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Old 12-19-2012, 11:36 PM   #1
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120V Inverter

I have a Yamaha 3kw inverter that I'm hoping to use to power a few circuits in my home in the case of an outage. I dont have a transfer switch, and I plan to turn off the main to avoid backfeed to the lines. I'll turn off all of the breakers except for the few I plan to run (fridge, furnace blower, one light circuit, and maybe a sump pump). All of the circuits I would run are 120V.

But their breakers are on both legs of the 240V service into the house...

The inverter only puts out 120V.

Im considering putting a 30A double pole breaker in the box that has a jumper between the two legs, that would be left off at all times (or left out of the box altogether) unless I was using the inverter. 240V circuits would be turned off.

Im also considering moving the breakers that Im looking to power over onto the same leg, and then just powering half the box with the inverter.

Great ideas eh? Let me know what you think.


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Old 12-20-2012, 03:47 AM   #2
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Whilst on a technical level it will work !
There are significant dangers with your switch idea !
What if some one who doesnt know plays with the switch ?
Please use a properly set up transfer switch ?


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Old 12-20-2012, 05:18 AM   #3
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I see serious problems with your idea

1) You NEED, at the minimum, an interlock kit to avoid unintentional backfeeding. You saying that you will turn off the main is NOT good enough.

2) If you feed multi wire branch circuits with the same 120V phase, you could possibly overload the neutral of the MWBC and create a fire hazard.

2b) If you move the circuits around in the panel to get the target circuits on the same phase you may end up unintentionally moving MWBC's to the same phase and, again, creating a fire hazard.
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Old 12-20-2012, 11:29 AM   #4
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And what if you get in a car wreck and die, then later someone else flips on that breaker "to see what it goes to"?

Not a good idea! Follow electrical codes (install transfer switch), then you will have a safe electrical system for you, your family, and anyone who may purchase your home at a later time.
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Old 12-20-2012, 06:05 PM   #5
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Very bad idea. If someone else flipped the wrong switch or you made a mistake in a outage, you could kill the guy from the power company trying to restore your power.

Manslaughter by gross negligence?
#1 - If you don't know what you're doing - get a licensed electrician!
#2 - If you follow my advice and something bad happens see # 1
Electricity bites hard, and it could be the last thing you feel... Good Luck!
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Old 12-20-2012, 07:43 PM   #6
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That 3k inverter will have a hard time starting either the blower or the compressor on the fridge...let alone both. And that's after you install the proper safety equipment. Buy some cords and a portable heater, or a portable generator...(still buy the cords).
Do you want it your way or the right way?
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Old 12-20-2012, 09:55 PM   #7
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Excellent points, thanks for the feedback!
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Old 12-21-2012, 10:49 AM   #8
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As an alternative, consider putting a NEMA 5-20 inlet on the side of your house. That's box with male prongs that you plug your generator into.

On the inside of the house, you can have one or more dedicated outlets that are connected to the inlet, but are not connected to the rest of the house wiring in any way shape or form. Then from the dedicated genny outlet(s), you can run temporary cord(s) to the fridge or whatever you need.

The above, so far as I know, is safe and legal. If you keep reading, things get hazy:

Code unfortunately doesn't allow a gas furnace to be supplied with a "temporary" cord to use the above trick. However, I have heard of people replacing the furnace hardwire connection with a twist-lock plug and receptacle on the theory that the twist-lock makes it a (more) permanent connection. Then, during a power outage, you just unplug the furnace from the twist-lock wall receptacle, and plug it into a twist-lock generator extension cord. Note, I've no idea if this is code, but it is certainly safer than backfeeding without an interlock.
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Old 12-21-2012, 06:38 PM   #9
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From a law perspective I know what you suggest to do is ILLEGAL with a generator, I don't know if it covers inverters, but regardless, it is the SAME DANGER. I would treat the inverter like a generator and install the proper interlock kit or transfer switch.

Another option is to consider an inverter-charger. It will run on AC and automaticly switch to batteries when the power goes out. You do not need a transfer switch as the way it would be fed is like this:

Breaker (ex: 30a) -> Inverter Charger -> Sub panel -> protected circuits

Basically the Inverter charger would have a AC in, AC out, and DC in. AC in from the supply breaker, AC out to the sub panel feeder, and DC in from the batteries. If you get an instant switch one (ex: <16ms transfer time) you can even have computer equipment or other electronics on there and it will be completely interrupted.

For a smaller inverter-charger, there would not be a sub panel but simply an outlet. But if you plan to make this a permanent installation may as well get a bigger one.

If you rather have more choice as to what you protect you can make it a manual transfer as well and treat it like you would a generator. Just be sure the batteries are always on some kind of charger to keep them topped up. Ensure the charger is off when you go to use it while the power is out, though. You don't want the charger trying to power the inverter when the power comes back. (would probably burn out unless it's a very expensive telco style charger capable of 100's of amps)
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Old 12-22-2012, 09:25 AM   #10
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Once apon a time I remember reading of an electrocution of a Lineman in a small town in Albeta. Some welder plugged in a portable welder, (A motor driven generator that is also equiped with a 110V power supply receptacle that he can use to run a grinder of a drill while he is out on a job). this 110 power was connected to a wall receptacle in a welding shop so they had lights and a small amount of power for the heaters. A lineman was killed when the transformers back fed 4160 volts to the high side line on the power poles. the lineman didn't have a chance! a transfer switch would only have helped if it was properly connected and used, but he as a welder and never heard of one! At the inquest they said the lineman should have been more thorough and was at fault, but he didn't go home to his family that night. How would you feel if you jury rigged that wiring improperly?


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