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Inverter Question.


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I know there are some pretty savvy electricians here. This question is exceeding my knowledge.....

 

I'm renovating a tiny house on wheels for friends with a tight budget.

 

120 power sources are inverter and shore power. Light load, no Air, Micro etc.

 

So, we need an either/or selector switch. Normally I would use a Blue Sea rotary switch, but those are $100 plus. Digging around the web I found someone who used a simple three way switch on the load side to create the either/or, and tied the three neutrals (shore/inverter/house) together.....

 

Is this OK? Intuitively I'm scratching my head whether it's a good idea to (if plugged into shore power and working off the inverter) potentially have the inverter's neutral tied to the grid neutral, but I can't say why that would be a bad idea..........

 

Of course if the shore polarity is accidentally reversed you are sending the hot side into the inverter neutral side.......hmmmm.

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Great questions Oscar, here's my take, but remember, I've been retired as an AC Distribution Design Engineer a longggggggggg time and am rusty so no warranty, consult the NEC or trained more current professional electricians or engineers or your local utility or authority is my best free advice.

 

 

WARNING,,,,,,,,,,,,,,,,,,DONT EVEN THINK OF CREATING OR ALLOWING THE POSSIBILTY TO REVERSE YOUR POLARITY. NO NO NO

 

DONT EVEN THINK OF EVER CONNECTING THE INVERTER OUTPUT TO THE UTILITY GRID (could kill someone)

 

MY (and Utility and NEC) PREFERENCE WOULD INCLUDE A SAFETY LOCK OUT SUCH THAT A UTILITY MAIN DISCONNECT

MUST BE OFF AND REMAIN OFF IF THE ALTERNATE SOURCE IS ON That arrangement, however, is typically used in the homes main

distribution panel (a simple cheap slider so the main must be off before the backfeed breaker is switched on) or in a stand alone actual

Transfer Switch. The reason is so there's no possible way the alternate source can backfeed voltage into the utility grid. Utility main must be

off and open before alternate source can be used

 

 

1) It is permissible to use a single pole Transfer Switch for 120 VAC Single Phase (you ONLY switch the Hot Ungrounded Conductor) in which you

 

a) DO NOT SWITCH THE NEUTRAL.

 

B) The alternate utility (genset or Inverter) isn't a separately derived source.

 

 

In such a configuration, you would indeed SPLICE AND BOND THE UTILITY AND ALTERNATE GROUNDED CONDUCTORS (Neutrals) TOGETHER

 

The Transfer switch could be as simple as a SINGLE POLE DOUBLE THROW 120 volt 15/20 amp rated switch. I have seen (but wouldn't use) household 3 way light switches used in such an application as they are SINGLE POLE DOUBLE THROW DEVICES and might be rated at 15 or 20 amps. I have also seen (what I would use) heavy duty better quality what are called double throw 30 amp "Safety Switches" that have a lockable lever and are in a metal NEMA 1 Enclosure like you may see powering a motor or an AC unit etc. So long as 120 volt and 15 or 20 or even 30 amp Single Pole Double Throw they will "work".

 

The Transfer switches two THROWS/INPUTS are the Utility and Alternate Source/Inverter,,,,,,,,,, while its single POLE/OUTPUT is what powers your unit. You simply select from EITHER Uitlity or Alternate as the input. Remember you ONLY SWITCH THE HOT UNGROUNDED CONDUCTORS and the Grounded Conductor Neutrals bond together. Don't forget the Equipment Grounding Conductor (Green/Bare) is bonded to a metallic switch enclosure, handy box etc

 

DONT GET POLARITY WRONG

 

CLOSING a simple cheap 15/20 amp 120 volt single pole double throw switch can "work" where you switch the Hots and bond the Neutrals

 

NOTE there are all sorts of ways to do this where the Inverter back feeds the homes panel via a backfeed breaker and a main breaker lock out device is used,,,,,,,,,,,,,a True Transfer Switch is used,,,,,,,,,,,,,,,,,or you can jury rig a 3 way light switch if you really want to go cheap,,,,,,,,,,,,,,and breakers and overcurrent and panel or panels arrangements I haven't even addressed MANY MANY WAYS TO DO THIS

 

I know what's safe and proper and best and NEC approved (I still remember the engineering basics) and also what will "work" but its your money and your risk and your choice. As long as you don't backfeed the utility and switch the Hots and bond the Neutrals and don't mix polarity you can get by and it will "work"

 

DO NOT READ INTO THIS I RECOMMEND THE CHEAPEST METHOD I'm ONLY saying what will "work" and what I recall as NEC approved with no warranty

 

John T

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Thanks. It's a very low load situation..... a few amps at most. I understand about wiring things correctly, and I can make sure I never back power the grid. The only risk I see is if you plug the (15A) shore power in an improperly wired (ie reversed) outlet, you are sending hot into the neutral side of the inverter...... I guess it could fry it.

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A double-throw three pole switch would be one choice for this, complete isolation between the shore and inverter power with no chance of feedback into the grid or toasting the inverter output, expensive though. Switching the third pole, the ground is unlikely to gain you anything.

 

A double-throw two pole switch would be my choice as it won't cause problems in any situation and prevents any dangerous feedback to the shore power or to the inverter output. Finding one rated for 30 amps is going to be the only issue.

 

Single pole options aren't anything I'd recommend, too much chance of something going wrong someday to be worth saving the few bucks difference.

 

 

If you are willing to pass up on the switch part and truly want low budget then two female outlets, one for the shore power and one for the inverter along with a male plug on the RV breaker box may be the way to go. Just plug the breaker box into the power outlet you want to use. You can even expand this by adding another female outlet if you want to use generator power. I've seen several RVs wired up this way and aside from the hassle of manually moving the plug you have no other downside from the other options.

 

 

With any switched setup you do still have the problem of possible overloads unless you add a sub-panel for just the loads you want on inverter power. If you decide to do a sub panel then just put the male plug on it instead of the main breaker box. With a sub-panel you might also be able to save a few bucks by using lighter duty plugs, sockets and cord. I lean this way myself, I don't want to save a few bucks up front then have the air conditioner and water heater kick on when connected to the inverter, that is not good and possibly expensive!

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Oscar, one other thing. You say its a "tiny house on wheels" A "homes" main distribution panel has the Grounded Conductor (Neutral) bonded to the Equipment Ground Buss while an "RV" DOES NOT, their Equipment Ground and Neutral are kept separate and isolated.

 

They sell those cheap easy testers that indicate if the ground and polarities are correct ya know.

 

A SPDT Transfer Switch (even if a cheap 3 way light switch) doesn't let the Utility and Alternate hot wires feed or back feed each other, but true it cant fix incorrect mixed polarity.

 

If you want to switch BOTH Hot and Neutral, then you need a Double Pole Double Throw switch.

 

In a 30 amp RV where a Generator is the alternate source, it has its Neutral bonded to its case/frame, its a Separately Derived Source, and there you switch BOTH the Hot and Neutral with again, NO Neutral to ground bond in the main panel.

 

They make auto transfer switches but they cost more.

 

See how there are so many different ways to do this?? No sense in covering all until we know more

 

Keep safe, don't backfeed, dont cross polarity you may get by

 

John T

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Thanks all. Stanley, I think I'll go with the outlet idea. In fact the inverter is one with outlets in it..... so a "shore power outlet" in the electrical cabinet next to the inverter and just take "the house" and plug it in either the inverter or the shore power outlet..... About the same cost as a box with a three way switch.

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Congrats Oscar, among all the different methods available, that ones pretty cheap.

 

BE ADVISED If the "tiny house on wheels" is to be treated as an RV and powered by your typical RV pedestal outlets, the AC distribution in the "tiny house" should NOT have the Grounded Conductor (Neutral) bonded to the Equipment Ground (The green or bare wire).

 

When you plug the "tiny house" into EITHER the Inverter or Utility, you are effectively plugging/switching/transferring BOTH the Hot and Neutral, i.e. kind of like a Double Pole Double Throw Transfer method. Had you used a Single Pole Double Throw transfer method, then it would have been like your original post where the Neutrals bonded together but were NOT switched. NOTE: The iron frame of the "tiny house" should have a bond to the Equipment Ground.

 

That being the case, the Inverter (or if you had a Generator) is to be wired and configured as a "Separately Derived Source" meaning its Neutral is to be bonded to its equipment Grounding Conductor. That means if you placed an ohm meter or continuity tester into the Neutral and Ground terminals on its receptacle outlet, there would be like 0 ohms and closed circuit continuity. Similar, if the Inverter is powered up and ON. a voltmeter with one lead in the Hot should register 120 VAC if the other lead is plugged into the Neutral orrrrrrrr the Ground. Also, the Inverter may have a grounding lug/terminal which should be wired to the iron frame of the "tiny house/RV"

 

Now that you decided on the method, the above is how to wire it. Hope this helps

 

John T

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John, I think I follow. Shore power would be a regular outlet in a garage. I'm planning on wiring it just like a S&B house, ie ground and neutral separated. It's a (wood) stick framed house on a steel trailer, so there is no steel frame to the house.

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Oscar, I guess it wont do any good to bond to a wooden frame then lol YES I believe you have it down quite well now.

 

YES, A trailer or RV, unlike a typical permanent residential electrical service, does NOT have a Neutral to Ground bond at the panel.

 

Regardless of a small say 15 amp or less load, if I was using a rubber type cord to plug up to shore power, I wouldn't use less then 12 Gauge 3 Conductor flexible cord with a quality (Hubbell etc. ) 15 amp 120 volt NEMA 5-15P Male Plug.

 

Good thought by Stanley for what is an inexpensive yet still safe and proper 2 pole (Hot and Neutral) transfer method. Much cheaper then the transfer switch method you first asked about. Many RV's I've owend used the same basic method. The Generator was configured as a Separately Derived Source having its Neutral bonded to its iron frame which went to a 30 amp 120 volt TT 30 R Receptacle which accepted the 30 amp shore power cord if no utility was available, IE 2 pole transfer, Hot and Neutral.

 

Congrats and best wishes

 

John T

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John, I think I follow. Shore power would be a regular outlet in a garage. I'm planning on wiring it just like a S&B house, ie ground and neutral separated. It's a (wood) stick framed house on a steel trailer, so there is no steel frame to the house.

Hmmm... Wood framed on a steel framed trailer. Call it a tiny house - it still is a trailer (portable, movable). Wire it like a trailer which it is.

 

And, I would wire for 30 amps with a breaker panel - hot water? electric (10 amps)? or straight propane?

 

Marinco 30 amp twist with a short cord - http://www.hodgesmarine.com/Marinco-Pigtail-Adapter-Plus-30a-Locking-To-15a-p/mac104spp.htm?gclid=Cj0KEQjw4fCqBRDM1ZKhk5jfo6IBEiQAZQ97OHVX6xSbHY6cKZJuc4EMRKEv7PgpxvXTUJNsFVDfpgoaArzk8P8HAQ&click=19 - with the inverter outlet 6" away in a rain cover outdoor outlet. A lot more options down the road!

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A few years ago I was trying to address a similar AC source switching issue with our TT. In my case I wanted to add a second wired-in 30A cord to the trailer. The usual post cord is still used to connect to a post when grid power is available. I purchased a 30A "RV extension cord" from WalMart and routed it permanently under the trailer to the front of the trailer where I hang a 6' loop of it on the tongue jack when not in use. Then if we stop for an overnight boondock or even a lunch stop and want to use the generator it's much more convenient to plug that cord into the generator which sets at the back of the truck bed. My switch problem was how to implement the permanent wiring of both 30A cords without having either of them back-feed the other cord and make it 'hot'.

 

After looking at all the options that I could think of I settled on using a pair of 30A Leviton DPST switches that I found at HomeDepot for about $12 each. I mechanically connected the two switches so they would switch simultaneously and electrically interconnected them so one would be in the OFF position when the other is in the ON position. The two mechanically interconnected switches operate as a single break-before-make DPDT switch. This ended up being a very convienient implementation with the switch(s) mounted in an indoor panel at my electrical distribution center. It was also much less expensive than any of the other approaches I evaluated.

 

---ron

 

 

 

post-6894-0-07533800-1432147222_thumb.jpg

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I mechanically connected the two switches

 

Thought about that too...... Then decided it would look a little too home made.....

 

Bill B.... no hot water..... 2cuF fridge, ceiling fan, two LED lighting fixtures and some phone charging. That's it.

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You can buy invertors that auto switch when the shore power is interupted. i have one. http://www.anchorexpress.com/promariner-pro-trusine-400-power-inverter-79400 If there is room to mount it behind the convertor as is my case, you can simply take the runs you want to run from the invertor and feed them through both it and the breakers. I did this with the run to my 110v TV, the 2 bedside outlets (for my CPAP) and the plug I use for a computer. On this invertor one of the outlets is ground fault , so i later intend to run my bath plug from it as well. The key is to have a heavy enough lead coming from your battery and you should have a tripable circuit breaker in that that trips before the current it can carry exceeds the limit. In my case there was a nice hefty one leading to the convertor already so i was able to tap onto that, I still put a circuit breaker in the loop though. I loaded the invertor up as high as i could and made sure the wire was not getting warm. That autoreset breaker on the back wall, I have now repalced with a manual reset. This is a 400 watt inverter. A 2000 watt woudl require a #4 wire back to the battery. I got away with #6. This inverter has a remote control panel you can see it the last photo. The light switch is to control my outdoor fridge and is not part of it. You can also see my surgeguard mounted in there in pic #2. lots of room back there, airflow, & easy access from a drawer above, I was lucky.

 

inverter2.jpg

 

inverter3.jpg

 

inverter1.jpg

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Be aware that most of the inexpensive stand-alone inverters (the ones you'll find in Harbor Freight, truck stops, etc) with only standard outlets and no way to attach permanent wiring, do not follow the source grounding rule that John T outlined.

 

They don't have hot and neutral lines, instead they put voltage on both sides of the plug. One pin is going +60 volts with respect to ground while the other is going -60 volts to ground so there's 120 volts between them.

 

This is fine in normal use, as long as you keep the neutral separate from ground. The problem is the ground wires are tied to the chassis, as is the battery minus cable. If you tie neutral and ground together you wind up shorting one side of the inverter to the battery negative cable.

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Good Morning Lou,

 

Thanks for posting that information regarding Inverter wiring configuration. I don't have any schematics or diagrams of all the different brands and types of Inverters to confirm how each might be configured. Many I've seen do indeed have that Grounding lug/terminal on their case/frame intended, I assume, to be bonded to the RV frame ground which is also the DC - ground.

 

When the alternate energy source is a genset instead of an Inverter, I (and the many RV's I've owned) would wire and configure it as a "Separately Derived Source" and have its Neutral NOT floating (NOTE some small gensets may or may not come that way from the factory you know, some may have a floating neutral), but instead bonded to the case/frame in which case the Transfer device must switch BOTH the Hot and the Neutral either by use of a 2 pole transfer switch (talkin 30 amp here) or by plugging/choosing the shore power cord into the utility pedestal outlet or the gensets outlet. Again, I would have to see an individual Inverters wiring schematic to confirm exactly how it is configured.

 

As you likely know but some may not, the RV's AC Distribution Panel, unlike a homes panel, has its Neutral and Equipment Grounding Conductor separate and isolated. IE NO NEUTRAL TO GROUND BOND THERE. The "Single Point Grounding" Neutral to Ground Bond is already in place in the RV parks distribution scheme.

 

Thanks again Lou, fun chatting with you.

 

John T

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  • 2 weeks later...

Lou, thanks. That was good information. As I stated above, this is not an RV it's a little house on a trailer, there is no DC other than to the inverter. There is no tie in to the frame, the wiring is strictly 120V and wired as a residential system. So, plugged into the house it's just an extension of the garage outlet. Plugged into the inverter there is no path to ground. I am now definitely sticking with either the dual pole switch, or the plug and outlet method, and based on cost it will be the latter for now.

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  • 5 months later...

Old post, but just went through this. An auto transfer switch is the easiest (and I'd argue safest) thing to do. A Parrallax ATS 301 cost only $73 from Amazon (not as pictured): I just repalced my orig (differnet amke) one (shore vs generator) and added a second (main panel feed (shore or generator) vs inverter to sub panel). Very easy to wire up (each lug is labled so as to identify which wire goes where), and it's safe. Really, for $73, you can't go wrong. There's my 2 cents anyway.

 

Here's the seocnd one (feeds the sub panel from main panel or inverter) (just roughed in as I hadn't attached the 10/2 wire to the main or sub when pic was taken).

 

upload_-1.jpg

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