When do you float the neutral in the subpanel?


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Old 11-24-14, 07:34 PM
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When do you float the neutral in the subpanel?

I'm having an electrician run 2-2-2-4 Al feeder to a 100A subpanel in a detached building. I'm running both 120V and 240V circuits and he's suggesting I 'float the neutral' as opposed to grounding at the subpanel. This is due to preventing feedback/to stop currents flowing the wrong way etc etc. I understand these things, but I thought code was to ground at the subpanel. He's a very respectable electrician in the area so I know he isn't trying to cut corners etc. In fact, he explained it and showed me how to do it so I don't doubt his intentions, I'm just confused as it seems to go against what I've read lately.

tl;dr, ground at subpanel or float neutral and run ground back to the house?

Thanks
 
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Old 11-24-14, 07:44 PM
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I'm not up to speed on the NEC, but every subpanel I've been involved in had an isolated neutral bar and bond just ran back to the ground bar on the main panel.
You only need to ground your neutral once.
 
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Old 11-24-14, 07:46 PM
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The neutral should always float in a subpanel. The equipment ground wire is connected from the subpanel ground bar back to the main panel ground bar. The subpanel ground bar is also grounded to the earth via a wire called a grounding electrode conductor to a copper plated rod driven into the soil. The neutral and ground are bonded together once, which must only be in the first panel after the meter.
 
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Old 11-24-14, 07:47 PM
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Great, thanks! And floating the neutral is the same as floating the ground, right? Sorry, I see different jargon used around and need to be clear. I see no reason logically why they're different.. essentially they're just being kept separate at the subpanel and joining only at the main panel. I think I understand it, at least.
 
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Old 11-24-14, 07:49 PM
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I have yet to hear the term "float".
And yes that is a good way to look at it, separated except at your main panel.
 
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Old 11-24-14, 08:30 PM
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is the same as floating the ground, right?
No ground and neutral are two different conductors. Ground is always bonded.
 
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Old 11-24-14, 10:58 PM
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Floating would be the same as isolated or not bonded to ground.
 
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Old 11-24-14, 11:02 PM
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I agree with Mr. Awesome, "float" is the incorrect word.

A "floating" neutral is one that has no reference to the "hot" conductors. In a standard single-phase 240/120 volt three-wire configuration the "neutral"; that point midway between the ends of the 240 volt transformer coil, is connected to the earth AND also becomes the "neutral" conductor, more properly called the "grounded conductor". It is ONLY at the source where the neutral-to-ground bond takes place and its sole purpose is to provide a low impedance (low resistance) path to allow the circuit protection (fuse or circuit breaker) to actuate in the presence of a fault condition between the hot line and any current-carrying conductor in contact with the earth.

By having the "ground" and the "neutral" conductors bonded ANYWHERE other than the source it provides a parallel path for returning currents on BOTH the neutral and the equipment grounding conductors. Under certain conditions this can be hazardous.

As a practical matter, the only place you are apt to see a "floating neutral" is on some models of portable generators.
 
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Old 11-25-14, 12:46 PM
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I agree with Mr. Awesome, "float" is the incorrect word.
And I also agree with Furd and Mr A. You wouldn't have a floating neutral at the subpanel anyway because it has been grounded at the main panel.
 
 

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