Connecting an inverter to a transfer switch

LandKurt

I want to do something the instruction manual for my inverter says not to. I want to hook it up to a transfer switch to power some circuits so I don't have to use extension cords when I need to run off batteries when the utility power is down.

I have a Samlex SSW-1000-12A inverter whose manual says: "WARNING: Do not connect directly to AC distribution wiring. This inverter is NOT grid interactive." That's not a surprise, I know better than to try a plug it into a live circuit, thus the transfer switch.

It also says: "Do not connect the power inverter to any AC load circuit in which the Neutral conductor is connected to Ground (Earth) or to the Negative DC (battery) source." That's more of a problem since my house wiring bonds neutral and ground, of course.

I'm guessing this is a fairly common problem. Is there a common solution other than "don't do that"?

Hooking my inverter to a transfer switch isn't as odd as it may sound. The transfer switch is an APC UTS6H, an automatic transfer switch with connections for both generator and battery backed UPS. This model is a varient for Honda that accepts a L5-30 120V connection for a Honda EU generator, I have an EU 3000is. The UPS connection is probably intended for a standard APC UPS, but I figure my inverter with 380AH of battery behind it will give me a lot more time before I need to run the generator and recharge the batteries.

I first hooked the switch to a standard 750VA UPS, which worked just fine. I then tried plugging it into my inverter. I immediately heard a pop and no power from the inverter. The GFCI outlet on the inverter tripped. The GFCI tripped even when hooked up through a switched off power switch since the neutral and ground weren't switched off. I finally used a two prong extension cord without a ground wire to connect the inverter to the switch. That works without the inverter complaining. It works, but is it a good way to handle the problem?

Other than the problem with the GFCI on the inverter I'm pleased with the setup. If power fails the switch automatically transfers the load from up to 5 circuits to the inverter. When I get out the generator and fire it up the loads are transferred to it and I can recharge the batteries. I can now run my gas furnace when the generator is on, though that's too much for the inverter. The switch is configured to only run the furnace circuit off the generator. Depending on how long I want the batteries to last I can configure the switch to just run the fridge circuit from the inverter or to include the TV room, computer room, and bedroom.

I'm just worried I've violated some important safety rule by hooking my inverter to the switch without a ground wire. I also haven't gotten around to making a proper ground for the battery system. Possibly once I do that I'll have the same problem since the ground on the inverter will then be connected to the same ground as the house wiring. Do I need to remove the GFCI outlets from the inverter or defeat it in some way?

mike95490

Re: Connecting an inverter to a transfer switch

Most Mod-sine inverters (I don't know what yours is) have an output leg, tied internally to the battery, 'cause it's cheaper to do that way. And if you connect it to a household electrical panel, 1 of 2 things can happen, the batteries get raised to 120V on one side, or the magic smoke comes out of the inverter. there may be a 3rd or 4th, beyond my imagination.

Follow the instructions in the manual - safety third you know !

RCinFLA

Re: Connecting an inverter to a transfer switch

The inverter has a push-pull DC-DC boost to produce about 155 vdc for a MSW inverter. This 155vdc then is fed to an H-bridge to chop the DC to +155,0,-155,0 volts for the MSW output.

The issue is the H-bridge reverses the polarity on both neutral and hot lines creating a problem for a grounded neutral situation. Some inverters have optical isolation for the feedback loop for the 155 vdc boost DC-DC converter. This allows the whole secondary HV side of the DC-DC converter to float with respect to primary battery side. These can tolerate a grounded neutral.

If there is not opto isolation for the 155v switcher feedback then, when you ground the neutral, the negative terminal on the battery becomes live with HV pulses with respect to output grounded neutral. Touch a terminal of the battery while standing grounded on floor and you will get zapped.

When an inverter is running asyncronously to the grid it is not a good idea to make an immediate relay switchover. If you have any AC induction motors, like a refrig compressor (which we don't recommend running from a MSW inverter), an immediate switchover may occur when the inverter happens to be out of phase with the grid. To an induction motor this creates a large surge current that can damage the motor. MSW UPS units syncronize their inverter to grid to allow a more immediate crossover.

LandKurt

Re: Connecting an inverter to a transfer switch

The inverter is a pure sine wave 1000 watt unit capable of surges to 2000 watts.

http://www.samlexamerica.com/products/productdescription.asp?ProductsID=4002

I've successfully run my fridge and sump pump on it.

I believe the automatic transfer switch does take a couple of seconds to switch from one power source to another. I certainly noticed a pause a couple times during testing, just not sure there was always a gap. Hopefully the won't be a problem from too quick a cut over.

My goal was to put together a system much better than a standard MSW UPS. I've found those useful but limited in past outages. I ran across the transfer switch after I had the inverter. What sort of specs would I be looking for in an inverter that could act as either a stand alone unit or power household circuits through a transfer switch?

Is it the GFCI outlet on the inverter that is the problem or is it something more basic?

icarus

Re: Connecting an inverter to a transfer switch

Most small inverters have a floating neutral, and if you wire them to a grounded neutral buss, through the transfer switch, you will find the inverter going up in smoke very quickly,,instantly in fact!

I know this for a fact, as I. Ironed up two of them in rapid succession before I figured it out!


So unless you can isolate the neutrals on the circuits that you intend to energize my guess is that you are going to have real trouble doing this. Some small inverters permit a grounded neutral, (Suresine 300 for example) Check the specs on your inverter.

Tony

solarvic

Re: Connecting an inverter to a transfer switch

I just Installed a Magnum MS-AE-4000watt 24 volt inverter. It isn,t advertised as a U)PS inverter but transfers at 16 hundredths of a second with the grid power switched off. My computer doesn.t go down. It doesn,t flicker the lights. Don,t know if you want to go that big but I can say I am very happy with it. I picked up mine used but if I was going to buy new I would get the pae model that suports AC coupling. :Dsolarvic:D

BB.

Re: Connecting an inverter to a transfer switch

Typically, TSW inverters are isolated and MSW inverters are not isolated between battery and AC output.

See word "typically". :roll:

Pick a couple inverters and read the manuals closely to see if they meet your needs (or not).

The one problem with common neutrals in branch circuits is that many homes are wired with Romex (Black/Red/White + ground)... If you use 120/240 VAC split phase, then a 120 VAC load on Black/White will balance the current on the White lead if an equal load is placed on Red/White.

If, instead you use a 120 VAC inverter, then the Black and Red load currents will ad up on the White lead:

• 120/240 VAC split phase; 15 amps on Black and 15 amps on Red (120 VAC) will add up to zero amps on White
• 120 VAC output with 15 amps on Black and another 15 amps on Red will add up to 30 amps on the White return wire--Not good.

So, if your home is wired with 2 hots and a return, you are correct, a 120 VAC only inverter can overheat the white/return cables if A) The inverter is large enough and the loads are on both Black and White, at 50% or greater of rated maximum current.


120/240 VAC split phase power source is (more or less) just a 240 VAC transformer with a Center Tap to provide a "neutral lead" for 120 VAC loads.



-Bill

LandKurt

Re: Connecting an inverter to a transfer switch

icarus wrote: »

Most small inverters have a floating neutral, and if you wire them to a grounded neutral buss, through the transfer switch, you will find the inverter going up in smoke very quickly,,instantly in fact!

I know this for a fact, as I. Ironed up two of them in rapid succession before I figured it out!


So unless you can isolate the neutrals on the circuits that you intend to energize my guess is that you are going to have real trouble doing this. Some small inverters permit a grounded neutral, (Suresine 300 for example) Check the specs on your inverter.

Tony

A floating neutral isn't necessarily a bad thing, right? My Honda EU generator doesn't bond the neutral to ground, so it has a floating neutral as is required for connecting to house wiring with a bonded neutral. The transfer switch manual cautions that the generator shouldn't bond ground and neutral but doesn't mention anything about the UPS connection. They probably assume you'll be using a standard APC UPS but don't mention it as a requirement.

Elsewhere I read that: "The term floating neutral is often used to describe a system where the neutral is not bonded to ground. Others also use it to describe two hot leads and a ground, with no neutral."

That sounds like a 60/120 split phase system with hot and neutral each carrying 60V with respect to ground. Sort of a half voltage version of a standard 240 volt circuit. If that's the way my TSW inverter works I can see I'm going to have trouble.

The good news is that so far I haven't released the blue smoke from my inverter though I have already rashly connected the hot and neutral to my transfer switch. Possibly this only works because my battery negative isn't connected to ground (I'd meant to get around to that). When I test with a voltmeter while there is a moderate load on the inverter I find a 120V AC potential between battery negative and a grounded pipe. That sounds like bad news unless it's some sort of minor induced voltage. Is there anyway to test if this is a meaningful problem?

If my inverter neutral is indeed hot with respect to the battery negative is there anyway to solve that? Would using an isolation transformer between my inverter and the house wiring solve the problem? It would be less expensive for me to buy something like a Tripp-Lite IS1000 http://www.tripplite.com/en/products/model.cfm?txtSeriesID=839&txtModelID=228 than to get a new inverter meant for a hard wired connection like http://www.solar-electric.com/sa2wa12vosiw.html or http://www.solar-electric.com/gtfx2524.html

Bill, I do understand about black/red/white branch circuits with common neutrals. I have a couple of those in my house wiring. I made sure to put only one side of any of those on my 120V transfer switch. I suppose that as the utility power returns the black wire could be powered by utility power and the red wire side by my backup source. The phase between the two sources would be random thus the two currents could add on the return neutral. But the condition would momentary, a few seconds at worse, until the automatic transfer switch noticed the return of utility power and disconnected the backup power and reconnected to line power.

icarus

Re: Connecting an inverter to a transfer switch

It also says: "Do not connect the power inverter to any AC load circuit in which the Neutral conductor is connected to Ground (Earth) or to the Negative DC (battery) source." That's more of a problem since my house wiring bonds neutral and ground, of course.

I don't mean to be overly cheeky, but what part of this don't we get?

Tony

mike95490

Re: Connecting an inverter to a transfer switch

on the inverter I find a 120V AC potential between battery negative and a grounded pipe. That sounds like bad news unless it's some sort of minor induced voltage. Is there anyway to test if this is a meaningful problem?

That's GOING to be a problem, if you had tied your battery - to ground, you would have fried the inverter.
READ THE MANUAL for the inverter, and follow it's recommendations.

LandKurt

Re: Connecting an inverter to a transfer switch

icarus wrote: »

I don't mean to be overly cheeky, but what part of this don't we get?

Fair question. The manual says not to do what I want to do. I'm being upfront about that.

The part I don't get is whether the inverter is completely unsuitable for what I want or whether there is a way to safely do it. Often there is a way around what a manual simplistically tells you not to do. For instance, would an isolation transformer between the inverter and transfer switch solve the problem because it wouldn't act as a direct connection to house ground?

Initially I though it might just be a problem with the GFCI protected outlets on the inverter. An overly protective safety feature that could possibly be safely bypassed for certain uses. It appears to be a deeper problem with the neutral and ground. Those sort of issues aren't easy to understand if the reading I've done is any indication. However, I enjoy reading up on difficult technical problems.

nsaspook

Re: Connecting an inverter to a transfer switch

LandKurt wrote: »

Fair question. The manual says not to do what I want to do. I'm being upfront about that.

The part I don't get is whether the inverter is completely unsuitable for what I want or whether there is a way to safely do it. Often there is a way around what a manual simplistically tells you not to do. For instance, would an isolation transformer between the inverter and transfer switch solve the problem because it wouldn't act as a direct connection to house ground?

Initially I though it might just be a problem with the GFCI protected outlets on the inverter. An overly protective safety feature that could possibly be safely bypassed for certain uses. It appears to be a deeper problem with the neutral and ground. Those sort of issues aren't easy to understand if the reading I've done is any indication. However, I enjoy reading up on difficult technical problems.

The only (safe) way around the problem is "galvanic isolation" meaning an isolation transformer in most cases. I have a 2kva unit on the output of my PROwatt SW 2000 inverter before it connects to a transfer relay on the furnace blower/controller, emergency lighting power circuit. It's not really needed for this inverter but I do have a backup MSW inverter that needs it.
http://en.wikipedia.org/wiki/Galvanic_isolation

RCinFLA

Re: Connecting an inverter to a transfer switch

Stray internal capacitance can cause a high impedance DVM to read AC voltage between battery terminals and ground.

Check battery terminal to ground with a small incandescent night light bulb to see if it lights. If it does then battery side is not isolated and neutral should not be grounded. If it does not light then you should also be able to ground negative side of battery without inverter blowing its fuse.

LandKurt

Re: Connecting an inverter to a transfer switch

I've contacted technical support at Samlex to get the official word on this.

First off they suggested that an SA-2000K model (http://www.solar-electric.com/sa2wa12vosiw.html) would be the recommended solution for what I want to do as it's suited for a hard wired situation. I pointed out that I'm not technically hard wiring the inverter but connecting with standard plugs and cords and pressed for more information. I'm told that an isolation transformer would likely work or I could replace the GFCI outlets which would be a warranty issue.

I'm told the neutral is not hot on the SSW-1000 model like it is on many other inverters. The manual states that the neutral is hot on the 300 and 600 watt models but doesn't say this about the 1000, 1500, and 2000 watt models also covered by the same manual. That may be why it cautions about connecting the neutral and battery negative, it would be a problem on the lower power units with a hot neutral.

So I need to decide whether to buy an isolation transformer at close to the same original cost as the inverter (but still a third the cost of the SA-2000K) or to try replacing the GFCI outlets in the unit and trusting nothing fries when neutral meets ground connected to battery negative.

Maybe I should try RCinFLA's night light test first to see if there is any appreciable current that wants to flow between battery negative and ground. I was afraid to even test that when I thought neutral might be hot, but it's not suppose to be.

BB.

Re: Connecting an inverter to a transfer switch

Just pull the GFI and put away in a safe place you can find later... If the inverter ever fails, just put the GFI back in before sending back for service. Unless they have one of those warranty seals.

By the way, did they say that the "neutral" of the inverter is bonded to frame/green wire ground?

-Bill

LandKurt

Re: Connecting an inverter to a transfer switch

The tech guy said the GFCI outlets were there to satisfy UL requirement and suggested the warranty would be valid if the unit was returned with the GFCI receptacle installed. I didn't notice any warranty seals on the unit, but I haven't looked closely yet.

I didn't ask about what the neutral was bonded to and he didn't say. I just asked if it was hot and he said it wasn't. He did mention that the neutral is hot on the PST series. I believe the manual states that the case is bonded to battery negative, but since the manual covers five different models in the SSW series I'm not sure if that applies to all models.

BB.

Re: Connecting an inverter to a transfer switch

The GFI outlet seems to suggest that the Neutral is bonded to the inverter case for proper GFI operation.

If you cannot lift the "neutral bonding" inside the inverter--it would be a poor candidate for connecting to a bonded electrical panel. Issues with ground loops.

-Bill

LandKurt

Re: Connecting an inverter to a transfer switch

RCinFLA wrote: »

Stray internal capacitance can cause a high impedance DVM to read AC voltage between battery terminals and ground.

Check battery terminal to ground with a small incandescent night light bulb to see if it lights. If it does then battery side is not isolated and neutral should not be grounded. If it does not light then you should also be able to ground negative side of battery without inverter blowing its fuse.

I am using a digital voltmeter. I don't know whether is high impedance or not. Another odd result it gives is measuring a 27 volt AC difference between the battery positive and negative anywhere in the system. I guess that's some sort of stray ripple current from capacitors, right?

When I make the two wire hot and neutral connection between inverter and switch and run a small load I get these measurements in reference to a grounded copper pipe: black battery negative reads 114v AC and -3.4v DC; red battery positive reads 128v AC and +9.2v DC. weird.

I tried the test with a small incandescent night light. I connected one side to battery negative and touched the other side to the ground pipe. Immediately the GFCI breaker on the inverter popped. I noticed no light from the bulb, but that GFCI may have reacted too quickly for the bulb to light. I reset it a bit late and the inverter ran fine again.

That 3.4v DC difference between the ground pipe and battery negative when inverter AC neutral is bound to ground through the transfer switch worries me. Maybe it would be best to use an 1:1 isolation transformer between the inverter and transfer switch. Then the inverter would be free to raise the AC output neutral to whatever it wanted to. The company tech says the neutral isn't hot but something odd is going on.

BB.

Re: Connecting an inverter to a transfer switch

LandKurt wrote: »

I am using a digital voltmeter. I don't know whether is high impedance or not. Another odd result it gives is measuring a 27 volt AC difference between the battery positive and negative anywhere in the system. I guess that's some sort of stray ripple current from capacitors, right?

Digital meters are typically around 1 MOhm to 10 MOhm (yes high impedence).

Depending on the type of meter, your AC readings will differ.

Cheap digital meters will typically read 0 Volts when in AC mode and measuring DC voltages. A capacitor blocks the DC components.

More expensive meters with True RMS reading capabilities will read the "real"
RMS (root mean square) voltage and current... If you measure a DC line and/or DC + AC voltage/current, it will read the actual "energy" available... So a 12 volt DC line should read 12 volts. If there is AC on it, then it will be the sum of the DC plus AC.

It is possible that you have a meter that reads DC * sqrt(2)...

When reading 27 volts AC, what was the DC reading (is this a 12 or 24 volt battery bank)?

If you read 27 VAC on a 12 volt bank, then there is something wrong (probably with the meter).

When I make the two wire hot and neutral connection between inverter and switch and run a small load I get these measurements in reference to a grounded copper pipe: black battery negative reads 114v AC and -3.4v DC; red battery positive reads 128v AC and +9.2v DC. weird.

The DC sounds about right... 9.2+3.4= 12.6 VDC -- Just about right for a pretty fully charged battery with light loads.

The AC reading--hard to tell...

I tried the test with a small incandescent night light. I connected one side to battery negative and touched the other side to the ground pipe. Immediately the GFCI breaker on the inverter popped. I noticed no light from the bulb, but that GFCI may have reacted too quickly for the bulb to light. I reset it a bit late and the inverter ran fine again.

An AC GFI will trip around 5 mAmps (0.005 amps) of current. And do it very quickly. It will turn off the outlet before you can light a filament lamp.

Still does not make complete sense... What was plugged in/connected to the AC outlet? In general, I would not have expected the filament lamp on the battery to earth ground to trip a GFI unless the load was somehow connected to earth itself...

If you connected the lamp from AC hot (or neutral) to inverter case ground, then I would expect the GFI to trip.

That 3.4v DC difference between the ground pipe and battery negative when inverter AC neutral is bound to ground through the transfer switch worries me. Maybe it would be best to use an 1:1 isolation transformer between the inverter and transfer switch. Then the inverter would be free to raise the AC output neutral to whatever it wanted to. The company tech says the neutral isn't hot but something odd is going on.

Isolation transformers cost extra money, add weight, and cost extra power (losses). It would be really nice to figure out what is going on with your inverter.

-Bill

LandKurt

Re: Connecting an inverter to a transfer switch

BB. wrote: »

Still does not make complete sense... What was plugged in/connected to the AC outlet? In general, I would not have expected the filament lamp on the battery to earth ground to trip a GFI unless the load was somehow connected to earth itself...

If you connected the lamp from AC hot (or neutral) to inverter case ground, then I would expect the GFI to trip.

I connected the night light between the battery negative (which I believe is connected to the inverter case) and a ground pipe which should connect to the ground in the service panel which is bonded to neutral which connects to the transfer switch the inverter was plugged into. So in effect I connected the inverter AC out neutral to the case/negative DC input and that tripped the GFCI.

The load plugged into the inverter was a CFL bulb and a small incandescent bulb totaling 63 watts with a power factor of 0.81. Without a load I don't get that AC reading between battery negative and AC neutral/house ground. The transfer switch was also plugged into the inverter but it's drawing no power, just providing a path connecting inverter neutral to house wiring ground.

LandKurt

Re: Connecting an inverter to a transfer switch

I've decided to go the safe route and use a different inverter.

I could have removed the GFCI outlets and replaced them with a standard unprotected socket. When connected that way it should have worked since the neutral isn't suppose to be hot in this model. It probably wouldn't have released the blue smoke from the inverter even with the battery negative grounded - probably.

I just didn't want to risk it. And since the cooling fan was having intermittent problems indicating a bad bearing I needed to return the unit for servicing anyway. So I took Samlex up on an offer to upgrade at a discount to an SA-2000K-124 that's suitable for hardwiring.

Since I'm going from a 1000 watt inverter to a 2000 watt inverter I've asked for the 24 volt version and I'll rewire my 4 6V batteries into a 24V bank. I don't like losing the flexibility of running any 12V compatible device, but it means I won't have to upgrade my main #2 AWG wiring and fuses. Maybe I can get a 24V DC to 12V DC converter to run miscellaneous small 12V loads.

I'll also need a new AC battery charger for recharging from generator in an outage when I want to put more of a load on things than my small solar setup can handle. It seems a shame to have a second smart charger in addition to the TriStar MPPT 60. Is there an inexpensive way to convert AC to DC that the Tristar would accept?

Dill

Re: Connecting an inverter to a transfer switch

funny you post this Kurt, I just got my setup working using a Xantrex Pro Sine inverter and a Reliance transfer switch, all my trials and tribulations http://forum.solar-electric.com/showthread.php?13518-240V-dual-phase-inverter

LandKurt

Re: Connecting an inverter to a transfer switch

Yes, I saw your thread, Dill. Reading it is one of the reasons I though the GFCI outlets might be my only real problem with my inverter. But the odd readings I was getting with my digital multimeter made me reluctant to put it to the test. So I'm upgrading my inverter.

I've seen those Reliance transfer switches around and was thinking of going that way. Then I saw the APC automatic transfer switch on the Honda generator site. It wasn't that much more expensive than the manual transfer switches and had the extra input plug for UPS/inverter. I couldn't resist getting it. The drawback to it is that the microprocessor control must give it more failure modes, so it's likely not as reliable in the long run. I hope it fails with its relays set to line power if the electronics fry. It also uses fuses internally rather than circuit breakers, which is kind of annoying in something that high tech.

Glad to hear your setup is working. I await my new inverter to see if I've finally got a working system that's safely grounded. Assuming I can get that tricky detail right.

vcallaway

Re: Connecting an inverter to a transfer switch

Have not been on in a while but this sounds like what I've done. I used an RV automatic transfer switch. You pass both hot an neutral through the switch (double pole, double throw).

Xantrex makes this little box that also does what you are asking.

LandKurt

Re: Connecting an inverter to a transfer switch

I got my upgrade inverter, a 2000W Samlex SA2000K-124. It plays better with my automatic transfer switch as it's got an option for hardwiring. It doesn't seem to have any problems with grounding the AC neutral.

My only trouble now is that it has a higher idle draw than the previous unit, just under 30 watts. I can't afford to leave it on all the time just in case there is a power failure. My solar setup can't afford 720wh a day just to idle the inverter. That kind of reduces the advantage of the automatic transfer switch if I have to go down and manually turn on the inverter when there is a power failure. I suppose I can turn it on when the weather is bad and an outage is a possibility.

It works pretty well with a decent load on it. I ran my desktop computer off it last night. The computer was drawing 170 watts from the inverter and that was drawing 210 watts from the batteries. So that was roughly 40 watts lost to the inverter. I expected it would be more when I saw the 30 watt draw at no load without the fans even running. The previous unit drew an extra 30 watts as soon as the fans kicked on at about a 20 watt load.

Cariboocoot

Re: Connecting an inverter to a transfer switch

Not sure how that Samlex unit turns on, but it's not too far-fetched to think of a relay that could activate it should the mains power fail (needs to be an isolated circuit that doesn't get fed power by the inverter, obviously).

LandKurt

Re: Connecting an inverter to a transfer switch

Good point. The inverter does have a remote option and the power switch has three positions: on, off, and remote. I'd need the technical spec to find out which pins on the RJ-45 connector control that function. Then I'd just need a device to open or close that connection appropriately when AC power drops at an unprotected outlet.

Jonesz

Re: Connecting an inverter to a transfer switch

icarus wrote: »

Most small inverters have a floating neutral, and if you wire them to a grounded neutral buss, through the transfer switch, you will find the inverter going up in smoke very quickly,,instantly in fact!

I know this for a fact, as I. Ironed up two of them in rapid succession before I figured it out!


So unless you can isolate the neutrals on the circuits that you intend to energize my guess is that you are going to have real trouble doing this. Some small inverters permit a grounded neutral, (Suresine 300 for example) Check the specs on your inverter.

Tony

Tony does the fact that the Suresine permits a grounded neutral mean that I can plug the shorepower cord of my RV directly into the output of the inverter without it faulting? I am looking to buy an inverter that will permit this so that I can power up my stereo and TV circuits at the same time. I don't see the need of a transfer switch in my case if I am careful to switch off my converter and any other circuits such as the MW and water heater. This is so that I can boondock on an infrequent basis. I have a cheap 750 W MSW inverter that immediately faults when I plug my shorepower cord into it.
Thanx
Jonesz

BB.

Re: Connecting an inverter to a transfer switch

[note the below response is correct in itself--but did not accurately reflect the question asked in the above post--so was wrong for Jonesz's question. -Bill]


NO.

Never plug the outputs of two inverters together or into other AC power sources ever--Unless:

• Grid Tied Solar Inverter
• "Stacking" inverters (models that are designed to stack together)
• Some Honda eu1/2/3000i gensets can parallel AC outputs and share loads

The Isolated output on many TSW inverters and a very few MSW inverters can mean that they share a common Earth Referenced Neutral (even a floating neutral is possible--but I would not recommend it). With one lead of an isloated inverter ground/earth/neutral referenced, the other power lead(s) are independent of any other power sources and will have issues is connected together with other AC power sources as they are not sync'ed together.

-Bill

Jonesz

Re: Connecting an inverter to a transfer switch

Hi Bill
Maybe I didn't make my question clear re plugging my shore power chord into the inverter output. I don't see how that is either plugging two inverters together or into another AC power source. No other AC source is connected while I am doing this. I am only using the inverter output as my AC source.I know from reading other forums that many RVers plug their shore power chord directly into their inverters to power up existing AC circuits in their RV's. I am thinking that it is basically the same as plugging the shore power chord into a power pedestal in an RV park or plugging the chord into a generator. What I don't understand is why the 3 inverters I have tried fault when I do this while a friends Black and Decker 750W MSW cheapo which I have borrowed seems to run my circuits with no problem. I am confused as I wish to connect an inverter to my RV the easiest way possible so that I can use the existing circuits, rather than run seperate circuits to different areas of my 5th wheel. I would need at minimum 3 seperate circuits to run stereo, LR TV and Bedroom TV (which are in different locations) plus satellite receiver. I am not being argumentative but am confused with your reply.
Thanx
Jonesz

BB.

Re: Connecting an inverter to a transfer switch

Jonesz wrote: »

Hi Bill
Maybe I didn't make my question clear re plugging my shore power chord into the inverter output. I don't see how that is either plugging two inverters together or into another AC power source. No other AC source is connected while I am doing this. I am only using the inverter output as my AC source.

Good Afternoon Jonesz,

Not trying to argumentative--Just want to be safe in the case we are misunderstanding each other. "NO" until I better understand what you are trying to do (safety first).

Re-reading the above line--I am now understanding you want to plug your RV's shore power input power cord directly into the stand-alone inverter?

Yes--That can be OK. Issues to check for. First, make sure that there is no Neutral to Frame Ground connection in the any of the RV's AC wiring. Normally, the neutral to ground to green wire bonding would have been done at the Shore Power receptacle wiring somewhere. The Hot/Neutral/AC Ground should all be floating (or close--you probably have some leakage current somewhere) with respect to each other (normal RV wiring plan).

And--if you have a DC converter/battery charger, you want to turn that off... Otherwise the inverter is trying to recharge the battery bank through the RV's AC to DC converter with battery bank power. Depending on the size of the DC Converter--that could overload a smaller inverter and/or smallish battery bank.

I know from reading other forums that many RVers plug their shore power chord directly into their inverters to power up existing AC circuits in their RV's. I am thinking that it is basically the same as plugging the shore power chord into a power pedestal in an RV park or plugging the chord into a generator.

I agree--But as commented above, you have to make sure that the DC Converter is turned off (breaker in AC panel) so you don't load the Inverter and waste battery power.

What I don't understand is why the 3 inverters I have tried fault when I do this while a friends Black and Decker 750W MSW cheapo which I have borrowed seems to run my circuits with no problem.

I assume this is all done with your existing RV Battery bank, just swapping out the inverters for testing. All AC connections are "the same" by plugging into the inverter 120 VAC output.

What do you mean by "faulted"? Many inverters these days have a GFI receptacle... So, if you have any sort of "ground fault" (AC Hot wire current is flowing back through an alternative path other than the Neutral/White wire back into the Inverter Receptacle), the GFI outlet will trip.

Or, there is the inverter faulting itself. Could be from over current on the AC output, 12 volt power droop on the input (<10.5 volts or so), or it could be a Neutral/White Wire to vehicle Frame/Chassis ground issue--Especially if the battery bank negative is also attached to frame/chassis ground.

I am confused as I wish to connect an inverter to my RV the easiest way possible so that I can use the existing circuits, rather than run separate circuits to different areas of my 5th wheel. I would need at minimum 3 separate circuits to run stereo, LR TV and Bedroom TV (which are in different locations) plus satellite receiver. I am not being argumentative but am confused with your reply.

Yea, was my fault--I misinterpreted what was "shore power connection"--Sorry.

I would suggest that you eventually get an inexpensive DC Current Clamp Meter--It would let you measure, for example, the DC input current to a "working" inverter (i.e., 20 amps at 12 volts = ~480 watts input to inverter). This DC Current Clamp Meter is "good enough" and has the rest of the normal DMM (digital multi-meter) type function for ~$60.

A Kill-a-Watt meter (or equivalent) is great for AC power monitoring of your RV and doing home conservation measurements too.

-Bill