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Open Roads Forum  >  Tech Issues

 > Generator interlock kit hookup-electricans?

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Bit Bucket

Brookings, Oregon

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Posted: 07/20/11 11:41am Link  |  Print  |  Notify Moderator

pkunk wrote:

The 2 electricians local to me haven't a clue when it comes to this situation with the generator or I wouldn't have asked here. They sell the devise (generator interlock) and it's UL approved so I was just trying to find out if..... there were any problems with the hook up. Obviously, UL hasn't covered all the bases. I do not want a manual or automatic transfer switch.
I appreciate all your time and even though you have mostly beat around the bush some, I'm inclined to try it, monitor what's happening, and if all goes well keep it as an emergency hookup.


The reason the more knowledgable seem to have "beat around the bush" is because that is exactly what we have done. We do not know all of the details of your situation or what your technical level is. It takes a lot of words and drawings and code references to explain exactly how and why things are done the way they are.

Most responsible professionals will only feel comfortable passing on exact answers for exact questions so as not to have something applied out of context. You seem to be willing to do things "almost right", meaning you are willing to take some chances in not having things exactly up to code. That is fine, it is your choice to do with your property what you see fit (as long as it only affects you) and I don't knock anybody for it when it's none of my business.

To explain a few of the aspects: Gen only being 120 volts - if it has to be this way then establish a procedure that ensures (1) before switching over your interlock breaker, turn off all of the 220 breakers in your house main. That will ensure you don't accidentally try to run a 220 volt appliance on 120 volts, which would burn up motors or worse.

Neutral sharing - it is common practice for most electricians wiring houses to save a little money in copper by having alternate phases of 120 volts sharing the same neutral. This works because alternate phases currents cancel each other out (let's leave out the why, it takes too long). If you unknowingly power up two of these such circuits that share a neutral with the same phase of power (which you will do in your situation) then the neutrals will no longer be cancelling but will be additive. Maybe this will burn your house down, maybe it won't, depends on how those circuits get loaded. You may operate that way for 100 years safely, maybe it will get you in the first 10 minutes, who knows?

Your decision not to use a transfer switch - the mandatory transfer switch used when connecting two seperate sources of power in a residence ensures that the mandatory neutral bonding that has to be at the source of power and only there is done correctly when switching between the two sources. Your switching of the circuit breakers doesn't accomplish this. This rule exists so that a situation doesn't exist where normal current flow that is supposed to going through the neutral wire doesn't end up also going through your ground wires. Given the right circumstances, this could easily create a shock hazard. Again, maybe nothing would ever happen, then again it could kill somebody within an hour, who knows?

There is much more to it than this, that is why sometimes we just say, you should find a qualified electrician to help you out. Not because we don't know what we are talking about or are blowing smoke at you, but because we care about you and your safety and do not want to unwittenly advise you into a bad situation.

Broccoli1

Los Angeles

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Posted: 07/20/11 11:46am Link  |  Print  |  Notify Moderator

pkunk wrote:

Broccoli1 wrote:

Fair enough.

A shared Neutral only carries the difference between the 2 120v Hot legs in a normal setting because the Hot legs are on each half of the panel.

Example:
Circuit A = 10 amps
Circuit B = 15 amps
Load on shared neutral = 5amps

using the Onan 120v ONLY genset
Circuit A = 10amps
Circuit B= 15amps

LOAD on Shared Neutral = 25amps

NO Circuit protection on Neutrals so there is no way to shut off the overloaded neutral

OK, I'm still not clear of the dangers. As long as I'm only powering my home panel, no one is in the MH, the 10g cord is intact and not in a puddle, what harm is there in having potential in the neutral?



It is not the 10g Power cord it is the 14g or 12g wiring INSIDE the house that the shared Neutral circuit uses since under normal circumstances that neutral wire will only carry the difference in the Hot Leg amps.

Of course if you never ever use those circuits in the house nothing really to worry about and in reality you probably won't BUT it is possible.

The Type of shared circuit is usually in the Kitchen where a Duplex receptacle has the Top and Bottom outlet on each half of the Bus bar's power but share the Neutral.

Turn on the Coffee maker and then the Toaster and that would be one scenario where the neutral would definitely be overloaded.

make sense?


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Bit Bucket

Brookings, Oregon

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Posted: 07/20/11 11:49am Link  |  Print  |  Notify Moderator

pkunk wrote:

Broccoli1 wrote:

Fair enough.

A shared Neutral only carries the difference between the 2 120v Hot legs in a normal setting because the Hot legs are on each half of the panel.

Example:
Circuit A = 10 amps
Circuit B = 15 amps
Load on shared neutral = 5amps

using the Onan 120v ONLY genset
Circuit A = 10amps
Circuit B= 15amps

LOAD on Shared Neutral = 25amps

NO Circuit protection on Neutrals so there is no way to shut off the overloaded neutral

OK, I'm still not clear of the dangers. As long as I'm only powering my home panel, no one is in the MH, the 10g cord is intact and not in a puddle, what harm is there in having potential in the neutral?



He is talking about the neutral in your house. In that potential scenario, you have more current in the neutral wire than it can handle, therefore it will melt in two. When it melts in two (probably somewhere deep in your walls) it may or may not cause your house to burn down.

pkunk

Questa, NM

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Posted: 07/20/11 12:08pm Link  |  Print  |  Notify Moderator

I would only be using this setup in the event of a power failure in the evening, before bedtime, and only for the use of florescent lights in the occupied rooms, maybe the fridge & TV. I would estimate no more than 15A total. Most of the wiring in the house is 12g as I used what I had on hand from my business. There are no split duplexes in the house. In a trial, if there were a problem, would I be able to feel hot neutral wires in the main panel?


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Bit Bucket

Brookings, Oregon

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Posted: 07/20/11 12:44pm Link  |  Print  |  Notify Moderator

pkunk wrote:

I would only be using this setup in the event of a power failure in the evening, before bedtime, and only for the use of florescent lights in the occupied rooms, maybe the fridge & TV. I would estimate no more than 15A total. Most of the wiring in the house is 12g as I used what I had on hand from my business. There are no split duplexes in the house. In a trial, if there were a problem, would I be able to feel hot neutral wires in the main panel?


For your trial, use a clamp on ammeter to read the current through any wire directly. You would be looking to make sure that 14 AWG wires have no more than 15 amps on them and that 12 AWG wires have no more than 20 amps on them. The amperage dictates the heat and I know of no "hand test method" I would rely on.

As stated by others be very careful, think, be safe. Know that there are problems with what you are trying to do and that myself (and probably others) as a fully qualified and experienced electrician do not endorse what you are doing.

Good luck!
I hope all turns out well...

pkunk

Questa, NM

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Posted: 07/20/11 12:48pm Link  |  Print  |  Notify Moderator

Bit Bucket wrote:

pkunk wrote:

I would only be using this setup in the event of a power failure in the evening, before bedtime, and only for the use of florescent lights in the occupied rooms, maybe the fridge & TV. I would estimate no more than 15A total. Most of the wiring in the house is 12g as I used what I had on hand from my business. There are no split duplexes in the house. In a trial, if there were a problem, would I be able to feel hot neutral wires in the main panel?


For your trial, use a clamp on ammeter to read the current through any wire directly. You would be looking to make sure that 14 AWG wires have no more than 15 amps on them and that 12 AWG wires have no more than 20 amps on them. The amperage dictates the heat and I know of no "hand test method" I would rely on.

As stated by others be very careful, think, be safe. Know that there are problems with what you are trying to do and that myself (and probably others) as a fully qualified and experienced electrician do not endorse what you are doing.

Good luck!
I hope all turns out well...

Thanks! That was the answer I was looking for. I'll hold no one responsible but my self.

Wayne Dohnal

Bend, OR.

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Posted: 07/20/11 01:17pm Link  |  Print  |  Notify Moderator

I keep dishonoring your request about non-electricians and if this post offends you I will delete it without any debate. If you're really serious about that 15 amp total, just use 15 amp breakers for the generator input and run a single power line from the RV. That gets rid of the shared neutral issue, and the parallel conductor issue (from the RV) which hasn't received much discussion. The only issue left then is the double neutral-ground bond, for which I have an opinion but would not want to influence anybody else with it.

Or, get a 2000 watt or smaller inverter generator and use that. The bonding problem would be gone, and the neutral overload problem would be gone. I have a 50 amp 240 volt generator input, and made up a cord to use the eu2000i with it. It has the standard 120 volt plug on one end, and the 50 amp twistlock on the other end, with both hots jumpered together. When boondocking, I use the Honda for battery charging even though I have a built-in Onan which is uncomfortably (actually obnoxiously) loud inside the RV. Your bigger Onan is probably less bad in this regard. And on our small acreage, I often find it easier to carry the generator to where I'm working than stringing a couple hundred feet of extension cord. Like most other tools, once you have it you find more uses for it.


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pkunk

Questa, NM

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Posted: 07/20/11 01:35pm Link  |  Print  |  Notify Moderator

Wayne Dohnal wrote:

I keep dishonoring your request about non-electricians and if this post offends you I will delete it without any debate. If you're really serious about that 15 amp total, just use 15 amp breakers for the generator input and run a single power line from the RV. That gets rid of the shared neutral issue, and the parallel conductor issue (from the RV) which hasn't received much discussion. The only issue left then is the double neutral-ground bond, for which I have an opinion but would not want to influence anybody else with it.

Or, get a 2000 watt or smaller inverter generator and use that. The bonding problem would be gone, and the neutral overload problem would be gone. I have a 50 amp 240 volt generator input, and made up a cord to use the eu2000i with it. It has the standard 120 volt plug on one end, and the 50 amp twistlock on the other end, with both hots jumpered together. When boondocking, I use the Honda for battery charging even though I have a built-in Onan which is uncomfortably (actually obnoxiously) loud inside the RV. Your bigger Onan is probably less bad in this regard. And on our small acreage, I often find it easier to carry the generator to where I'm working than stringing a couple hundred feet of extension cord. Like most other tools, once you have it you find more uses for it.

You're saying that if I run just one cord (say from the 30A on MH), split it to 2 15A breakers feeding both sides of the panel (like a double pole w/ no joining pin), I'll eliminate the shared neutral issue? I'll still monitor the amps.
My Onan is fairly quiet and I only run it when the batteries need charging or sometimes when we desire air. A Honda would be a nice thing to have but at this point in time an unnecessary expense. Around the property here I use a 5000W inverter hardwired into the dual batteries in my Dodge.

Wayne Dohnal

Bend, OR.

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Posted: 07/20/11 03:44pm Link  |  Print  |  Notify Moderator

pkunk wrote:

You're saying that if I run just one cord (say from the 30A on MH), split it to 2 15A breakers feeding both sides of the panel (like a double pole w/ no joining pin), I'll eliminate the shared neutral issue?
That's what I said, and now that you rephrased it I see that it's dead wrong. This would still let you draw a worst case 30 amps on a shared neutral. If you came off the 20 amp genny breaker and didn't have any shared neutrals smaller than 12 ga. I think you'd be ok, but I have to admit that I just proved to myself (again) how easy it is to overlook some detail when doing design on the fly. I apologize for the bad info. I think the bottom line is that a 120 volt genset isn't a good match for a 240 volt system if it has any shared neutrals. Makes me think that it's a no-no to have a shared neutral in a 50 amp RV, since they are frequently used with a 120 volt source via an adapter.

Edit: Or how about powering just one side of the panel? (Dang, I need to quit thinking about this!)

* This post was edited 07/20/11 03:52pm by Wayne Dohnal *

pkunk

Questa, NM

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Posted: 07/20/11 04:37pm Link  |  Print  |  Notify Moderator

That may be what I'll have to do. Now to figure how to have the necessary lighting circuits on just one side.

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