Emergency backfeed failed

JoshK · May 2015

I'm only looking for troubleshooting advice of what when wrong in the past, I know this is a bad idea and anything added is for educational purposes only...

It's my belief that I can use my 4-prong dryer outlet to power the 2 rails in my breaker box and then only turn on 120v breakers. Never a 240v because the rails will be in phase.
I shut off my main breaker AND all the individual breakers for inital verification. I powered the Left and Right prong on the dryer outlet with the hot wire from the inverter. I connected the neutral to neutral, and then the two green ground wires together. That was all verified with a continuity tester. When I plugged in the inverter, it should have energized the two rails and been ready to flip on one breaker. Instead the inverter immediately went into overload and shut off. Why would that be?

Photowhit · May 2015

Do you have live power?

You say backfeed, if you want to back feed you have to have an inverter designed to back feed and sync up to the sine wave of the grid.

Brlux · May 2015

So if I under stand what you are saying you had you service main breaker switched off killing the utility power to the whole house. You switched off all the individual breakers to the house, plugged in the back feed jumpered plug and when you switched on the breaker to that back feed plug your inverter triped even tough there were no other breaker loads switched on?

My thought is that your problem is with your ground pin. Your inverter likely does not support bonding the ground pin to the neutral line which is what happens to those two lines in the breaker box. If your inverter still works, you may be able to get it to operate by not hooking up the ground pin on your back feed jumper.

JoshK · May 2015

Yea the main breaker was also off to disconnect me from the grid the whole time. The only breaker not off was the dryer one, I guess I should have started with that off too...

Brlux wrote: »

If your inverter still works

Yes it does, nothing was harmed in the process.

Brlux wrote: »

Your inverter likely does not support bonding the ground pin to the neutral line which is what happens to those two lines in the breaker box.

You just might be a genius, that would have never crossed my mind. I will give it another try soon. Thanks you guys

MarkC · May 2015

I have designed a similar setup using a small, but very efficient (120 volt) APC UPS to power selected 120 volt users that are a "mix" of the two legs of a conventional home breaker panel. This would only be used under a extreme emergency (no fuel left, no grid left, etc, etc.) and carefully controlled environment for long term sustainability - not my typical off grid setup (Priups generator with 240 split phase outputs and SMA SPS for any small 120 volt loads). I have used a modified NEMA "L14" connection system. The following article shows the wiring schema that is typical for this. My (admittedly somewhat limited) knowledge of this schema indicates that the main issue is the current capabilities of the neutral wiring is the main concern - that under certain load conditions may need to be sized for the full 120 volt amperage loading - which is not a problem for me as the total load is only a few hundred watts. The issue of the inverter "bonding" is somewhat of a "mystery" to me. My system is bonded at the critical breaker panel that will be fed by this inverter. However, I've not had a chance to try this yet (wiring is complete - but no testing).

Need to get "BB" to look at this one. If he does not pick up, I'd recommend you send him a private message to look at this thread.

Mark

http://www.samlexamerica.com/support/documents/WhitePaper-120240VACSingleSplitPhaseandMultiWireBranchCircuits.pdf

BB. · May 2015

Is this a TSW/PSW inverter (true/pure sine wave) or a MSW (modified square wave) type?

MSW inverters do not generally like Neutral to Earth grounding on the AC output and having the battery bank grounded to earth ground (more or less, creates a dead short through the inverter).

With the AC Mains turned off, and the all the other branch circuits turned off. Does the inverter work? Then turn on the AC Branch Breaker for the inverter (no other breakers on). Does it still work?

Then turn on one branch circuit breaker at a time (work/not work per circuit).

If this is a PSW/TSW inverter--I am not sure why you are having a problem (in general, TSW inverters are find with a ground referenced neutral on the AC output and Grounded battery bank negative bus).

Hmm... Perhaps you have a polarity problem. Many TSW inverters have a green wire to AC neutral ground bond. If you have the AC inverter Hot/Neutral reversed--The ground bond "neutral" inverter connection and ground bond neutral in the main panel could be reversed and give you a dead short (i.e., Hot from inverter connected to neutral in main panel, and neutral from inverter connected to main panel hot)..

-Bill

inetdog · May 2015

JoshK wrote: »

I'm only looking for troubleshooting advice of what when wrong in the past, I know this is a bad idea and anything added is for educational purposes only...

It's my belief that I can use my 4-prong dryer outlet to power the 2 rails in my breaker box and then only turn on 120v breakers. Never a 240v because the rails will be in phase.
I shut off my main breaker AND all the individual breakers for inital verification. I powered the Left and Right prong on the dryer outlet with the hot wire from the inverter. I connected the neutral to neutral, and then the two green ground wires together. That was all verified with a continuity tester. When I plugged in the inverter, it should have energized the two rails and been ready to flip on one breaker. Instead the inverter immediately went into overload and shut off. Why would that be?

One possible problem (not the cause of the inverter shutdown) with that is if your installation uses what are called MultiWire Branch Circuits (MWBC). These combine two opposite phased with a same sized shared neutral wire to feed two 120V circuits. This saves on wire cost and labor, but means that your installation has the potential to send double the rated current through the neutral of an MWBC. In normal operation the two circuits neutral currents oppose each other and partially cancel.
Also, for the same reason, if several circuits are run through one conduit the derating according to the number of Current Carrying Conductors (CCCs) may not work out either. That is usually more of a regulatory than a safety problem, unlike the double neutral current which could actually start a fire.

As for the shutdown, does your inverter have a GFCI built in? What happens when you try to power only one of the two panel buses?

JoshK · May 2015

Well the cable seems to have grown legs and run away. This never used to happen before I was married. It must be a coincidence...

JoshK · May 2015

Alright, so I made a simple 120v cord to do the same with a standard outlet, and this time I did not connect the ground to the inverter output. It powered half the panel as expected. I learned my LED bulbs buzz a bit on running on Modifed Sine Wave, but light up normal. Same with the GFI outlets. I feel this is probably hard on them...?

JoshK · May 2015

So I improved my 120v cord into a 'double-death' cord as I now call it. I am lucky enough to have two outlets next to each other in the garage that are on different rails. So using my double-death cable I have powered the whole house from my inverter in the garage.

Thanks again to Brlux for nailing the issue!

Brlux · May 2015

Give us some information on the inverter you are using?

When I was a kid I helped my folks build their house. We could not get a power pole and they would not hook up power till all the final inspections were done. We used a little 2400W sears generator 120V 20A with a 5hp brigs engine to run the place for a year while building it. Once all the electrical was in we used 2 lengths of romex with 120V male plugs at each end, the first was used to plug the gen in to a single receptacle dedicated 20A circuit for the central vac system in the garage and the second cord to jumper between 2 different 120V receptacles on different phases of the panel. It worked surprisingly well running all the lights in the house, plus drills, table saw and whatnot as needed to get the house done.

JoshK · May 2015

My inverter is a Whisler Pro 1600. (3200w for 30 seconds) Attachment not found.

Johann · May 2015

As far as I know, some inverters do funny things to create the 120 volts. Some inverters will create 60 volts on the hot leg and 60 volts on the neutral, but combined it still will be 120 volts. It is like a 120 volt single phase with an center tap ....60 volts on each leg(neutral and hot) and then the ground is your center tap. So if you have a true ground ( a actual ground that is wired inside the inverter etc etc ) on the inverter you would read/measure 60 volts on the hot and 60 volt on the neutral toward the ground side of the inverter.

If you hook such inverter up like the OP did, you would short circuit or overload the inverter between the neutral (60 volts ) and ground since neutral and ground are joint together in the breaker box.

Note: Some inverters will put out 120 volts on one leg and will have a true neutral and ground.
Other inverters may not even have a ground hooked up inside them, like my china inverter that burned up last year after almost 5 years of use.

JoshK · May 2015

Yea I see what you are saying, I think that's it. I'm not so impressed with this Whistler Pro 1600 anymore.

Brlux · May 2015

I have seen some tear down videos of those Whistler Pro1600 and it is the oddest design I have seen. Looks like it is way more complicated to assemble than it needs to be. My theory is that a poor mechanical design does not inspire confidence in the electrical design. I have had had an older Whistler PP1500AC for the past 15 years. It doesn't get regular service but when I have used it, it has always worked great.

Here is the first of the videos detailing repair. In the #3 video he says it will not operate his refrigerator which is about 160W running. He has had no problem running the fridge on other 600W inverters.
https://www.youtube.com/watch?v=qG60rPmSqxc

Sawman · April 2021

I have a generator hooked up to my off grid cabin through a dryer plug jumped across that powers my 60 amp panel box. I am now putting in solar and am thinking of plugging my inverter the same way. So i can unplug one and plug in the other . The generator is a Yamaha 3000 watt inverter. I see no problem with this. Any responses are welcome.

BB. · April 2021

Doing this does create problems... It is called a "suicide plug" because the leads are hot if the genset/inverter is running and can shock/electrocute somebody. So from the "safety" point of view--That is not a good thing.

The second issue--In the USA (at least), there are two styles of Dryer plugs. One is a 3 prong (L1 / L2 / Neutral) and the other is a 4 prong (L1 / L2 / Neutral / Safety Ground). If you have a 3 prong, changing over to a 4 prong plug at least gives you a true green wire safety ground (instead of using the plug's neutral as both White wire neutral and as a safety ground). Doing that, at least, allows the idea of separate neutral/ground wires from the genset/inverter to carry through to the house wiring.

NEMA outlet chart:
https://acupwr.com/pages/nema-chart

NEMA 10-30R and 10-30P "3 prong" 120/240 VAC L1/L2/N (N is assumed tied to safety ground "upstream" of outlet at power source)
NEMA 14-30P and 14030P "4 prong" 120/240 VAC + L1/L2/N+Safety Ground

https://www.homedepot.com/p/GE-6-ft-3-Prong-30-Amp-Dryer-Cord-WX09X10004/203497480? (3 prong cord)
https://www.homedepot.com/p/HDX-6-ft-30-Amp-4-Prong-Dryer-Power-Cord-HD-601-004/202532733? (4 prong cord)

I always wondered with 3 prong 120/240 VAC split phase dryer cords are still "legal"... Violates that whole idea of safety grounding the dryer sheet metal--Especially given the dryer is next to water (washing machine and sink).

For example, 4 prong allows you to have the same green wire ground for the generator frame/AC inverter chassis and choose where to do the "neutral to earth ground" bonding. This choice is a bit confusing--In North America, that bonding choice is usually inside the Main panel (neutral bus bar grounded to panel sheet metal/safety ground bus). In larger generators/AC inverters, Neutral to Safety Ground are (usually) tied together by default (around 3,000 Watts or so seems to be the default--But you can find gensets that do not follow that rule).

Functionally, the AC power system works fine either way (panel bond or genset bond, or even bonded in both places). The reason this is not usually done is because NEC (national electric code) does not allow the green wire to carry normal current--Only current if there is a short circuit. And second, the whole issue of GFI (ground fault interrupter) outlets. If you have a GFI outlet on the generator and tie N+G in side the genset/inverter and carry H+N+G to the AC panel which is also N+G bonded--The genset/inverter GFI outlet will usually trip (because N and G are in parallel and sharing current flow--Which looks like a ground fault to the GFI).

After I have typed all of the above... The short answer is "it will work" with the dryer plug (3 or 4 prong). Just a "safety" issue with "suicide" plug on genset/inverter side.

Longer term, if you installed a manual transfer switch or even a simple relay transfer switch--You could have two normal blade plugs--One for genset and one for AC inverter--And not do the "suicide plug" anywhere.,

https://www.solar-electric.com/search/?q=transfer+switch (transfer switches)

Use a manual transfer switch, or a relay type (for example, AC inverter connected to "default/AC mains" input, and genset connected to GEN input--AC inverter connected normally, and switches over to the generator when generator is running).

If you only have one power source at a time on your system... You could use normal "blade plug" on the cabin input, and a normal "socket plug" on the genset/inverter/extension cord and do away with the whole "suicide plug/cord".

If you ever plan on using a 120/240 VAC electric dryer with a larger genset (typically 6 kWatt or larger)--You would want a separate circuit from the genset to the main panel, so you can run the dryer too.

You would only "revisit" the cabin "blade" plug if you added another source of power (bring utility power to the cabin, want backup generator + AC inverter power sources always connected)... Then a transfer switch or suicide plug decision would be needed.

If/when you purchase a new AC inverter--You can look at the more sophisticated models that have a Genset input and their own internal transfer switch... As well as AC Battery charging, Generator Support (AC inverter "shares" heavy loads with small genset--Pretty nifty), or even Grid Tied (hybrid) Inverter if you ever get Grid Power and want to "sell" excess energy back to the utility (GT may or may not be legal with your utility).

-Bill

Emergency backfeed failed

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