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Request for Comment on Portable Generator Connection Logic

Request for Comment on Portable Generator Connection Logic

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Old 08-01-17, 09:33 AM
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Request for Comment on Portable Generator Connection Logic

Request for Comment on Portable Generator Connection Logic
1. The term "ground", has multiple meanings based on where it is used. The two grounds are sometimes disambiguated by calling one earth ground. Unlike neutral and the two hot legs, it is not a source of power, but rather a conductor.
a) The earth ground is when a conductor attaches to a ground rod that uses the dirt/earth as a conductor. On one end the power company's transformer bonds it's neutral to earth ground, and on the other end, the house's circuit panel bonds its neutral to earth ground. With a neutral also running between the transformer and house, this becomes a parallel circuit for neutral When people say ideally there should be no current on the neutral, they are not referring to earth ground.

b) The wired ground, or simply ground, is where ideally there should not be any current flowing. The ground wires (uninsulated) connect to the ground pin on receptacles (round) and metal case of directly wired equipment, and run back to the circuit panel where they connect to a bus bar inside of the panel that is connected directly to the metal case of the panel. Also connected to that same bus bar is earth ground, the neutral that runs to every outlet (long slot, white wire), and the neutral that comes in from the power company (normally an uninsulated wire). When you have a device with touchable metal surfaces, such as a microwave, the case of the appliance is connected to the wired ground. Under normal conditions, neither the hot circuit nor neutral touch the case. If there is some type of failure that allows a hot wire to touch the case, the power can take a low-resistance ground wire back to the circuit panel and instantly trigger the breaker, which could save a person's life who is in contact with the case. Many devices are double-insulated making it impossible for that to happen. Those sport two-prong plugs because the case ground is not required for them.
2. The earth and people are relatively high-resistance conductors, but it only takes a current between 100 and 200 mA (0.1 to 0.2 amps) to kill someone, which isn't even close to what it takes to trip a breaker. There are certain situations where a body and the environment can combine to create low enough resistance to enable 120v to push lethal amounts of current through someone. This is usually around water. This is addressed by GFCIs. On a class A GFCI (residential), if the power difference between the hot and neutral differ by more than 6mA, (the 6mA had to have found another path back, such as through someone's body to the ground wire or earth ground) it trips and shuts off the circuit within 1/40th of a second, (Only 1 1/2 cycles of the 60hz sine wave.) (Class C, D, and E trigger at 20mA) An important point is the GFCI does nothing if you get between the neutral and the hot and there is no other path for neutral to ground. It only trips if the hot finds a way back to neutral without taking the proper neutral wire. The only time GFCI is necessary, and can work, is when there is a potential parallel path back to the neutral of the power source.

3. Generators are configured basically 2 different ways with regard to ground. Portable generators are generally configured with "bonded neutral" for use with extension cords and not connected to house wiring. Whole-house standby generators are normally configured with floating neutral, which does not bond neutral because they connect to the house's circuit panel which has a neutral bond.
A. Bonded Neutral: This is how most portable generators are shipped. As with your house, the neutral is bonded to the generator frame/circuit panel, and again like your house, the frame/circuit panel is bonded to earth ground. This is the OSHA and NEC approved configuration for portable generators because the assumption is they will be used with extension cords, not connected to a house's electrical system. This is referred to in the NEC code 250.35(A) as a separately derived system (SDS). When the neutral conductor of an alternate power source is solidly connected to the service supplied system, it is not considered a separately derived system. 250.20(B).
1) Extension cords from generator: When properly configured with an earth ground, this configuration functions identically to the house's electrical system as long as it does not run through any of your house's electrical system. If the generator has GFCI outlets, they will function as designed because the earth ground enables the alternate neutral path back to the generator to create the neutral imbalance to activate them.

2) Circuit Panel Connection: This appears to mimic the power-company-to-house connection where the neutral is bonded to ground at both ends of the connection, thereby creating a parallel path for neutral between them. However, the 240v connection between the generator and house's circuit panel also passes a ground wire that ties the generator frame/circuit panel to the house circuit panel thereby creating a 3rd and low resistance path neutral. Since the power company's transformer and the house's circuit panel both use bonded neutral, the main circuit panel in the house cannot employ a GFCI main breaker because it would immediately trip due to the imbalance created by the parallel neutral path through earth ground. It has been common for generators to not provide GFCI protection on the 240v connection, and NEC does not require it below 1000A. It is also not uncommon for generators, such as the Honda construction generators, to have a common GFCI for all receptacles, including the 120/240 connection that is used between the generator and house panel. That makes sense too as you might want this if you need to plug in a 240v appliance and GFCI. Insulating the generator from earth ground will not solve the problem alone because in addition to the parallel path of neutral through the earth ground, there is a ground wire in the 120/240 circuit between the generator and panel that also provides a parallel path for neutral back to the generator. The common work-around is to disconnect the ground at the generator inlet, however, any place that disconnects the ground from the generator will do. Miswiring the inlet is not an approved method of course, but it requires no changes to the generator or cord and can easily be reversed later if you switch to a generator with a floating neutral where you would want the ground to prevent any potential from building up at the generator. Better and approved ways to fix this for a bonded neutral generator would be to switch the neutral bonding location with something like the inexpensive Siemens breaker panel with a tri-breaker and generator lockout, or a special transfer switch which additionally switches neutral.
B. Floating Neutral: This is the NEC approved method of connecting standby generators, which are classified as alternate power sources that will always have their neutral connected. This is the opposite of a separately derived system (SDS). Here the assumption is that the neutral bonding will be done at the premise's circuit panel, not at the generator.
1) Circuit Panel Connection: Floating Neutral is the approved configuration of connecting a generator to the house's circuit panel, and is used by dedicated, whole-house standby generator systems. The neutral is bonded one place, at the circuit panel. It can use the ground wire of the 120/240 receptacle between the generator and house connected because there is on parallel paths back to neutral at the generator. If the generator is equipped with GFCI that monitors both hots and the ground between them, it will work as designed. If it only has GFCI that monitors each of 120v plugs, they will also work as designed.

2) Extension cords from the generator: Floating neutral is NOT OSHA or NEC approved for floating neutral in this configuration. No neutral bonding would exist in this configuration. That means there also is no place for an alternate path to neutral to create an imbalance that would trigger a GFCI outlet. However, without an alternate path back to the generator neutral, even if you touch a hot wire, there is no risk because there is no path back to the generator's neutral, neither through the earth ground nor even touching the generator's frame. You should still ground the generator to earth to ensure no potential can build up between the generator and earth.

Summary Thoughts:
1. Earth grounding the generator frame makes sense to eliminate any stray potential that might build up between the generator frame and earth.

2. The main problem with more than one neutral bond is if the generator has GFCI on the 120/240 receptacle, it will cause the GFCI to trip and it drop the connection between the house and generator.

3. Floating neutral is how whole-house standby generators are configured. Their transfer switches do not switch neutral because the neutral bond only exists in one place, and neither do nearly all separately purchased transfer switches.

4. While not according to OSHA or NEC, I concur with many others that floating is safer even with extension cords. While it is true that GFCI will not work, it is also true that it eliminates most of the reason for needing GFCI. Consider having the generator chassis connected to a suitable ground rod, and you are a few feet away, standing in a puddle of water with bare feet, holding one of the live wires in your hand. There is no return path to neutral either through earth ground or by touching the generator. You would NOT want to try that in a bonded neutral configuration because now a return current path exists through your feet to the grounded chassis, through the bond jumper, and to the neutral conductor. The only thing that could save you would be GFCI. I would still want to have the option with GFCI on at least the 120v outlets anyways, but it's not easy to come up with a scenario where bonded neutral is safer, especially if the generator does not have GFCI outlets.

5. Transfer vs. lockout:
a) Aside from neutral-switching transfer switches, they both provide the same functionality.

b) The pluses of a manual transfer switch is it separates and pre-balances circuits for use when the power is out. The downsides are they add cost and complexity. Moreover, the circuits that are predefined are the only ones you can use, and you are limited by the number of circuits and amps that the transfer switch supports. It requires rewiring your circuits and running them to another box on the wall.

c) The pluses of a lockout is it is much simpler, cheaper, no boxes on the wall, you can still balance the power between legs by simply moving a few breakers around, you can still designate which circuits to turn on when under generator plus it allows you to add or remove circuits when you change generators or fuel, and you can change on the fly without rewiring anything. (E.G. Don't use the Microwave, we're going to turn the AC on now. The downsides are???

d) If I wanted to switch the neutral, I could either replace the current panel with the inexpensive Siemens circuit panel with generator lockout and breaker that switches neutral, and have all of the advantages of a lockout, or buy a special transfer switch with the third pole to handle switching the neutral, and live with typical transfer switch limitations, extra box, extra wiring, and complexity.

e) If I had a generator that started automatically when the power went out, I would want an automatic transfer switch. If I had a big enough generator, I would want something that simply switched the mains.
 

Last edited by IT_Architect; 08-01-17 at 10:29 AM.
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Old 08-01-17, 10:07 AM
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I think the tl;dr version of your post is that you are asking why a neutral-ground bonded generator with GFI is safer (according to OSHA and NEC) than a neutral-ground float generator.

I think while in theory, keeping hot/neutral completely isolated from earth ground is good protection. There's no potential difference between the hot/neutral and ground, so no risk of shock.

The problem comes up when things aren't as you planned. What happens when you drop the drill into a puddle. All of a sudden, the neutral is connected to an earth ground, and you lose all the safety. As they say, '*** happens', especially on a jobsite. Thus the idea of GFI protection is pretty close to foolproof.
 
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Old 08-01-17, 11:21 AM
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This looks like a continuation of your already running thread on generator grounding.
Is there a reason why you started a new thread ?

http://www.doityourself.com/forum/el...grounding.html
 
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Old 08-01-17, 12:54 PM
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@Zorfdt What happens when you drop the drill into a puddle. All of a sudden, the neutral is connected to an earth ground, and you lose all the safety.
That would not be a good situation. There would still only be one way back to neutral, which means GFCI would not trip. I'm glad I posted my thoughts.

@PJmax This looks like a continuation of your already running thread on generator grounding. Is there a reason why you started a new thread ?
Yes, because it was long and all over the place, and would be a challenge for someone to learn from. This is a logical summary of what I've learned on that thread and my own thoughts and research. Based on Zorfdt's comments, there was still a flaw in my logic.

Are there any safety issues if the generator is configured bonded neutral to feed the panel? (Provided there is no GFCI on the 120/240v plug and there is on the 120v plugs)

Thanks!
 
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Old 08-01-17, 07:29 PM
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Use of a tri-breaker to switch the neutral in a backfed interlocked panel or using a transfer switch that switches the neutral will not solve the problem of double bonding neutral and ground while using a generator with a hidden neutral/ground bond. It would be necessary to rewire the service panel as a subpanel i.e. with grounds and neutrals put on separate bus bars.

Many homeowners will, for simplicity's sake, elect to wilfully not so revamp the service panel and also elect to wilfully not dig into the generator to forcefully unbond it. . The remaining alternatives are to unhook the green wire from the inlet to prevent ground fault interrupter trip, or, if there is no affected GFCI, to simply allow the double bonding and resultant violation of the NEc to exist and persist. I believe that there is no safety hazard.
 

Last edited by AllanJ; 08-01-17 at 07:58 PM.
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Old 08-01-17, 08:26 PM
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Consider the purpose of the bare or green-insulated equipment grounding conductor. Its sole purpose is to provide a low impedance return path to facilitate the actuation of any overcurrent device when there is a ground fault on the so-called "hot" conductor.

When connected to a standard service panel an equipment grounding conductor back to the the portable generator would serve to trip the overcurrent device (not all generators have conventional circuit breakers) AT THE GENERATOR. Since in most cases the branch circuit overcurrent device at the service panel will have a lower amperage rating than the generator protection it is the branch circuit protection that will actuate first.

As a practical matter the ONLY ground fault that would be protected for by using the fourth conductor ("grounding") between the service panel and the generator is the interconnect cable itself. Use a type SO** cable for this interconnect and protect it from physical damage and the solid ground connection between the generator and the service panel is all but irrelevant.
 
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Old 08-02-17, 02:54 AM
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@AllanJ Use of a tri-breaker to switch the neutral in a backfed interlocked panel or using a transfer switch that switches the neutral will not solve the problem of double bonding neutral and ground while using a generator with a hidden neutral/ground bond. It would be necessary to rewire the service panel as a sub-panel i.e. with grounds and neutrals put on separate bus bars.
Thank you for correcting me on that.

@AllanJ ...The remaining alternatives are to unhook the green wire from the inlet to prevent ground fault interrupter trip...
I would think that wouldn't work unless the generator frame were also not earth grounded, and touching the generator would cause the GFCI to trip?

Thanks!
 
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Old 08-02-17, 02:57 AM
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@Furd Consider the purpose of the bare or green-insulated equipment grounding conductor. Its sole purpose is to provide a low impedance return path to facilitate the actuation of any overcurrent device when there is a ground fault on the so-called "hot" conductor...When connected to a standard service panel an equipment grounding conductor back to the the portable generator would serve to trip the overcurrent device (not all generators have conventional circuit breakers) AT THE GENERATOR.
With both the panel and generator bonded, perhaps to the same ground rod, I would think that wouldn't be any different than if the equipment grounding wire were left connected?

@Furd Since in most cases the branch circuit overcurrent device at the service panel will have a lower amperage rating than the generator protection it is the branch circuit protection that will actuate first.
If the breaker in the panel is sized to a 30A inlet (7200), and the generator is a 5500/6500, the generator would be less.

@Furd As a practical matter the ONLY ground fault that would be protected for by using the fourth conductor ("grounding") between the service panel and the generator is the interconnect cable itself. Use a type SO** cable for this interconnect and protect it from physical damage and the solid ground connection between the generator and the service panel is all but irrelevant.
The generator came with such a cable except it was 3 conductor and wired using ground instead of neutral. I've since rewired the cable to use the neutral connection for the 3rd wire.

Thanks!
 
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Old 08-03-17, 06:51 AM
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With both the panel and generator bonded, perhaps to the same ground rod, I would think that wouldn't be any different than if the equipment grounding wire were left connected?
Either way a parallel path for "return" currents would exist. The amount of current on each path would be determined by the impedance of that path.

If the breaker in the panel is sized to a 30A inlet (7200), and the generator is a 5500/6500, the generator would be less.
I was referring to the BRANCH circuit breaker, not the circuit breaker through which the generator supplies the panel. This latter CB is primarily a disconnect and has no bearing on the subject being discussed. General service branch circuits (lighting and convenience receptacles) are limited to 20 amperes by code.

Bottom line, there really is no code-approved method of connecting a portable generator to premises wiring. There are several work-arounds that minimize any hazard and what works best for you must be determined within your specific requirements.
 
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Old 08-03-17, 07:48 AM
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I broached the subject a bit on the "other thread".
Basically, ungrounded power conductors have a safety benefit on "small scale" installation.
Grounding one of the conductors has safety benefit on "large scale" installations.

So, the question is what is small, what is large?

Lighting is a risk, and accidental contact with a different, high voltage source is a risk.
So, let's say I'm going to power a carnival with a 250kW "portable" generator. Do I want to bond the neutral?
I tend to think YES, due to the geographic spread of the system and the risk of a lightning strike hitting the Ferris Wheel and having no defined grounding path for the power conductors in the system.
If I'm going to power a skill saw at a job site, I would not bond the neutral to earth.
If I'm going to power my house after a hurricane, it really doesn't matter, since the house neutral is ALREADY bonded. If I bond or don't bond, it really doesn't matter. It would mean some ground/ green wire current takes some neutral current on the way to the genset, likely not a big deal.
 
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Old 08-03-17, 09:43 AM
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Maybe the easiest and best way to handle this is to swap the generator I just picked up for one with GFCI, and install a SPST switch with "House Connect" and "Extension Cords". The truth is, 90%+ of the use will be connected to the house in standby mode. Nobody tailgates or camps with a 200lb+ generator.

Another option is to leave it bonded and put kids plugs in the generator 120v outlets while it is connected to the house which will also keep the junk out of them. I have the ground disconnected by default between the house and generator but it would be grounded to the panel anyway through the neutral. The generator will have no ground rod, just sitting in a plastic shed. I can add GFCI to the garage outlets. If it ever does get used in a remote location with extension cords, they can take the plugs out.
 

Last edited by IT_Architect; 08-03-17 at 10:37 AM.
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