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Old 07-17-2012, 07:24 AM   #1
east_tn_emc
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Default Generators, Transfer switches and "switched neutral"

All,
I am trying to wrap my head around hooking up a generator to my house electric. I have been looking at transfer switches and the generator interlock kit so you can backfeed the panel thru a circuit breaker (it interlocks the main breaker with the generator backfeed breaker so only ONE can be closed at a given time).

There has been a lot of debate on the internet and on here about the use of the interlock kits and the neutral feed...and how transfer switches will "switch" the neutral, but the interlock kits do not switch the neutral.

I read this, and understand this to mean, that the neutral feeding back to the utility company is switched, or open, to prevent any connection between the generator, via the transfer switch, and the power company feeds. Is this correct?

I have a couple of questions...
1) why would that matter when the neutral is tied in to grounding rods at the house and at each and every pole within my utility company (as confirmed from a retired utility company lineman I know).

2) I found the wiring diagrams and install procedures for the Gentran transfer switch. If you look at the last page where you can see the wiring diagram, it looks to me like the neutral and ground from the generator inputs are tied, via bus bars, directly to the neutral and ground bus bars in the circuit breaker box. How exactly does this switch the neutral?

I am trying to do this safely, but do not see a downside or unsafe condition possible with the interlock kit related to the neutral. Convince me otherwise.

Okay, cant attach the pdf because if size...here it is:

http://www.gen-tran.com/assets/pdfs/MTSinstall.pdf
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Old 07-17-2012, 08:43 AM   #2
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Default Re: Generators, Transfer switches and "switched neutral"

I use an interlock on my panel for a generator. I called my local building inspection department to verify that it would meet code and it does. The thing is that all jurisdictions may not allow an interlock device so you should call your local electrical inspector to see if you can use one.
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Old 07-17-2012, 09:25 AM   #3
EOC_Jason
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Default Re: Generators, Transfer switches and "switched neutral"

I think because some generators have a floating neutral or maybe some codes require complete isolation perhaps for accidental back-feed reasons?

But to answer your question, the neutral doesn't have to be switched in the breaker panel.


During Hurricane Ike a few years ago, we were without power for over two weeks. Well, after a while you run out of things to do, and one of the things on the overdue to-do list was to re-wire the main breaker panel. The old panel didn't have a main, which was one major reason we wanted to swap it out.

Of course at the same time we also replaced the meter can and wiring between the can and the panel... Talk about sweating bullets hoping the power wouldn't come back on! (It was another week before they trucks even started showing up in our neighborhood!)

Long story short, at that time we put in a disconnect box that runs to the main panel. Also coming out of the disconnect is a long SO cord with a plug that goes into the 220V on our generator. So basically all we have to do it throw the main off, then throw the breaker for the disconnect, fire up the generator, and throw the disconnect and the panel gets power. I know technically you should have an Interlock kit, but since there are only two people that would be messing with the breaker panel, it's fine for our use. If we ever sell the house it's easy enough to remove the cable for the generator and then they have a plain old disconnect box for whatever use.
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Old 07-17-2012, 11:22 AM   #4
Highbeam
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Default Re: Generators, Transfer switches and "switched neutral"

You don't worry about the neutral. Just don't even think about it. Follow the installation instructions of the UL and NEC approved transfer devices or interlock kits and you'll be fine.

The neutral can be considered an extra ground for most things.

What's really confusing is the whole floating vs. tied neutral at your genset. Should you be grounding the genset to its own rod. What about when powering an RV trailer with no ground rod anywhere and rubber tires?

You can analyze these things to death if you want or you can just follow the directions.
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Old 07-17-2012, 11:31 AM   #5
aandpdan
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Default Re: Generators, Transfer switches and "switched neutral"

Quote:
Originally Posted by east_tn_emc View Post
There has been a lot of debate on the internet and on here about the use of the interlock kits and the neutral feed...and how transfer switches will "switch" the neutral, but the interlock kits do not switch the neutral.utility company (as confirmed from a retired utility company lineman I know).
Not all transfer switches switch the neutral.
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Old 07-17-2012, 12:03 PM   #6
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Default Re: Generators, Transfer switches and "switched neutral"

Basically if the genset has the neutral tied to the ground (EGC), switch the neutral. If the genset neutral is floating (not tied to EGC), neutral is not switched. In all cases, follow the rules of the AHJ.
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Old 07-17-2012, 07:37 PM   #7
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Default Re: Generators, Transfer switches and "switched neutral"

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Originally Posted by Highbeam View Post
You don't worry about the neutral. Just don't even think about it. Follow the installation instructions of the UL and NEC approved transfer devices or interlock kits and you'll be fine.

The neutral can be considered an extra ground for most things.

What's really confusing is the whole floating vs. tied neutral at your genset. Should you be grounding the genset to its own rod. What about when powering an RV trailer with no ground rod anywhere and rubber tires?

You can analyze these things to death if you want or you can just follow the directions.
this is the sort of advice that gets people killed. the neutral is most certainly not an 'extra ground'
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Old 07-17-2012, 07:49 PM   #8
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Default Re: Generators, Transfer switches and "switched neutral"

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this is the sort of advice that gets people killed. the neutral is most certainly not an 'extra ground'
Ditto - what he said. For sure!
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Old 07-18-2012, 11:34 AM   #9
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Default Re: Generators, Transfer switches and "switched neutral"

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Originally Posted by Alchymist View Post
Ditto - what he said. For sure!
For the intents of this thread, why not? How about some helpful information.
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Old 07-18-2012, 11:47 AM   #10
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Default Re: Generators, Transfer switches and "switched neutral"

Well, pretend the power line got cut outside for some reason. You have your generator running but the neutral is still tied to the panel & thus to those cut lines outside. There is the ever so slight chance that someone could get a nice shock (or worse)...

However, in most systems I've seen (and as you said in your interlock setup), the neutral is not switched at all. For safety's sake make sure you have your grounds.
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Old 07-18-2012, 11:55 AM   #11
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Default Re: Generators, Transfer switches and "switched neutral"

Quote:
Originally Posted by mrb View Post
this is the sort of advice that gets people killed. the neutral is most certainly not an 'extra ground'
Quote:
Originally Posted by Alchymist View Post
Ditto - what he said. For sure!
Quote:
Originally Posted by Highbeam View Post
For the intents of this thread, why not? How about some helpful information.
The neutral is a current carrying conductor for all 120 volt circuits, and current imbalance on 240 volt circuits. The EGC is for safety, and should never carry current except in the case of a fault condition. For isolation from the POCO lines the rule about neutral switching I posted above comes into play. Another rule of thumb - If you don't know what you're doing consult a knowledgeable electrician - the life you save may be your own! Helpful enough?
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Old 07-18-2012, 12:11 PM   #12
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Default Re: Generators, Transfer switches and "switched neutral"

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Originally Posted by Alchymist View Post
The neutral is a current carrying conductor for all 120 volt circuits, and current imbalance on 240 volt circuits. The EGC is for safety, and should never carry current except in the case of a fault condition. For isolation from the POCO lines the rule about neutral switching I posted above comes into play. Another rule of thumb - If you don't know what you're doing consult a knowledgeable electrician - the life you save may be your own! Helpful enough?
Yes, that's much better. I'm assuming you mean ground when you say EGC. What exactly does EGC stand for?

Of course I know the neutral carries current, terminology is the problem. Been working on lots of 12 volt DC stuff in RVs lately where the ground wire carries current as well.

My genset has a floating neutral as most all portable gensets do. There is also a provided ground lug in case you want to ground the chassis I guess.
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Old 07-18-2012, 12:16 PM   #13
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Default Re: Generators, Transfer switches and "switched neutral"

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I'm assuming you mean ground when you say EGC. What exactly does EGC stand for?

My genset has a floating neutral as most all portable gensets do. There is also a provided ground lug in case you want to ground the chassis I guess.
EGC - Equipment Grounding Conductor - the proper terminology for the "third wire ground".

And no, not all gensets have a floating neutral. Otherwise transfer switches that switch the neutral would not be needed.

PS: We haven't even touched on "separately derived circuits", but I'm not opening that can of worms; not really relevant to consumer products.
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Old 07-18-2012, 11:27 PM   #14
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Default Re: Generators, Transfer switches and "switched neutral"

Switching the neutral is not necessary in MOST residential applications. If you don't know what you need, consult an electrician. You need a single neutral grounding point.

Ground Fault Protection is pretty important... make sure you don't create a path that can bypass it.
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Old 07-19-2012, 10:13 AM   #15
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Default Re: Generators, Transfer switches and "switched neutral"

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Should you be grounding the genset to its own rod.
When it's powering my house - no. I had my house wired by electricians and my generator plugs into the house with a four wire cord. The ground for the whole house is in the transfer panel.
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