inverter to AC breaker box (off-grid)

Re: inverter to AC breaker box (off-grid)

You won't have a problem with current carrying capability on the neutral.

Your biggest potential issue is whether the Samlex MSW inverter you have is neutral plug side (the wider prong blade) groundable.

Most MSW inverters have an H-bridge output with no isolation on the HV DC boost ground to battery ground. This allows some of the electronics on the AC H-bridge driver side to be run from the battery. A cheaper approach then having an separate, isolated secondary side generated low voltage DC source to run the H-bridge MOSFET's gates.

The result is when you ground the neutral there will be an AC voltage to ground on the battery terminals. You will get a shock if you touch negative terminal of battery when standing on the ground. If you also ground the negative terminal of battery the inverter fuse will blow or inverter will blow out.

Re: inverter to AC breaker box (off-grid)

I think most of the new breaker boxes have insulated neutral bus bar.

This is required in a normal auxilary box setup to meet code of single ground bonding point back at the main breaker panel. The floating neutral in auxilary box is white wired over to neutral bus bar in main box which has the single point ground in main box case.

Re: inverter to AC breaker box (off-grid)

I don't know the voltage isolation spec but the GE boxes I use have a half inch plastic base the whole length of bus bar which the neutral bar clips into. Some have two plastic standoff posts at either end of the neutral bus bar.

There is a a provision for ground bonding at the end of the bus bar with a lug provided attached to the metal box.

Re: inverter to AC breaker box (off-grid)

Yes, but can you be sure that anything that is plugged in at any outlet is neutral isolated from ground ??

Re: inverter to AC breaker box (off-grid)

The other way it's commonly done is with a copper strap - 1 end screwed to the box and the other end crimped/squished to make it wire-like. You just bend it up, stick into one of the wire holes in the bus bar and tighten down the screw if you need to bond neutral.

Re: inverter to AC breaker box (off-grid)

BB. wrote: »

Because the "Neutral" is already connected to "ground" inside the inverter... So you do not want/need to connect your neutral elsewhere.

The manual should give you a diagram of how to ground the metal chassis of the inverter and the battery bank.

-Bill

One the model I posted, the neutral is floating. This is why there is a waning about the longer prong (neutral) being a electrocution hazard.

tallgirl has stated why, as in respect to the inverter not to bond the neutral and ground toghther.

Re: inverter to AC breaker box (off-grid)

bobdog wrote: »

So, as I understand this, the neutral IS bonded to the ground (just at the box), even with a new isolated neutral buss bar. So in that case what's the point of having an"isolated" (ergo insulated) neutral buss bar? As a friend says, "What does it buy you?"

Neutral must be bonded to ground, but ONLY at one point - the main "service entrance" panel.

Sub-panels tapped off the main panel must NOT have their neutral bonded to ground.

Due to this, almost all panels nowadays come from the factory with the neutral bus isolated - BUT, provision is made to allow the installer to bond that neutral bus to ground if required (if the panel is used as the "main" panel).

The two main ways this is done, is to either;

A) At the factory they drill clean through the neutral bus bar, and the box, and then include a couple of long screws. Then, if need be, the installer can run the screws through the bus bar and screw them into pre-drilled holes in the metal box.

or

At the factory they take a piece of flat copper strap and crimp one end so it's "wire-like". Then they screw the flat end of the strap to the box next to the neutral bus bar. If the installer then needs to bond neutral to ground, he (or she, TG) can simply bend it up and stick it into the neutral bus bar and tighten down a screw.


For your off-grid application, you would install the 120v AC system just as though it were to be fed from the grid - i.e., you WOULD bond neutral to ground at the main panel.

Which is apparently a problem for the inverter you have, since it is not designed to be attached to such a system - it has neutral and ground bonded inside of it.

Some generators have this same problem, though some are designed so that you can "lift" (disconnect) the neutral to ground connection and thus make them correct for connection to a properly grounded 120v system.


Okay...now...for your situation - little off-grid cabin. I tend to approach such things are roughly equivalent to an RV.

You first asked about tying the inverter to both "legs" or "phases" of your panel. Well...if there are NO 220v circuits in your panel and NO 220v feed from a generator or grid - then there really isn't any problem with feeding both hot bus bars from one single-phase 120v source. Just tie the two bus bars into one and call the whole thing a "120v panel".

No problem - other than, of course, the potential (pun intended) problem of overloading your inverter. But that's not a safety issue.

The other issue is grounding/bonding.

Now, an RV is not grounded - unless connected to shore power. And your inverter would be perfectly OK to use in an RV.

So, (insert rapidly spoken legal disclaimers about don't do this at home and do as I say, not as I do and so on and so forth, ad infinitum) why not just treat your cabin as an RV for wiring purposes and NOT bond neutral to ground in the main panel? I.e., allow the inverter to serve as the neutral to ground bond - since it IS already built for that.

Just bond the chassis of the inverter to ground and leave the neutral bus in the panel isolated. There is still a neutral to ground bonding, it's just in the inverter instead of in the panel - which should be fine, since the inverter is the source of the power anyway.


If you were in an RV and the 120v source was a generator - the neutral to ground bonding would be done in the generator.

Re: inverter to AC breaker box (off-grid)

Okay, with those inverters you definitely do NOT bond neutral to ground - and it's not bonded to ground in the inverter either. You do however bond the chassis of the inverter to ground and you would also bond the chassis of the electrical panel to ground.

I.e., connect the metal box of the panel to the ground rod, and also connect the chassis of the inverter to the ground rod.

But DO NOT bond neutral to ground in the panel. Make sure your panel has an isolated neutral - i.e., the neutral bus bar sits on plastic mounts so it doesn't touch the metal box.

Also, when you stick neutral wires through the holes in the bus bar - DO NOT make them overly long and let them touch the box.


The REASON why you don't bond neutral to ground with that inverter is because the neutral IS NOT neutral - it's hot! If you bonded hot to ground it would just short the inverter out.

From the manual:

http://www.samlexamerica.com/customer_support/pdf/Manuals/PST-100S-12A_24A_Manual_Dec2005.pdf

"CAUTION! : In these NEMA 5-15R receptacles, the current carrying conductor connected to the longer rectangular slot is isolated from the metal chassis of the inverter. Hence, when the metal chassis of the inverter is connected to the earth ground, the longer rectangular slot is not grounded to the earth ground. The longer rectangular slot is, therefore, not a “neutral”. Do not touch this slot as it will be at an elevated voltage with respect to the metal chassis / earth ground and may produce an electrical shock when touched."

Due to that, I WOULD NOT use that inverter hooked to an electrical panel UNLESS there was a GFCI between the inverter and the panel. Some of their inverters do have a GFCI option. For your application I would ABSOLUTELY get that.

Without GFCI I would say, "definately do NOT feed your panel with that inverter".


Having said that - they do say in the manual:

"If an electrical breaker panel / load center is fed from an inverter and this panel is also required to be powered from additional alternate AC sources, the AC power from all the AC sources like the utility / generator / inverter should first be fed to a manual selector switch and the output of the selector switch should be connected to the electrical breaker panel / load center."


Okay...that's to prevent paralleling (blowing up your inverter by backfeeding it power from a grid or generator), but it IS a tacit approval of connecting the inverter to a panel.


They also say:

"Do not directly connect the hot side of the 120 VAC of the inverter to the two hot legs of the 120 / 240 VAC electrical breaker panel / load centre where multi-wire (common neutral) branch circuit wiring method is used for distribution of AC power. This may lead to overloading / overheating of the neutral conductor and is a risk of fire."

What they are referring to is what is called a "3-wire circuit". It's two hots (coming off of two breakers) sharing a single neutral. Okay, so don't make any 3-wire circuits - use a dedicated neutral for every hot...problem solved.


So...

* NOT ALLOWED: 220v feeding into the panel from either grid or generator.
* NOT ALLOWED: 220v branch circuits.
* NOT ALLOWED: 3-wire branch circuits.
* GFCI REQUIRED. (Buy an inverter with the GFCI option or install a GFCI between the inverter and the panel.)

Step 1.
Make sure there are no 220v branch circuits or 3-wire branch circuits.

Step 2.
Tie the two hot bus bars together to make the panel a 120v panel.

Step 3.
Ground the chassis of the panel but DO NOT bond neutral to ground.

Step 4.
Ground the chassis of the inverter.

Step 5.
Feed the inverter AC output into a GFCI and then feed from the GFCI into the panel.

Step 6.
Kick the tires and light the fires.

Re: inverter to AC breaker box (off-grid)

bobdog wrote: »

Excellent analysis of the issue DWH. I appreciate the work you have put in, especially reading and then giving advice. I read the same things, but your interpretation makes it much more clear.

When I was a boy I read something in a "Mel Bay" music book which I have never forgotten; "To learn, teach."

I'm here to learn just like eveyone else so doing what I do helps me as well. I'm mercenary, not altruistic.

So, I hope I have it right.
1) jump the two hot (main) bus bars together to make one hot, 120v panel
2) run the one "hot" wire from a circuit (in my case the black wire) into the breaker, "plug" the breaker into the buss (panel)
3) the white or neutral wire of the circuit goes to an isolated neutral buss bar
4) the green to ground

AND from the inverter:
1) the black wire from my 'Plug' goes to the hot, main buss
2) the white goes to the isolated neutral
3) the green (or ground plug wire) goes to the ground buss

Finally, ground the panel, but NOT the neutral bar

And either get an inverter with a GFCI or put one between the inverter and the panel

Yup.

Make sure you have one neutral (white) per hot (black) - no sharing neutrals.

Also, again, be careful when stripping the neutrals and sticking them in the neutral bus bar. The bus bar will likely be very close to the side of the box - maybe 1/2" or less gap. Keep the stripped end of the neutrals just long enough to penetrate the bus bar, but not long enough to touch the box.

If I had 2 inverters, couldn't I simply use one side of the panel for this inverter/circuits (not jump the 2 sides) and the other side for the other inverter/circuits? Or would I run into neutral buss issues?

To rig that, you would need to have two neutral bus bars isolated not only from ground, but also from each other. You would also have to make CERTAIN, that you didn't get any mixing. I.e., If a hot is on "Inverter A" you better make sure that the neutral doesn't get stuck into the neutral bus bar for "Inverter B".

But that's just hypothetical since by doing something like that you are starting to get into realm of "dodgy and possibly dangerous" because even though you know how the system is rigged, the next guy won't unless you make extraordinary effort to label everything with WARNINGs.

(Actually, this discussion is ALREADY bordering the realm of dodgy and possibly dangerous. I have no doubt that some of the engineers here (there are many) are cringing and just itching to tell me to shut up. And so, I must insert the next paragraph...)

Better to have two small panels each fed by their own inverter or even better to buy a bigger inverter so you don't need two to begin with. I DO NOT RECOMMEND FEEDING ONE PANEL WITH TWO SEPARATE INVERTERS UNLESS THE INVERTERS ARE DESIGNED TO BE PARALLELED WITH EACH OTHER. In fact, I strongly advise against it.

(EDIT: just read the 3-way circuit line and you said for every hot make a dedicated neutral. Does that mean for every HOT breaker I have a neutral bar? I'm sure I'm mis-understanding.)

No, it means for every hot wire you have to have a neutral wire back to the bus bar.

For example, you can buy "3-wire" romex with one white, one black and one red (and one bare ground, but it doesn't count as a "wire"). You can wire it so that each hot (red and black) goes into a separate breaker and thus the two circuits (say one for upstairs lights and one for downstairs lights) share the same neutral (return path).

That is a perfectly legit, legal and common thing to do (under certain circumstances - it is forbidden for other uses). However, it must not be done with those particular Samlex inverters.

If you are wiring things up yourself - use only "2-wire" romex (black, white and bare ground). That way no matter what, you will have a neutral for every hot.

Given all of that, I assume not all inverters, let alone pure sine wave inverters require this setup? And doesn't this leave me with no place to run power "back" to the source as if I had a 'real' neutral? Maybe this is NOT the inverter for my cabin then.

I'm not sure what you mean by that "run power back to the source" statement.


As for the differences in inverters, I would have to defer to the engineers to answer that. My understanding is limited. From what I understand, there are:

* MSW
Modified Square Wave - sometimes referred to as Modified Sine Wave which is usually marketroid doublespeak, but sometimes means that the square wave has so many steps it is really VERY close to a sine wave.

* PSW
Pure Sine Wave - which seems to me to usually be a highly stepped square wave which comes very close to being a sine. I suppose if you zoom out then it really is a sine.

* The Good Stuff
I have no doubt there are some really nice and expensive inverters that really do put out an absolutely true sine wave. Some of these put out 220v split-phase and are designed to feed electrical panels.


I have a feeling that when dealing with low cost inverters, there probably isn't all that much difference. How many have a "neutral" that is actually another hot? Dunno. Probably a lot of them.


Quite often here in the forums we see a conflict between "the right way" and "the cheap way". You are certainly not the first to try to setup an off-grid power rig "on a budget" (hell, I'm a cheap bast**d myself).

Let me ask you this: WHY do you keep wandering off into the dual inverter train of thought?

Is it because you have a need to supply 220v to something?

Or is it just because you want more wattage available? Like maybe you'd like to start out supplying the panel with 1000w today and maybe add another 1000w later? Or maybe you're thinking that two lowbuck 1000w inverters will cost less than a 2000w bigboy?