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PA_Bound
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# Posted: 30 Jan 2012 09:50
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I know I'm not the sharpest tack in the box, but usually with enough research I can learn just about anything. But I've read the posts and reviewed other on-line sources and when it comes to eletrical grounding where portable generators are attached to permanent structures, I just can't get my head around this.
I'm trying to determine where the electrical system in my cabin should be grounded (attached to a physical ground rod). I have a portable generator, powering a inverter/charger, through a standard electrical panel box. Both the generator and inverter have ground lugs and, of course, the panel box is typically where grounding is done in a permanent structure. My understanding (correct me if I'm wrong) is that you should only have one ground point in the system. So which of the three do I use as the ground point? (BTW,... I don't need to worry about any inspections).
I've attached a drawing to help detail my configuration.
Eletrical system overview
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MtnDon
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# Posted: 30 Jan 2012 11:22 - Edited by: MtnDon
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Actually the big thing to avoid is something you have not mentioned. That is the neutral-ground bond. In a typical AC system there is a "bond" between the white neutral wire and the bare(or green) ground wire made in the AC load distribution center. There should only be one bond in the system. The problem comes in when a generator is introduced. There is no industry wide standard whether or not the generator shall or shall not have the generator neutral and ground wires bonded within the generator.
You should examine the wiring schematics for each piece of equipment to ascertain if they have an internal neutral-ground bond.
Often generators with chassis mounted receptacles have internal bonds. It's best to have the neutral-ground bond at the main AC panel, so bonds in equipment must be removed.
The same neutral-ground problem can arise with an inverter; some bond and some do not.
Grounding is best accomplished by driving an 8 ft copper clad rod into the ground and connecting that to a ground block or bus using a single #6 copper wire. Then each piece of equipment is connected to that ground block. This would include a charge controller if/when PV modules are introduced to the system. There is nothing wrong with having a second ground rod in the system. If there is a second ground rod it should be connected to the first ground rod with #6 wire. The second rod should not connect to the ground block/bus or to any other piece of equipment. A second ground rod is actually required if you wanted to check the efficacy of the grounding system. With two rods there should be a minimum ten foot separation.
The DC side of the system should also have a neutral-ground bond and there should be a ground rod as well. The NEC does not require a ground rod in a 12 VDC system, but it is a good idea nevertheless.
Is that grey box between the generator and the inverter/charger a transfer switch? Or does the inverter have an internal transfer switch? If the inverter/charger does not have an internal transfer switch there should be a transfer switch. The generator connects to one input side. The inverter/charger AC output connects to the other input side. The transfer switch output then connects to the AC load/distribution center. That way only one source of AC can be feeding the cabin AC system at a time. No danger of mix ups.
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PA_Bound
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# Posted: 30 Jan 2012 15:59
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@MtnDon... Thanks for the input. This is starting to become clearer.
Beginning with your last question first, the inverter I'm using has an internal automatic transfer switch so the "grey box" is only operating as an AC load center.
So, after your comments, here is what I'm thinking-
1. Drop an 8' grounding rod into the ground next to the generator and attach it to the generators ground lug. The generator documentation states the "Neutral is bonded to frame". I'm assuming that the ground lug is also wired to the frame, making the Neutral also bonded to Ground (via the lug). I will need to confirm that however. If my assumption (and understanding) is true, this would be the one Neutral-to-Ground bond.
2. In the AC load center, I will not bond the neutral and ground there. I'll simply pass each of them through.
3. At the inverter, its documentation indicates that when running on an AC source it will pass that power through, open it's own internal bonding relay, and depend on the AC source to be properly bonded. So no bonding issue there, either.
Now I just need to figure out this is all works when running off of batteries.
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Bzzzzzt
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# Posted: 30 Jan 2012 16:59
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Personally speaking, (as an electrician) I would drive the ground rod and ground THE FIRST PANEL or disconnect box. The neutrals and grounds SHOULD be bonded together here in spite of the fact that they are bonded together both at the generator and in the inverter.
In my system, I have a panel box with a cord that will plug directly into the generator or into the inverter. THIS box is where all your boding between neutral and ground should be done. It should then be separated throughout the cabin.
Think about it this way. If you had an electric pole supplying you with power, they set a transformer and at that transformer, by code, they bond the neutral to the pole ground wire. You get a bonded system coming into your cabin. Instead of this setup you have a generator of your own so as to cut out the middle man. You're doing EXACTLY the same thing as you would otherwise do if you bond the neutral and ground together at the first panel or disconnect.
I'm a licensed master electrician in 2 states. I deal with this stuff on a regular basis. You'd be surprised at how many people think that the end of the world is coming in December and want generators hooked up to their houses.
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MtnDon
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# Posted: 30 Jan 2012 18:12
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Q: regarding the inverter/charger. When the generator is not running and AC power is being supplied from inverting the DC battery power, where is the neutral-ground bond? Does this inverter still connect the neutral or ground back to the generator where the neutral-ground bond has been made. In other words does the cabin wiring then still have an effective neutral-ground bond? Asking as a non-electrician, but reasonably well versed diy'er.
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Bzzzzzt
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# Posted: 30 Jan 2012 19:46
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I was looking at the diagram here and that is a complex system there. I honestly would bond the neutral and ground together at the panel shown after the generator and the DC converter thingy doesn't make a lot of sense to me. Why would you want to have a bunch of DC stuff if you already have AC?
What makes the most sense to me is to have solar (or whatever is charging your batteries) feed the inverter and change it to 120V and feed the same panel feeding the generator. Granted, it won't run the stuff as long, but the wire sizing and stuff would be a lot more manageable and cost effective.
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MtnDon
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# Posted: 30 Jan 2012 21:20
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Quoting: Bzzzzzt What makes the most sense to me is to have solar (or whatever is charging your batteries) feed the inverter and change it to 120V and feed the same panel feeding the generator. Granted, it won't run the stuff as long, but the wire sizing and stuff would be a lot more manageable and cost effective.
I totally agree with that. I went through months of pre planning and had started with the premise I would run many things off DC. The more I looked at it and added up wire costs, etc. etc. the less and less I liked the idea of much DC. I still have a few (3) DC lights, but that was only with an eye to having some light quickly and readily available if the inverter crapped out.
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PA_Bound
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# Posted: 30 Jan 2012 21:57
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A couple thoughts on my design:
1. There is no DC in the cabin itself. The only DC is into the inverter, where it is converterd to AC and supplied to the second panel in the drawing.
2. No solar in this design, so most of the time this will be running off of battery. The generator performs 3 distinct functions- run the 240v well pump (not depicted in the drawing), drive an 120v receptical in the cabin for a window air conditioning unit and finally to charge the batteries.
3. I'm still trying to determine how the inverter handles neutral-ground bonding with each input source. I understand how it works with the generator, but I need to put some time into how it works on batteries. My suspician is that inverter closes the relay and establishes a Neutral to Ground bond in this case, but I have to confirm that.
@Bzzzzzt- One counter to your comments that I am also trying to reconcile is in the inverter manual, where it clearly states that "the AC panel must not have a permanent neutral to ground bond installed".
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PA_Bound
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# Posted: 30 Jan 2012 22:07
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@MtnDon- to answer your question... from the manual:
The neutral conductor of the inverter/charger's AC output circuit is automatically connected to the safety ground during inverter operation. When AC utility (or generator, in my case) power is present and the inverter is charging, this connection is not present, so that the utility (or, again, generator) neutral is only connected to utility (generator) ground at the source.
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MtnDon
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# Posted: 30 Jan 2012 22:09
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Quoting: PA_Bound "the AC panel must not have a permanent neutral to ground bond installed".
If you violate that the magic smoke will be released from the inverter and it won't work anymore. I found that out with a cheap cheap small inverter. The warranty worked though.
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Bzzzzzt
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# Posted: 31 Jan 2012 16:48
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Well if your inverter says that then you should follow that. Ground the generator itself and the inverter itself and you should be good to go. That's the goofiest setup I've encountered, but I'm not saying it's wrong. The paperwork that comes with the device is what you should follow.
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