neutral ground bonding - using coach to power the house

[Seayfam]

Let me try and explain how wiring is done in Alaska by code. I personally think every state should be done the same.
The power company brings in 4/0-4/0-2/0 to the meter loop mounted on your house or shop. The two 4/0 are hot and the 2/0 is neutral. In your meter loop you have two bars that your 200a breaker connects to. You also have a neutral bar that the 2/0 wire connects to. At that neutral bar you run a #6 bare ground wire to 2ea 5/8 x 8 ground rods drove in 6' apart, and you loop your ground wire, from the first rod to the second rod and back to the first rod. "This keeps your neighbors low voltage neutral from entering your house"

Inside the house the breaker panel can have another 200a breaker in there, but not required. You have a isolated neutral bar, you have a ground bar and your two hots. From your ground bar, you run another #6 bare ground wire to either a good plumbing or gas ground, or you add another ground rod set up. Then your neutral goes back out to your meter and is grounded there. This prevents any low voltage from any of your appliances to be connected to your grounds in your house. Otherwise why have a ground lug on any of your outlets.
If you are not set up this way then you can get shocked by touching any metal appliance or light fixture in your house. Also could be bad on your computers.

Hopefully this explains the neutral bonding issue for anybody that may be confused.

Gary

[kevink1955]

Just read something that joged my memory

Less than 10 years ago electric dryers and stoves used to be wired with 2 hots and a neutral, the neutral was allowed to be bonded to the appliance case and serve as a ground. If you buy a new appliance today they still provide instructions on how to connect it to a 3 wire cord.

In the 3 wire setup you have the potental a shock hazard if the neutral becomes open as the appliance case could become live.

I do not see that happening in a multiple bond like we have been discussing. Just some electrical trivia.

[Tim Strommen]

... Tim says a copper clad steel rod driven down to the water table, is that correct?...

I've never heard this particular recommendation before.  Driven ground rods are generally 8' long and driven in a minimum of 6'.  It is important to get them below the frost line for best conductivity.

That said, again I do not see the need for an additional driven ground in anything we've been discussing.  Once the grounded case of the generator has been connected to the existing driven rod that is already part of the house's electrical system via the generator umbilical, the grounding requirement has been met.

If you already have a sufficient Ground, you shouldn't need to add a ground for the generator - yes, I agree with Sean.

NEC Chapter 2, Article 250.34 deals with Vehicle mounted generator grounding.
NEC Chapter 2, Article 250.52 deal with the ground electrode itself - it is not a full explanation of the concept, just enough to be generally "safe".

A better explanation of grounding and rods is available in NFPA 780, IEEE-142, MIL-HDBK-419A, and MIL-STD-188-124B (updated to Note 3 - the physical spec I quoted for the ground rod is in this guy in section 5.1.1.1.4).

The IEEE stuff is not free just like NFPA's stuff, but the MIL info can be found for free (unclassified) at this site: https://assist.daps.dla.mil/quicksearch/

-T

[Sean]

Still very confusing to me.  Most commercially available generator transfer switches do not switch the power company neutral.

That's because if you use the proper generator, no neutral transfer is required.  See my original post on this topic.

The reason the neutral must be transferred in the case of using a generator built into a bus is that there is a ground-neutral bond in the bus.  Along with the one already inside your house, you now have a double-bond situation.

This means that current will be flowing on the ground system, which you never want.

So, again, to be clear:  If you are using a household backup generator, properly configured and installed, then a normal household transfer switch is OK.

If you are using your bus to power your house, you need a switch that also transfers the neutrals.

-Sean
http://OurOdyssey.BlogSpot.com

[thomasinnv]

I think to further clarify what Sean is saying about the Neutral/ground bonding issue (not that it really needs clarification, but anyway): An rv generator has the ground and neutral "bonded", or "connected together" at the generator itself.  a household standby generator has the neutral and grounds "isolated" or "not connected together".  this is why a transfer switch for a standby generator in a house or other type of permanent structure does not transfer or switch over the neutral feeds...no reason to.  every source of power must have a neutral to ground bond.  in permanent structures like your house it is accomplished at the point of connection, usually the main breaker panel.  mobile homes can be an exception.  mobile homes usually require the meter to be mounted on a remote pedastal, and the main breaker panel inside the home.  In this case the neutral/ground bond is accomplished at the remote meter pedestal, and the main breaker panel in the mobile home has seperate neutral and ground bars.  there is still only one neutral/ground bond in the entire system.  in an rv when plugged into shore power, the neutral/ground bond is accomplished at the source of the power supply...the main breaker panel at the house.  still only one neutral to ground bond.  when running the rv generator to power the rv, the neutral to ground bond is accomplished at the generator.  next is where we differ between rv's and permanent structures..... any proper rv transfer switch also transfers the neutral line.  by doing so we still have only one point that the ground and neutral are bonded together.

If you are trying to power a mobile home as described above from an rv generator the setup would be fairly simple to achieve.

Sean, any reason why a 3 pole transfer switch couldn't be used right before the main panel to accomplish this?  by isolating the neutral and ground in the main panel and placing the bond at the shore side (utility supply) of the transfer switch?  would that be a code violation?  if not that would seem to be a simple way of solving the multiple bonding issue, and ensuring that 2 power sources are never able to be cross connected.

[rv_safetyman]

Kevin, you asked how my generator is wired in the bus.  As best as I recall, I bring the 4 wires out of the generator and have the hots connected to 40 A breakers.  I then run the circuit to the transfer box.  I will have to check to see where I connected the 50 amp cord (for the auxiliary power supply).

I did not get out to the main power panel to see if I have space for an extra breaker.

Jim

[Seayfam]

I have a quick question for Sean
I just need to clear one thing up on a code issue I have. Are you considering the main panel, the distribution panel in the house that all the breakers are in? Or are you considering the main panel, at the service entrance on the outside of the house? This is where most of our main breakers are at. That way if there is a fire, the firefighters can cut the power before going inside. From everything I can gather, most of your main breakers are inside in the same panel as all the other breakers. And there is no main outside at the meter.

I just need to clear this up so I know where all the neutrals and grounds are bonding in your area.

Thanks

Gary

[buswarrior]

DING DING DING!!!

Quote:

.... you now have a double-bond situation.
This means that current will be flowing on the ground system, which you never want.


What happens if power is able to flow on the ground system, to more than one destination?

The power has a choice of routing, and a choice of destinations, which is quite dangerous.
All the places where the neutral and ground are connected together become destinations.

Having the ground and neutral connected together in more than one place creates the conditions which may allow power to flow in all the metal that we thought was safely grounded... pipes, the metal case of tools and appliances, the electric boxes, and outside, the body of the coach.

With the ground and neutral connected together in more than one place, the power has a choice of more than one destination, and paths that we have assumed were not possible, or at least, we assumed were safe routes for the power to take, become possible.

Picture the family dog in the middle of a field, with the family around the perimeter. Everyone call the dog... will the dog go to the same person every time? Can we predict the dog's path? (forgetting for the moment the bias to go to whoever feeds it!)

Now, if the dog was in the middle of the field and only one family member was present and called the dog, the dog has only one choice of destination.

We only want one destination for the power.

The safety message is:

A person could become a better path for that power to follow, a short-cut of sorts, an easier path for the power than staying in the wires and metal and going back via the "proper" or expected, route, if that person was to be touching the wrong combination of things.

Becoming the path for power to return is electrocution.

How about some plausible examples of how we might get electrocuted with more than one neutral/ground connection?

happy coaching!
buswarrior

[Len Silva]

Typically, unless it is connected to shore power, the bus itself is not grounded, as it is sitting on rubber tires.

Even with all the precautions you might take with this connection, I think I would run a large (maybe 2/0) cable directly from the house ground to the bus frame, even directly to the generator frame.

[Sean]

...
Sean, any reason why a 3 pole transfer switch couldn't be used right before the main panel to accomplish this?
...

That is actually the preferred solution.  However, it does no good to install a 3-pole switch ahead of the main panel unless you first ensure that the neutrals in the main panel are isolated.

Depending on how the house was originally wired, the ground/neutral bonding point may be in this main panel, and when it is, it is common for electricians to simply use the same bus bar for both the grounds and neutrals for all the branch circuits.

In such an arrangement, it can be a challenge to re-wire the panel with separate, isolated bus bars.

I have a quick question for Sean
I just need to clear one thing up on a code issue I have. Are you considering the main panel, the distribution panel in the house that all the breakers are in? Or are you considering the main panel, at the service entrance on the outside of the house? This is where most of our main breakers are at. ...

By definition, a main panel is the first panel after the transformer.  Any panels downstream of that are considered sub-panels.

As you note, some builders put (and some jurisdictions require) the main panel outside at the service entrance, sometimes in the same enclosure as the meter.  Special partitioned enclosures are used for this where the meter part of the panel can be sealed by the power company, but the breaker portion is customer-accessible.

With such an arrangement, the neutral-ground bond is likely inside this panel.  The panel inside your house, which is fed from a breaker in this outside panel, is technically a sub-panel.  If properly installed, this sub-panel will have an isolated neutral bus.

Note that the neutral-ground bond may not be in a breaker panel at all -- depending on power company, local codes, and builder practices, it might be in the meter enclosure at the service entrance, or even at the transformer.  In this case, all panels downstream of the bond location will have isolated neutral bars.

-Sean
http://OurOdyssey.BlogSpot.com

[Seayfam]

Thanks
That solved all the issues I was having. I do know a lot of people that have a main breaker inside and they consider it the main panel, when infact they have one out at the meter. Hopefully this cleared things up for others with the same situation.

Gary

[kevink1955]

I would have to say that on long island 99.9% of services have a main breaker inside at the panel, this breaker is the service disconnect and the bonding takes place in this panel.

The other .1% that have outside breakers (either part of the meter or hung under the meter) are wired that way because the NEC limits the length of unfused service conductors that can be run inside the structure before they hit the main breaker.

The length of the unfused conductors is up to the inspector, most around here allow 4 to 5 feet as the maximum inside. The bonding takes place at the outside breaker and you have to treat the inside panel as a subpanel. That means runing 4 conductors (2 hot-1 neutral-1 ground) between the outside and inside panels. There is no bonding done on the inside panel and the neutral and ground land on their own connection bars

[artvonne]

  This does get confusing, seems many have lil bit different ideas to accomplish the same thing.

  If im understanding this properly.....

   The main line (from pole/meter) neutral is grounded (bonded) at the service panel/main breaker. IE; all circuit grounds and neutrals return to the main panel and are bonded, generally with a bonding screw that connects the neutral and ground BUS bars.

   Sub panels are NOT bonded, all circuit grounds tied to the sub panel ground BUS return to the main panel via seperate ground wire, and additionally, sub panels may have ground rods connected to the sub panel ground BUS bar.

   A transfer switch generally only switches HOTS, not neutrals. Neutrals pass through, no bonding of neutral to ground?

   A home generator is NOT bonded.

   An RV generator IS bonded.

   To then connect an RV generator to your home/transfer switch, should you then UN BOND the generator?? Would it then be ideal to run a 4 wire 50 amp type plug, or three wire 30 amp with a seperate ground wire to the house/transfer switch connection?

[Kenny]

One must be careful in saying that a home generator is NOT bonded.

A home generator could be a generator that was designed solely for the purpose of residential power backup in which case it is likely to to NOT have a Neutral Bond.

A home generator could be construed as a portable general purpose generator which is likely TO have a Neutral Bond that needs to be unbonded when using it as a residential power backup.

There is no rule of thumb here - You should check the generator you are about to use for residential backup to insure the neutral is NOT bonded..... The bonding is done in the residential Main power panel only

[Seayfam]

How about some plausible examples of how we might get electrocuted with more than one neutral/ground connection?

Lots of ways... #1 for me would be the distribution panel (breaker panel in your house could become HOT)

Gary