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Joined: Nov 2000
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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?


-Virgil
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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(resqcapt19)
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Quote
Originally posted by resqcapt19:
[B]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.
B]

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
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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(resqcapt19)
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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.


Bill
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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
Joined: Nov 2000
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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)


Don(resqcapt19)
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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?


-Virgil
Residential/Commercial Inspector
5 Star Inspections
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Joined: Oct 2000
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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.


Bill
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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)


Don(resqcapt19)
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