battery charger and the vacuum cleaner!)

The neutrals than land on the neutral buss bar in the 120vac breaker panel. This is then in turn grounded outside the building to the building ground. Works perfectly.

If your inverter won't support the grounded neutral I suspect that you may have to isolate the ground in the panel some how.

Tony

PS I looked at the Samelex site, and I can't find a 700 watt inverter either msw or pure sine. Please post which inverter you are planning to use. If it is a msw, I can almost guarantee that it won't work with a grounded neutral.

bryanl

Re: small cabin inverter/backup generator grounding

don't worry about it. What you've got, in essence, is a 'parked' RV not connected to the power grid.

just switch the neutral and hot (or use a proper plug) and let the inverter or genset worry about it. For power levels less than 5kw, having neutral bonded to frame ground isn't an issue and the inverter or genset will (should) take care of GFI hassles. Let the source device and its installation guide tell you how to wire things and, if using an external switch, keep all special wiring connections (if any) in front of the switch.

I do think it might be a good idea to put in an earth ground, just one, that is connected to the frame ground at your box.

mxsolar

Re: small cabin inverter/backup generator grounding

Tony,

Here is the link to my inverter's website: http://www.samlexamerica.com/products/productdescription.asp?ProductsID=19001

I am not really sure what you mean when you ask if my inverter supports grounded neutral. This is from the inverter manual:

"The neutral conductor of the AC output circuit of the inverter is automatically connected to the safety ground during inverter operation"

If I understand this correctly, since there is a neutral to ground bond inside my inverter, I cannot make a neutral to ground bond anywhere else in the system. That means I have to isolate the neutral from ground at the AC load center.

Assuming this reasoning is correct and moving forward to the generator side:

Since my generator does not have its neutral and ground internally connected, I need to make this connection somewhere other than in the above mentioned AC load center. Does that make sense?

I have an idea but I am not sure it will work:

I am thinking about running a power cord from the generator to an independent receptacle nearby the IOTA transfer switch. Inside the receptacle I will bond neutral to ground with a jumper cable. I will then connect a second power cord to this receptacle and the three wires on the other end to the transfer switch.

BTW, in case you are wondering, both the inverter and generator will have their own independent grounding rods connected to their respective chassis.

I hope this is clear.

Thanks for your input.


Ernesto

BB.

Re: small cabin inverter/backup generator grounding

mxsolar wrote: »

This is from the inverter manual:

"The neutral conductor of the AC output circuit of the inverter is automatically connected to the safety ground during inverter operation"

If I understand this correctly, since there is a neutral to ground bond inside my inverter, I cannot make a neutral to ground bond anywhere else in the system. That means I have to isolate the neutral from ground at the AC load center.

From the wording of the manual--it seems to state that the inverter Neutral/Safety ground connection is closed when the inverter is on, and open when the inverter is off (un-powered?).

If you have a transfer switch option--Samulex specifically states that the Neutral/Ground connection is closed when the inverter is operational, and open (not connected) when the inverter is off.

It appears that your setup will use the Neutral/Safety bonding of either the Inverter, Genset (if it has one), or external shore power box. You should not need to connect a neutral bus bar to frame/safety ground in your wiring box.

If you wish to bond the generator Neutral to earth, then do this at the genset behind a multi-pole transfer switch (hots and neutral switch).

-Bill

icarus

Re: small cabin inverter/backup generator grounding

I think Bill has got it,

T.

mxsolar

Re: small cabin inverter/backup generator grounding

Bill,

You got it exactly right but I still have one question.

In reference to the last part of your response: "If you wish to bond the generator Neutral to earth, then do this at the genset behind a multi-pole transfer switch (hots and neutral switch)."

Any suggestions on how to accomplish this? I would like to leave my generator's internal connections untouched.

Thanks.

Ernesto

BB.

Re: small cabin inverter/backup generator grounding

Are you going to do this with a 120 VAC genset (hot/neutral) or a 240 VAC (hot A / neutral / Hot ?

Assuming 120 VAC and an Iota 30 amp automatic transfer switch with 20 second delay (to let the genset warm up)... It is a two pole transfer switch. You can ground the neutral at the inverter and ground the neutral somewhere in the generator side wiring -- and the Iota 2 Pole transfer switch will transfer both the Hot and Neutral leads as a pair.... So you will never end up with a double neutral / ground bond in your system.

I presume you would connect the relay coil/time delay to the genset (genset power hot to coil, generator selected. Generator power goes away, relay turns off and falls back to inverter power).

Now--if you had any "critical" devices (i.e., computer without battery backup, DVR, etc. turned on when you shut the genset down, I would not expect a "clean" power transfer). You might wish to put a On/Off switch on the generator output so that you can turn off the AC power from the genset cleanly (no voltage/frequency sag as genset turns off) and if you had heavy loads--you can let the genset cool down for a few minutes before shutting down.

I would assume you would put Generator only loads (AC battery charger, small A/C unit, etc. on the generator side of the transfer switch).

By the way--I would probably try to have one ground rod near the power enterance of the cabin instead of two (one at the cabin and another 50' away at the generator shed). If you have two ground rods and a near by lightning strike--you will generator a voltage gradient across the ground and induce currents into your AC Leads and Safety ground leads.

My two cents, I would run the ground lead from the gen shed to the cabin ground rod.

Dwh did a very nice generator grounding post in another thread awhile back... I would read it carefully:

dwh wrote: »

...(I'm going to go by the NEC, even though I realize that Baja is not legally subject to the US NEC.)

Since the house is not grid-connected, the generator (and the solar system) constitute a "separately derived system" (no permanent connection between building neutral and grid neutral), in which case a ground is *required*. I prefer a ground rod (or two) over a Ufer, so that's what I'd use (I would even if there WAS already a Ufer ground).

Okay, first of all, I would make sure the house was properly wired - at least at the main load center. I.e., I would make sure that all the safety grounds were into a bus bar, and I would bond the neutral to the ground *at that point*.

This is important, since the neutral to ground bonding should happen *at ONLY one point*.

I would also make sure that in any sub-panels (and junction boxes and device boxes, etc.), the neutral and ground were NOT bonded.

I also would NOT assume that there is any sort of acceptable ground just because the building is concrete.

Then, I would drive a ground rod at the house (8' copper-clad 5/8"), and check the resistance. If the resistance is 25 ohms or less, then one ground rod would be sufficient.

I will assume that the incoming water line is metal.

Then, I would run a single unbroken *solid* copper wire of some stupidly large size - #4 at least, and maybe even #2. That wire would run *unbroken* like this:

house panel -> water pipe -> ground rod -> generator frame

If the resistance of the ground rod is more than 25 ohms, then we need a second ground rod, and since the two rods have to be at least 6' apart, I would drive the second rod at the generator for convenience. (Assuming that the gen is 6' away - most are.)

In that case, the path of the unbroken solid wire would be:

house panel -> water pipe -> ground rod 1 -> ground rod 2 -> gen frame

Now, according to the Kohler 17RES installation manual:

http://www.apelectric.com/v/vspfiles/pdf/kohler17res.installation.pdf

"1.8.3 Grounding

Ground the generator set. The grounding method must
comply with NEC and local codes. Connect the
grounding strap to the generator set ground lug,
terminal GND inside the controller compartment.

Kohler generator sets are shipped with the generator
neutral attached to the generator in the junction box.(emphasis added)

At installation, the neutral can be grounded at the
generator set or lifted from the ground stud and isolated
if the installation requires an ungrounded neutral
connection at the generator. The generator set will
operate properly with the neutral either bonded to
ground or isolated from ground at the generator.(emphasis added)

Various regulations and site configurations including the
National Electrical Code (NEC), local codes, and the
type of transfer switch used in the application determine
the grounding of the neutral at the generator. NEC 2002
Section 250.20 is one example that has a very good
explanation of the neutral grounding requirements for
generators."

Okay, since the bonding of neutral to ground should only happen *at one place* (we're doing it at the house panel), and since the Kohler ships by default with the neutral bonded to the frame - I would unbond the neutral from the generator frame at the generator end. This way the neutral-ground bonding will happen ONLY at the main house load center - which is the proper place for it.

Essentially, I would treat the building wiring as a separate system and make it proper unto itself - regardless of what the AC source is; grid, solar or generator.

-Bill

mxsolar

Re: small cabin inverter/backup generator grounding

Bill,

My genset is a 120 VAC (Honda EU2000i) so you couldn't have described my situation any better. Thanks for your great input along with the post by dwh which was also very useful.

bryanl

Re: small cabin inverter/backup generator grounding

I think there is a size issue here that is being ignored.

Talking about a 2kw (peak) genset means the considerations are not in the usual 'whole house' situation but rather in the portable situation where connections are made to power sources with plugs. The rules (code) are different for these two situations.

This is seen by the manual quotations that indicate that neutral to chassis ground bonding is handled inside the genset. That means that the transfer switch and entry panel should not mess with making neutral to ground connections for that device.

Some older equipment, especially inverters, may not function if you start connecting its outputs (often because the chassis ground is shared with the battery power).

The discussion about earth grounds is related to the one about multiple points of bonding between neutral and chassis ground. In general you only want this sort of connection to occur at one place to eliminate problems with circulating currents.

Context makes a difference. Some gensets allow you to get into internal wiring so you can use them either as whole house backup sources with appropriate service entrance neutral to ground bonding. The Honda 2000si is not in this class.

Unless you have an avocational interest in making your life complicated, I suggest taking the simpler route. Use the proper paradigm for what you are actually trying to do. When you are under 5kW and completely disconnected from the grid and using power sources that have plugs on them, you don't need worry about earth grounding or neutral to chassis ground bonding as a major safety or code issue.

mxsolar

Re: small cabin inverter/backup generator grounding

Bryan,

Are we talking about the same genset (Honda EU2000i)? I read the manual and the only thing I can find about this topic is the following:

GROUND SYSTEM
Honda portable generators have a system ground that connects generator frame components to the ground terminals in the AC output receptacles. The system ground is not connected to the AC neutral wire. If the generator is tested by a receptacle tester, it will not show the same ground circuit condition as for a home receptacle.

I am not an electrician nor I want to make my life complicated, I just want advice making a safe electrical installation.

Thanks.

hillbilly

Re: small cabin inverter/backup generator grounding

Off topic here, but did you by chance take an Advanced PV course online from SEI last spring? I think we were classmates.... either way good luck on your project.
Saludos,
HB

BB.

Re: small cabin inverter/backup generator grounding

It is unfortunate the Samulex grounds the neutral... It would be better to just have the central Neutral/Ground bonding in your main power panel.

I wonder if you can safety (and within warranty) disable the Samulex ground Bonding.

In reality, with the small samulex wattage rating, I would not have a heart attack with Neutral/Ground bonding in the main panel and at the Samulex--You would get some shared current in the green and white wire--not the end of the world.

But, it would not be an issue to leave the generator neutral floating either...

In the end, neutral bonding at the Samulex and at the generator side of the transfer switch will meet your needs nicely and have consistent neutral bonding whether generator / inverter operation (and transfer switch will keep the "two neutral bond points" from interacting.

As long as you use good AC wiring practices and components you are doing fine.

-Bill

mxsolar

Re: small cabin inverter/backup generator grounding

HB,

That is me! I am glad you remembered.

Thanks and good luck to you too.

mxsolar

Re: small cabin inverter/backup generator grounding

Bill,

I thought about disabling the neutral to ground bond inside the Samlex inverter but I don't want to mess with warranty issues. Besides, as you mention, this is such a small system that it probably won't hurt to have the neutral to ground bond at the AC load center again.

Thanks for your input.

EB

icarus

Re: small cabin inverter/backup generator grounding

Bill (or others)

Can you explain, in simple terms the real world difference (s) between bonded neutral, floating neutrals etc in gennies as well as in inverter. I have a pretty good idea how bonded neutrals work, but I am a bit cloudier on floating neutrals. It might be a help for others.

Tony

RCinFLA

Re: small cabin inverter/backup generator grounding

The Honda generator needs to have neutral floating. Only the larger EU6500i, which is also a 240v/120v inverter/gen, can have neutral grounded.

If you measure AC voltage at gen plug you will find about 60 vac from hot and neutral plug prongs to generator ground prong.

Sounds like the Samulex only grounds neutral when transfer switch is on inverter output. This would allow you to keep the neutral floating on your house wiring when on Honda inv/gen.

House breaker box needs to float the neutral bus bar.

Other option would be to put an isolation transformer on generator. I would only do this if there is a utility grid connection which requires neutral grounded at breaker box/ service entrance by regulation code.

bryanl

Re: small cabin inverter/backup generator grounding

re: "Are we talking about the same genset (Honda EU2000i)?" - yep.

This topic has a lot of discussion on RV forums regarding this genset as that genset is very popular in those circles. The isolated power leads means that the little 3 light proper wiring indicators show fault when on the gensets and that confuses folks.

One of the interesting points that comes up in this neutral to ground thing with the Honda 2000i is the difference between Canadian and US codes.

The isolated power leads do mean that a high impedance voltmeter will show half voltage between either power lead and chassis ground. That voltage doesn't mean you can get any current. This is related to the 5kW boundary, though, as when power levels start to get past that the induced currents can be an issue in some circumstances.

When I mention portable gensets with plugs and 5kW power levels, I am describing factors that are relevant to the code as I understand it.

BB.

Re: small cabin inverter/backup generator grounding

Just think of a center tapped transformer. 120 VAC to center tap (A and B legs) and 240 VAC A to B legs.

The maximum center-tap current is A to center-tap or B to center-tap loads. If you have a 10 Amp A-CT load and a 10 amp B-CT load--the Center-Tap actually carries near zero Amps.

Notice, the only "short circuits" you can have are involve at least two of the three legs... A to earth, B to Earth, CT to earth shorts will involve no dangerous voltages or currents. A transformer (or un-earth genset/inverter) provides a "floating" output.

So, the above is the relatively safe (Floating AC and DC circuits).

Now, grounding the neutral/center-tap AC system... We are looking for protection of many types of failures. Say a 12,000 volt line crosses a 120/240 volt line. If the CT was floating, then the entire 120/240 volt home power lines would get energized to 12,000 volts and possibly not even blow a fuse anywhere (lots of fire and smoke in people's homes though).

If the 120/240 lines are ground referenced, then a line cross will cause lots of smoke and sparks on the pole--but the homes will be relatively safe.

And you can look at other floating vs ground referenced faults... For example, a floating 120/240 VAC circuit could have the A lead grounded (no current flow)--and now the B lead is now 240 VAC above ground (and the CT/Neutral is now 120 VAC with respect to ground). If the CT/Neutral was ground referenced, an A lead short to ground would pop the breaker and everything would remain at predictable voltages.

Another reason for ground referenced neutrals was the way the old filament lamps are wired. The outside base of the lamp socket (which is easy to touch) is supposed to be connected to Neutral--(usually through a polarized two prong plug or hardwired lamp fixture).

So--from my understanding of the history and goals of Neutral Grounding, that is why it is done that way here...

In other countries they do things differently. In Germany, the wiring is not polarized (don't know which lead is hot/neutral--I don't even know if Germany grounds one of their 230 volt leads, or if they center-tap ground the 230 VAC transformer secondary at the pole/home).

Modern stuff is now usually double insulated (two insulative/physical barriers) between the AC input and the DC output (like computer power supplies), plastic cases (which don't require the third wire grounding), etc.

And even then, I have seen countries in the past (such as Iceland?) that did not allow double insulation but required a "ground screen" (tied to the green wire) between primary and output (such as on a computer power supply).

Home generators do not have to worry about 12,000 volt mains crosses. And if 120 VAC, it does not matter much if a floating lead gets grounded... The other end is only still at 120 VAC above ground.

Inverters, we have the MSW which are usually (by the way the design is implemented) referenced to the battery bank--and if the battery bank is grounded, the MSW inverter 120 VAC output is ground referenced too (but you cannot ground the "neutral" because the "reference" flips between plus/minus on the battery bank and will create a high amperage current path for battery current to ground through the inverter's internal FET/MOSFET switches.

TSW inverter--again, like the genset, no high voltage lines to cross, floating output with respect to battery input, can usually ground reference the neutral.

It is interesting (aka boring) and confusing.

Anyway, I will stop here--before i dig myself deeper. :roll:

-Bill

BB.

Re: small cabin inverter/backup generator grounding

By the way, the 60 VAC between A or B and ground is frequently just filter caps across the output to reduce electrical noise... Not much current (should be less than 5 mAmps of leakage).

-Bill

icarus

Re: small cabin inverter/backup generator grounding

Thanks Bill,, I THINK I got the gist of it. It explains why you can chose which leg to ground with the Suresine 300 inverter.

I've got one more question for you. I have a series of buildings that are fed through a central "power house" In this shed are 2 stationary generators, 1 120 vac Onan the other a 240 Lister Diesel. These feed a central fuse box/distribution panel though a double pole triple throw switch. Also wired into this switch is a #12/2 wire with ground leading to a male plug. This allows me to feed the buildings with either of the older stationary gennies, or a newer smaller honda eu.

The central fuse box is bonded to the frames of the two big gennies, and this box is in turn grounded to earth ground with ~#6 wire. (The genny frames are also bonded tot he same earth ground system.

Now here is the question. When I plug the Honda Eu in and turn the 3 way switch, it energizes the buildings just fine. (Each of the "big gennies does as well) I also have a honda Ex series gennie that I can plug into that line and it works as well. The problem is if I plug in a 120 vac 2900wt Mitsubishi generator. As soon as I plug it in, it faults and trips its own out put breaker.

The question is, if the Onan and the lister work fine with bonded neutrals, and the honda Eu works with a floating neutral, and the Ex honda works with what I assume to be a floating neutral, what do you suppose the deal is with the Mitsubishi?

I am away from home for a month, so I can't confirm the details of that gennie, so I don't know how it is wired.

Any ideas?

Tony

BB.

Re: small cabin inverter/backup generator grounding

My first guess is that there is a Black/White wire cross-over somewhere your wiring or Genset to common wiring connection.

I would fire up the Mitsubishi and use a meter/load to see that the Neutral is really "neutral" with respect to the ground plug (and not hot).

If I recall correctly this was a problem child you inherited (swapped out voltage regulator boards because of ground current problems?). Perhaps one or more of the Mitsubishi outlets have swapped their hots/neutrals (assuming it has a neutral to ground bond). When you plug into the building wiring, you end up with neutral bonding in two places (genset or genset wiring and main panel--and with the crossed 120 VAC leads, you end up with both power leads neutral/ground bonded).

-Bill

icarus

Re: small cabin inverter/backup generator grounding

Good idea, I will do that.

I am not sure how that might have happened as there would be no need to change any wiring to change the control board. And you are right, this is the gennie I got for $50. I use it to power the bigger saws, or to move around as a construction gennie. It is a great gennie now that we have solved the voltage regulator issue.

I'll repost the results later when I get home, and I shovel out the shed. I won't need this 'till sometime this summer.

Tonh

hillbilly

Re: small cabin inverter/backup generator grounding

Well Bill, you've done it again. Every time I start thinking that I understand this stuff a bit better, you make a post that does a pretty good job of reminding me that I don't. You and others here have such a wealth of knowledge, it's pretty much a daily reminder that there is always more to learn...
The more I learn the less I know,
HB