I'm under the impression that for proper generator hook-up, the grounded conductor must be opened along with the hot conductors to completely separate the main system from the emergency circuits...I've yet to find anything in the NEC that supports this; the part about separately derived systems and Emergency Systems (Article 700) is a little ambiguous when it comes to the residential situations. My neighbor- a retired Master mining electrician- is happy to have his generator hooked up to a double pole breaker in the panel, where he kills the main and backfeeds the buss with the generator. I personally don't like that. I've got mine set up this way...
> A 50A 2 Pole breaker feeds a 50A Four-wire Receptacle. A #6-3 w/#8G Cord and plug feeds the Emergency Panel. Another four-wire 50A receptacle is fed by a #10-4 SOW cord that connects to the generator, protected by the generator's circuit breaker. One simply plugs into the appropriate receptacle for either main or emergency power...nice and safe...Right? (Cheap too...) But I'm suspicious that there are still codes being violated by either means... Can one feed a 50A receptacle with a 20A Breaker by code? I can't find it... but some thing tells me "no".

At any rate... pardon my ignorance in this topic. I've seen pictures of residential transfer switches that appear to be two 2-pole mains connected with a bar to make them essentially one unit with only one pair being closed at any time. (Seems to me that under the right short-circuit conditions...say a shorted buss with both main and generator power available... said breaker(s) would oscillate between the two positions and never really protect the circuit... first opening one breaker which closes the other ad infinitum... )I have to assume that this is safe and legal and that I'm just missing something somewhere...

Any comments?

There is no requirement to break the grounded conductor (neutral) at the transfer switch, but if you don't transfer the neutral, you must disconnect the internal neutral to frame or ground connection at the generator. In most small residential use generators this connection is not easily removed.
The handle of a braker does not operate with enough force to reset another breaker when it trips. Also it is very very unlikely that a generator can provide enough short circuit current to cause a breaker to trip on short circuits. A braker requires 500% to 600% current to trip on a short circuit. The generator cannot provide this amount of current. It may trip after time on overload, but most likely the engine will stall out due to the high electrical load.
Don(resqcapt19)

Don,

In the manufactured generator transfer switches with the 4, 6, 10 etc.. circuit capabilities that are commonly used in residential applications, I do not believe the Neutral wire is switched. There is 1 neutral wire that gets connected to the Neutral bar in the Panelboard and that is shared by all when the generator is in use.

If the Neutral is not bonded to the Generator frame you will get voltage fluctuations on 120v circuits being fed by the generator unless both legs are perfectly Balanced. The first time I saw this I called the Generator manufacturer and was told to open the Generator and bond the Neutral to the frame.

Bill,
It is a violation of 250-142(b) to use a 2 pole transfer switch on a 120/240 volt system and have the grounded conductor bonded to the generator frame. The bonding will still exist in a correctly connected system, but the path is longer. If you are using a 2 pole transfer switch, the neutral must be kept isolated from the grounding conductors and an equipment grounding conductor must be run to the generator frame. The bonding path is from the generator neutral to the house neutral to the house main bonding jumper and back to the generator via the equipment grounding conductors.
I also don't understand how bonding will make any difference in any voltage fluctuations.
Don(resqcapt19)

Don,

I'm not sure of all the terminologies involved, (Trying not to mention a Brand Name) but the Equipment I am talking is a Manual transfer switch with say 6 single-pole/double-throw switches. It is prewired and gets attached to a residential Panelboard. It has a short length of Greenfield with a switchleg pair for each circuit and 1 common neutral. There was no ground wire.

There is a twistlock (4 wire)120/240 volt power supply cordset that gets plugged into the 240 volt outlet on the Generator. It was some time ago so I don't remember the exact situation, except that the voltages were very different L-N on the different legs.
It could have been a situation where there was only a 3 wire 240v outlet on the Generator if that makes any more sense.

Don,

I just realized that I didn't mention that My Comments were referring to Portable Generators in the 4000w - 5000w range. Does that make a difference?

Bill,
If the generator is intended for portable use the application of 250-34 will require that the grounded conductor be bonded to the generator frame. When you connect this generator to a transfer switch that has a grounded (neutral) conductor solidly connected to the building neutral, you must, according to 250-142(b) remove the neutral connection to the generator frame and MUST use a 4 wire connection between the transfer switch and the generator. Any other type of installation is in violation of the NEC.
I am aware of equipment that is on the market that does not comply with the NEC rules.
Don(resqcapt19)

Pardon the spelling error of "transfer" in the subject... not sure how that happened... (how embarrassing)...

Information overload...

So either breaking the ground and neutral (4-pole)OR Not (2-pole) is acceptable?

Ok... what happens if the neutral/ground connection in the generator loosens due to vibration, and the neutral/ground is connected to the grid, a 240V series circuit results sending 240V out onto the grounded conductors... won't this cause a dangerous situation?

Getting back to the "flawed" theory of the oscillating DP Breaker... Is there a central "OFF" position between the two positions? What would such a breaker due assuming an infinitely large current available from both the generator and the grid?

Finally...

Who has the safer connection...me or the miner?

Sparky,

I think that what you were looking at was actually 2 seperate double pole breakers that had a mechanical interlock that prevented both being on at the same time. I agree with Don in that I don't think the tripping of one would have enough force (or travel) to set the other.

Sparky,
If the grounded conductor opens at the generator it is no different than if it opens at the utility transformer. You cannot build an electrical system that is 100% safe from all failure modes.
As to the breaker oscillating, this will never happen because, in addition to one breaker not being able the operate the second one via a handle tie, a generator cannot produce enough short circuit current to cause a breaker to open.
Don (resqcapt19)

Thanks guys...
So to reiterate... Both methods are acceptable according to the NEC? That is.. Breaking the Neutral or not...?

That's really the back bone of my questions... the other thing was a tangent... but then I digress... again...lol

I'm aware that the BEST method is to use a UL approved transfer switch... But I'm still in question as to who has the better homemade system (remember my neighbor and I are both qualified electricians and we keep an eye on our unorthodox contraptions...) He with the backfed breaker... or myself with the elaborate ( and fail-safe , if I may add.....) cord and plug system...Easy on the flames... neither of us would approve a customer getting such a setup...but I do want to know what you think.

Sparky,
Unless there is some kind of mechanical interconnect between the main breaker and the back fed breaker at your neighbor's house, his installation is in violation of 702-6. If there is an interlock then both systems are OK, except that the grounded conductor to the generator frame must be removed in his and must be inplace in yours. You must also have a grounding electrode system connected to the grounded conductor at your generator.
Don(resqcapt19)

Sparky,

Of the two, I would sleep better with your arrangement. His is a lot simpler, but more prone to accidents. It is potentially a very dangerous situation. I probably don't have to elaborate on ther dangers if the main was left on or there was some leakage through the main back to the pole.

Does anyone know what would happen to the generator if it was hooked up, breaker was turned on and utility power was also present?

This Topic was moved from the General Discussion Area to the NEC Discussion Area.

[This message has been edited by Webmaster (edited 12-03-2000).]

Great question, Bill...
In fact, I was going to ask that myself...
Ya beat me to it.

What could happen if the generator and grid tried to occupy the same system?

Would the timing of the phases be critical in this? I realize that any timing would be short-lived with the flucuations of a portable generator...

And if so, would the potential be anywhere from 0 to 480V (given a 120/240V system) depending upon how the phases lined up?

Thanks again for your input guys...and thanks for taking the time to "spell it out" for me...

Sparky,

We have several people that frequent this board that should know the answer to that, hopefully they'll pop in soon and tell us about it.

If you try to connect the generator output to the utility, there may be serious damage to the generator. When you tie the generator in parallel with the utility the utility will instantly try make the phase angle of the generator match that of the utility. If the phase angle between the 2 sources is not within about 5 degrees, the utility power will try to make the generator mechanically match the utility phase angle instantaneously. This will result in as a minimum of a tripped breaker, but may actually cause the generator shaft to break. When generators are run in parallel with a utility source or other generators, special paralleling equipment is used to match the phase angle before closing the parallel switch. I am not talking about phase rotation here, but rather the actual point on the sine wave where the two sources are when they are tied together. It goes without saying that the phase rotation and voltage must be matched in addition to the phase angle. All most all utilities require reverse power switching in any generator that operates in parallel with them. This is to prevent back feeding their system. Most of the time it you try to back feed a dead utility system you will just stall out the generator due to the high load, but under some conditions you will create a very hazardous situation for any utility personnel working on the lines.
Don(resqcapt19)

Thanks Don,

That was interesting, I had never heard anything on the subject but assumed that it wasn't good. What happens if the Utility power is fed into the Generator? (Generator not running or stalled)

Wow... You're answering my questions now before I ask them!

Seriously...

I'm to understand that a generator connected to the mains would backfeed the transformer sending line voltage out on the line...

I've always felt that the loads on the rest of that line would be too much for a small generator to compensate for...and as you said, except for some circumstances...it will.

Thanks again for such great and thorough answers, Don.

Just this fall I went to a call, a homeowner had a small 100A RONK dpdt t-switch between the meter and his main panel at his home. He had a small 5kw generator made into it. At his insistence, he had me watch as he cranked up his generator, and threw the t-switch. The house panel died, and the generator simply wound down rather quickly and stalled. The owner exasperated, claimed that was all he'd been getting repeatedly.On investigation, you guessed it, the t-switch was miswired, sending the utility right into the generator. So I corrected it, metered out the generator and ran it to his satisfaction. I suppose, reading some of the reply's here, that it was a rather lucky situation, especially in that I was metering said unit while it was croaking....

In response to the best "homemade" system......if you have a chance to go on a call to a farm, take a good look around...