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Generators & Electric Motors General Discussion

Unswitched Neutrals on Two Gensets


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  #1  
Old 08-09-2007, 08:00:26 AM
Jim Davis Jim Davis is offline
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Default Unswitched Neutrals on Two Gensets

I am preparing to install the second genset with the second transfer switch. Right now my neutral from genset 1 is unswitched and bonded right on through from the genset to the utility. I assume this genset's windings see unbalanced load from the house when it is idle. Now I plan to have another genset with another transfer switch to switch between the two power sources and I am planning to have both gensets neutrals bonded on through to the utility. What I am wondering is what effect the unbalanced load from the genset on line running will have on the idle genset next to it? The only bond from neutral to ground is at the service entrance.
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Old 08-09-2007, 10:46:03 AM
Jim Rankin Jim Rankin is offline
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Default Re: Unswitched Neutrals on Two Gensets

With an open transfer switch, the windings of the generator don't see any load at all, balanced or not. There is no complete circuit if the neutral is the only connection to the house/other source of power.

Once the transfer switch connects the generator to the load, then the balance of the load across the lines comes into play and the neutral becomes very important to "split" the 240 volts properly into two 120 volt circuits. Without a good neutral connection, the 240 line-line voltage will be divided according to the total load placed on each line.

So if you have a lot more amperage on one line than the other, without a good neutral connection, the voltages you 120 volt appliances will see might be 180 and 60 depending on which line they were on.

Hope that helps and didn't confuse.

Keep asking and clarify your question if I have confused things.
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Old 08-09-2007, 12:22:26 PM
armandh armandh is offline
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Default Re: Unswitched Neutrals on Two Gensets

just be sure the white is not connected to ground any where but the one bonding point [first disconect.] this may require a change in your distribution panel to be isolated. [should have been done with the first gen set.]
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Old 08-09-2007, 12:52:49 PM
Jim Davis Jim Davis is offline
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Default Re: Unswitched Neutrals on Two Gensets

Jim I think I got it and it clears up a lot at once. If the hot legs are not there with it then the neutral is not doing anything. I appreciate the help. Armand the neutral is bonded to ground nowhere but at the service entrance (my main panel) and that is from the advice I gained here. Thanks a bunch.
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Old 08-09-2007, 07:38:29 PM
armandh armandh is offline
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Default Re: Unswitched Neutrals on Two Gensets

the newest meter boxes have disconects and may be grounded there but as long as it is only one place you are good to go
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Old 08-10-2007, 11:28:33 AM
Steve Dawkins Steve Dawkins is offline
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Default Re: Unswitched Neutrals on Two Gensets

Jim,

We have several installations at telecommunications facilities where there are two generators and two transfer switches. One transfer switch is used for utility/generator just like any normal standby application. The second transfer switch is generator/generator. Both generators connect to this switch and the output (load) of it goes to the generator side of the other switch. This setup provides redundant generator protection in case one unit fails during a utility power outage. The idle generator will start and come on line if the other unit fails to start or shuts down for any reason. This feature was used extensively during a hurricane a few years ago when utility power was out for over a week. The primary genset would run out of fuel and the other unit would start and assume the load.

On all of these installations, the neutrals are solidly connected and the only grounding point is at the main service for the builidng. The neutral of each generator is isolated from ground.

Drive a ground rod for each generator and attach it to the frame of the generator. If the ground rods are less than six feet apart, they must be bonded togethe per the NEC. If the generators are fairly close together, just drive one rod and connect both gensets to it.

Last edited by Steve Dawkins; 08-10-2007 at 12:44:02 PM.
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Old 08-10-2007, 01:12:21 PM
Jim Davis Jim Davis is offline
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Default Re: Unswitched Neutrals on Two Gensets

Nothing like hearing from a pro that has seen it work first hand. Thanks a bunch Steve!
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