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Author Topic: 2 bonded neutrals  (Read 8100 times)

Chris Delcambre

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2 bonded neutrals
« on: September 20, 2013, 02:12:47 pm »

Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris
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TJ (Tom) Cornish

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Re: 2 bonded neutrals
« Reply #1 on: September 20, 2013, 03:29:50 pm »

Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris
The result is any ground conductors before the second bond (your distro) potentially become current-carrying - there are effectively two neutral wires returning current - the neutral, and the ground.  The amount of current flowing on each of these wires will be in proportion to the path resistance, and in the event of a neutral wire failure, all return current would be on that ground wire.

Your portable distro should not have a ground-neutral bond unless that's the only bond in the system.
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Rob Spence

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Re: 2 bonded neutrals
« Reply #2 on: September 20, 2013, 03:56:28 pm »

Hi there,

I understand alot of electrical stuff and how to hook something up. However I do not understand the workings of electricity. Can there be 2 bonded neutrals in a system???
Example:
My main portable power distro has a bonded neutral and then I connect it to a generator that also has a bonded neutral. What would the result be????

Thanks,
Chris

There can as you say in your setup (and as we see around here, often does) but there should NOT be!

Sub panels should not bond neutral to safety ground.
Since the generator has the main panel (think of it as the service panel), the bond should only be there.

You should remove the bond in your sub panel (distro).

Edit: clarify the wording

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« Last Edit: September 20, 2013, 04:14:46 pm by Rob Spence »
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John Roberts {JR}

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Re: 2 bonded neutrals
« Reply #3 on: September 20, 2013, 04:09:19 pm »

Indeed it is incorrect to short ground to neutral in multiple places. I once read about an unusual (studio) wiring scheme where a local independent ground system was tied to neutral at some distance from the main panel (trying to clean up studio ground noise). I sent an email to that author questioning the safety of that wiring and he never responded to me.

The major concern about commingling neutral and ground paths is that a loose or open connection of both back to the generator could make your distro ground and any plugged in product chassis grounds hot....  if ground was not connected to neutral at your sub panel, the hot neutral would be relatively harmless to humans in the area.

Admittedly this anticipates multiple faults at the same time but in life stuff happens. 

JR

Note: I am not an electrical wiring expert, so take my advice with a gain of salt. but i do grok how the electrons can misbehave.
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Mike Sokol

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Re: 2 bonded neutrals
« Reply #4 on: September 20, 2013, 04:16:46 pm »

It can (and as we see around here, often does) but should NOT!

Sub panels should not bond neutral to safety ground.
Since the generator has the main panel (think of it as the service panel), the bond should only be there.
Yes, that's correct according to the NEC. However, be aware that many (if not most) smaller (under 5 KW) inverter generators have a floated G-N bond. That includes all modern Yamaha and Honda generators up to 5,000 watts. In that case, you'll want to create a G-N bond in your distro (think of it as a sub-panel).

Now, this G-N bond doesn't have to be a complicated thing. Here's an article I wrote specifically for RV owners who want to run their trailers and motor homes from a Honda/Yamaha portable generator. Since those generators have a floated neutral, and their RVs have isolated neutrals, then adding a simple jumper plug at the generator itself is an easy and safe solution. See http://www.noshockzone.org/generator-ground-neutral-bonding/ for the article.

So, if you isolate the G-N bond in your distro (it's supposed to be isolated anyways for connecting to building power that DOES have a G-N bond), then that will work as-is with any large generator (over 5 KW) which should already have a grounded neutral. However, if you're plugging into a 3KW Honda Inverter generator, just stick your G-N bonding plug in the unused generator outlet, and it will provide the neutral to ground bond.

How to tell if your generator has a G-N bond? Just plug in a simple 3-light cube tester and see if it shows a proper ground. If it shows open ground, then plug in your G-N bonding plug and try it again.
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Jonathan Johnson

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Re: 2 bonded neutrals
« Reply #5 on: September 21, 2013, 01:22:24 am »

The result is any ground conductors before the second bond (your distro) potentially become current-carrying - there are effectively two neutral wires returning current - the neutral, and the ground.  The amount of current flowing on each of these wires will be in proportion to the path resistance, and in the event of a neutral wire failure, all return current would be on that ground wire.

Your portable distro should not have a ground-neutral bond unless that's the only bond in the system.

In this scenario (where the generator has bonded neutral and ground) you have two options:
  • Leave it bonded at the distro, but disconnect the ground conductor that goes to the generator. Connecting other distros or equipment directly to the generator could result in ground loop hum.
  • Break the bond at the distro. The grounds and neutrals in the distro should go to separate buses; the neutral bus should be insulated from the chassis of the distro except by the REMOVABLE bond. I think I would prefer this method, as it seems safer and complies with codes.

It may be an option to install an SPDT switch to selectively bond your load grounds to either the neutral or the upstream ground connection. Or you could just make up a "bonding adapter" as Mike suggests and use it when needed, leaving the ground and neutral unbonded in the distro.
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Jerome Malsack

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Re: 2 bonded neutrals
« Reply #6 on: September 27, 2013, 08:36:07 am »

You should look carefully at the switch before you install that because It will need to handle the 20 amps current that can be there.   
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Chris Hindle

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Re: 2 bonded neutrals
« Reply #7 on: September 27, 2013, 08:58:11 am »

... or It may be an option to install an SPDT switch to selectively bond your load grounds to either the neutral or the upstream ground connection. Or you could just make up a "bonding adapter" as Mike suggests and use it when needed, leaving the ground and neutral unbonded in the distro.

Switches = BAD
Adapters = Useable

A switch can be set to the "wrong" position too easily.
Adapters, while sometimes clunky and able to get "lost", are put in place when needed, and not likely to get installed/removed by accident.
The real question is, will the fire marshall/inspector accept you workaround, or declare it N/G ??
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Jonathan Johnson

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Re: 2 bonded neutrals
« Reply #8 on: September 27, 2013, 05:10:43 pm »

The real question is, will the fire marshall/inspector accept you workaround, or declare it N/G ??

Always comply with the code enforcement officer. If that is not acceptable, present the code enforcement officer with engineering drawings (from an electrical engineer licensed in the state or jurisdiction where the installation is located) showing your preferred method. Then comply with the code enforcement officer. They will usually go with the engineer's specs, but they do have the power to override the engineer.
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Craig Hauber

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Re: 2 bonded neutrals
« Reply #9 on: October 05, 2013, 03:53:18 pm »

Yes, that's correct according to the NEC. However, be aware that many (if not most) smaller (under 5 KW) inverter generators have a floated G-N bond.

What's "floated" mean?  Is the ground simply not connected to anything but the metal of the generator housing? 

I have an older style pullstart Generac and haven't actually checked it out but need to use it soon for a homecoming float.  (which brings up a whole other grounding set of questions as the metal framed float trailer is grounded to the vehicle's battery and the whole assembly is on rubber tires.  what happens when generator-run equipment is tied to inverter-run equipment (or car-audio equipment) through the grounds of audio cables?
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