neutral ground bonding - using coach to power the house

[Tim Strommen]

...There is a LEGAL AND SAFE way of doing this...

And more to the point "published"...

Article 250 Grounding and Bonding
Article 445 Generators
Article 551 Recreational Vehicles and Recreational Vehicle Parks

Almost all of Chapter 7 but at least:
Article 700 Emergency Systems
Article 702 Optional Standby Systems
Article 705 Interconnected Electric Power Production Sources

And this too...

-T

[rv_safetyman]

My name is Jim and I am at this meeting because I need help.  

You guys really got my attention!  Yes, I made up what has been called a "suicide cable" (male/male) on another board (yes, you guys got me doing some research).  I plugged it into a 50 amp dongle on the bus and a welder outlet in the shop.  Opened the main breakers first.  Obviously it worked and I did not die (nor anyone else).

I tried to research transfer switches for the house end, but it looks like you would have to rewire a good part of your house.  In my case, I have two service boxes (one outside for big circuits and one in the kitchen area for the small circuits - outlets and lights).  Would just not be practical to try to accommodate a transfer switch.  

In a like manner, running extension cords from the shop to the house is not practical, as it is well over 50 feet to many things I would need to power.  Plus I need 220 for the well.

So, here is my plan - let me know what you think:  

1)  I will buy a small service box with perhaps room for 6 breakers.  The feed for that box will be a 50 amp male flexible cord about 10 feet long.  

2)  When I did the wiring to my shop, I ran a second set of 100 amp service wires to the shop.  One end is in the main house service box (not hooked up) and the other end is in the shop, also not hooked up.

3)  The end in the main service box will be hooked up to a 50 amp outlet mounted just below the main service box in the proper enclosure.

4)  I will put a 50 amp connector (male) on the shop end.  This end will be plugged into a 50 amp extension cord from the bus.

5)  From the new service box, I will drop a 220 outlet for the well pump and two or three 120V outlets (all mounted below the box).  The new service box and outlets will be in the furnace room where the well pump service, furnace, and freezer are located.

6)  I will put outlet boxes and plugs in the normal circuits for the furnace and well pump, so that I can unplug them from the normal service and plug them into the new service panel/outlets.

7)  I will make up, or buy, heavy duty extension cords to run upstairs where I will hook up a couple of lights and the fridge.

 When I need to hook everything up I will plug the shop end into the RV extension cord, plug the new service panel into the 50 amp outlet at the main service box (the one connected to the RV) and then plug in the furnace, well, freezer and extension cords at the outlets from the new service panel.

9)  This system will completely separate the generator from any public power supply.  I will ground the service panel.  The only issue that I can think of is the bonding at the generator.  I don't think that will be an issue, but need input.

If you think about it, this is just a fancy set of extension cords, but executed a bit better for my needs.

BTW, my generator is 10 KW wired for 220.  Thus I only have 40A 220V available, but that seems to be OK for what I need to run.  This scheme will also let me balance the load fairly well.  The output from the generator is connected to 40 amp breakers, so the system should be well protected.

Thanks for looking at my scheme.

Jim

[kevink1955]

Jim, could you fit another breaker in your main box outside for the generator feed from the shop. You could then interlock the utility main with the generator breaker with a kit from the link below. This would power up the entire service and your kitchen panel that I assume is a sub panel of the main box outside.

You would have to shed some of the large loads in the outside panel, A/C units, electric water heaters etc.


http://www.interlockkit.com/

I am assuming they make a kit for your existing outside box, this setup will assure that the main has to be open before you could close the generator backfeed breaker

[Tim Strommen]

I like what Kevin said.

Also, if you only have a few "emergency loads" and no space left in your box (common), you can move a few circuits to a main-lug sub-panel and place this device between the main panel and the sub-panel.  This should also save you some wiring duplication, and reduce the marking complexity (you need to indicate where a back-up powered circuit gets its power).  If you don't have a direct line of sight to the generator, some type of pilot light would be a good idea to show that the gen input is live.

This seems like a perfect topic covered by NEC Articles 700, 702, and 705...

I don't get what's so complicated with AC power - it seems fairly straight-forward.

Although, I do believe the details get more complicated if the generator has its own Ground (as it should, 10-foot or longer by 3/4" 12-mil copper-clad steel-rod driven down to the water table), and the Neutral is correctly bonded at the generator (should be).  Sean can check me on this, but I recall that Neutral must be bonded to Ground at the point of service entry for a typical residence (mostly because you need to provide ground locally at the site and null out the Neutral line from the transformer - the power company doesn't supply a ground from the pole, all GFCI devices should be downstream from the bonding point).  If you don't isolate the Neutral at the main breaker when you power the generator input, both the ground and Neutral will split the Neutral return power and this will pop a generator's GFCI (as it should.  GFCI looks for balance in the Hot an Neutral - any difference is presumed lost to Ground, no current should be "missing" - just 10mA can kill a person).  This means that you would need to separate both Hot-legs of the main input plus the Neutral - but leave Ground from the main connected (it should never be isolated!!).  Remember that the Ground-wire and the enclosure and conduit bonding is there to catch stray current and bring it back to a point where it can be properly controlled (i.e. GFCI) - incorrectly isolating the Ground can lead to a person or some other potentially damagable item completing the ground circuit otherwise - the worst being a person working on a system and touching both open ends of a circuit with different/separate hands causing the current to cross the chest cavity.

This same thing is done in Inverters with a Generator input - the individual power source devices have the Neutral bonded to Ground at the earliest point.  When a generator input powers the inverter, it must isolate the Neutral or GFCIs pop (current incorrectly carried by ground).

-T

[rv_safetyman]

Kevin and Tim, neat concept.

I don't get what's so complicated with AC power - it seems fairly straight-forward.

Tim, if you have ever seen a mechanical engineering student in an EE lab, you would not make that statement.

If I understand the concept, if the supply breaker from the generator is on, the InterLock demands that the main breaker is off via the steel plate.

I assume that I would tie the neutral and ground from the generator to the bars in the main breaker box.

Early in this thread, it sounded like using the main breakers to isolate the house from the grid was a no-no.  Apparently if there is a mechanical lock system then using these breakers is acceptable.  It is minus 10 degrees tonight, so I will wait until tomorrow to look.  I am pretty sure I have an extra slot.

On another board, someone mentioned that the neutral was still tied to the grid and that could cause a problem.  Comments?

I am still a little lost of the bonding issue.  Seems to me that the InterLock does not resolve that problem.  Tim, it sounds like you are saying that if you provide a good earth ground to the generator, then the bonding is not an issue. 

You guys are slowly getting this EE challenged ME up to speed. 

[libby]

A quick question for the experts, Tim says a copper clad steel rod driven down to the water table, is that correct?  I just asked cody how deep the water table is here and he didn't know so he called Larson Well drilling and asked bob larson, bob said generally 25 to 60 foot deep, thats a heck of a rod, was something else meant?

[Sean]

...
9)  This system will completely separate the generator from any public power supply.  I will ground the service panel.  The only issue that I can think of is the bonding at the generator.  I don't think that will be an issue, but need input.
...

Well, now we are back to the original question that started this thread.  Everything you propose here will work, so long as the neutral in your new sub-panel is isolated from the ground.  When you connect this sub-panel to your coach generator, the neutral and ground will be bonded then only at the generator, which is as it should be.

I would not worry about additional grounding, such as a driven rod, for the generator so long as there is a good ground connection at the new sub-panel.

...
I assume that I would tie the neutral and ground from the generator to the bars in the main breaker box.

Early in this thread, it sounded like using the main breakers to isolate the house from the grid was a no-no.  Apparently if there is a mechanical lock system then using these breakers is acceptable. ...

On another board, someone mentioned that the neutral was still tied to the grid and that could cause a problem.

The neutrals must be isolated from one another, otherwise you will suffer the multiple-bonding issue that can be an extreme hazard.

Interlocking breakers are a legitimate means to separate the services, but you must use multi-pole breakers and use one pole for the neutral.  That means a two-pole breaker for a 120-volt service, or a three-pole breaker for a 120/240-volt service.

I am still a little lost of the bonding issue.  Seems to me that the InterLock does not resolve that problem.  Tim, it sounds like you are saying that if you provide a good earth ground to the generator, then the bonding is not an issue. 

Yes, the bonding is still an issue, no matter how many driven grounds you have.  As I just wrote, an interlocking breaker system is only acceptable for this purpose if the neutrals are also interlocked.  I probably should have made that clear earlier in the thread when I agreed that such an interlock is acceptable.

... 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.

-Sean
http://OurOdyssey.BlogSpot.com

[Fred Mc]

So in a standard 200 amp house main panel does the main disconnect (which is 240v) shut off the hot and neutral when it is shut off? How can you tell if it disconnects the hot and neutral?

Thanks

Fred

[Seayfam]

So in a standard 200 amp house main panel does the main disconnect (which is 240v) shut off the hot and neutral when it is shut off? How can you tell if it disconnects the hot and neutral?

Thanks

Fred

No it doesn't, it just shuts off the two hots. Even the main breaker at the meter loop only disconnects the two hots.


Gary

[bevans6]

A standard house panel has a two pole main breaker which interrupts the two 120 volt feeds only.  It does not interrupt ground or neutral.  In a standard house main panel neutral and ground are bonded in the panel itself.  That should be the only bond for the house.

My understanding of the way a generator panel works is - you have a separate panel from the main house panel, that supplies a subset of the house (could be all of the house, but usually a subset of important branches  is selected to run from the Generator).   that panel has no bond between neutral and ground.  It is connected back the main panel AND to the generator by interconnected breakers that connect both 120 volt live feeds and neutral from whichever supply is in use at the time.  the generator panel gets both it's supply and it's neutral bond from it's source, be that the main panel or the generator.  It does share the common ground at all times.  The Interconnected breakers must be physically unable to connect both supply choices at once, and must always connect all the feeds to the selected supply - all of the two live feeds and the ground simultaneously.  It can be a manual or automatic switch system

Now - I wrote all that out to see if I actually got it.  Did I?

Brian

[Lee Bradley]

Being a believer in Murphy; I had a large knife switch installed years ago. The house is connected to the common with the power company on one side and the generator on the other; power company and the generator can't be connected. No failed breakers to cross-connect, no systems to remember to have in the right configuration, just move the switch from power company to generator. Since my generator is not big enough (at this time) to run the whole house, I have to switch a couple of breakers to manage the load. Worst case if I forget is a tripped breaker on the generator.

[Len Silva]

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

An open neutral is an absolute disaster in a house, causing all manner of equipment damage, possible fire and shock hazard.  The increased risk of an open neutral by introducing a breaker or switch into the mix seems risky to me.  If there were such a switch, I would want to be very sure that it made the neutral before making the hots.

I have seen it happen where the kitchen stove, refrigerator, and several televisions, along with multiple small lamps and appliances were destroyed by an open neutral.  Fortunately, the power company accepted responsibility and paid for everything.

There is no way to be sure that the neutral coming into the house is not grounded.  You can add an isolated neutral bar in your panel but it still might be grounded at your neighbors house if you share a transformer.

[bevans6]

there is no neutral coming into my house.  I went outside and looked  there are only two hot leads coming from the street.  Neutral is created by bonding to ground within the main panel, only.  No need to switch "power company neutral" since it doesn't exist.

for me, at my house, anyway.


edit - I went outside and looked again,  the bare wire is indeed brought into the meter base.  Not for nothing do i hire electricians to do this stuff for me.

Brian

[Fred Mc]

So just to be sure, I built an interlock device similar to the one referred to previously.(http://www.interlockkit.com/) which PHYSICALLY prevents you from having both the main breaker and the breaker receiving power from the generator on at the same time. With the main breaker shut off is that a failsafe way of assuring that you won't get backfeeding into the utilities lines? I guess to be doubly sure you could remove the meter head but the utility might just frown on that. 

Thanks

Fred

[kevink1955]

Brian

Are the 2 hots wraped (suspended) by a bare wire, the bare wire is the utility neutral. Unless you have a 30 or 60 amp 120V service there has to be a utility neutral. Looking back at your post you said 2 hots so you must have a 240V service and that must have a utility neutral.

Jim

How does your gennerator connect to the bus electrical system now, is it direct wired to the circuit breaker panel or is it cord and plug connected and is it a 4 wire plug?  If it is cord and plug you may be able to remove the generator bond and replace it with a bond in the circuit breaker panel. Then when you unplug the generator from the bus and plug the cord to your house in the bond will be at the house panel. You would only have 1 bond that way and it should be code legal.

I still think we are making more of the double bonding issue then we should, worse thing that can happen is you have a paralel path where both conductors will carry part of the neutral current. Since both conductors are sized to handle full load I do not see the hazard. I see more kill potential in not having the main/generator breaker interlock.

If you have a copper city water service in your house take an ampmeter reading on that conductor. Bet you find several amps flowing on the water main, thats the utility and some of your neghibors geting their neutral thru your water main ground. Water guys around here put a jumper cable across the water meter before they remove it so they do not get shocked. New installations require the jumper by code so even the code expects some form of extra bonding.