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Old 28-01-2019, 19:29   #1
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Plugging inverter into 120 outlet

During hurricane Sandy, we tossed the breakers and we plugged the house generator into the 120v to keep things running. In hindsight this may have been illegal due to safety of linemen, however, a double-ended plug did work to power the house and keep the refrigerator running just fine.

I have an inverter that is decent quality, though it only offers the 120v outlets not any way to connect it to the 120v system.

Would it be "acceptable" to power outlets this way for convenience, or just way too dangerous? I suppose the double-ended plugs themselves offer some level of danger just by potentially shorting or touching a live plug.
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Old 29-01-2019, 00:56   #2
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Re: Plugging inverter into 120 outlet

would it work? yes. Is it correct. No.

Run a plug from the inverter to a transfer switch at the ac panel.
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Old 29-01-2019, 01:13   #3
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Re: Plugging inverter into 120 outlet

There are inverter, that can synchronize to the sine wave of a grid connection (grid tied inverter) and can supply energy back to the grid, but they are expensive.

There are some, that have power assist functions to pass through and team up with other sources like weak grid or generators. They have extra AC inputs, the outputs are not connected to the grid, but only to the loads.

Some can be configured in parallel mode with same size, same manufacturer devices for higher outputs or 2/3 phase configurations. Some generators - like the honda - can also be paralleled.

Some better brands simply sense the output and shut off as protection if there is external power.

But most simply die when accidently connected to the grid.

Usually you would install a grid priority relay after a inverter or generator if they do not have a built in transfer switch logic.
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Old 30-01-2019, 04:56   #4
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Re: Plugging inverter into 120 outlet

Quote:
Originally Posted by odonnellryan View Post
During hurricane Sandy, we tossed the breakers and we plugged the house generator into the 120v to keep things running. In hindsight this may have been illegal due to safety of linemen, however, a double-ended plug did work to power the house and keep the refrigerator running just fine.

In Australia we call them "Suicide cables". I used one once but I worried one of the kids might get hold of it so I chopped a plug off as soon as I finished with it.

I also wondered whether I could have killed a linesman too but even non technical people said any power I produced would quickly be used up by the first fridge/light/fan/stove in the houses on the power distribution circuit. And they said the power would have dissipated after a few hundred meters of power-line anyway. That seems logical?


NOTE
I just did an online calculation of voltage drop (240V) over 1000 ft and it would only be about 8%.....I thought it would be a lot more.
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Old 30-01-2019, 05:37   #5
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Re: Plugging inverter into 120 outlet

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Originally Posted by coopec43 View Post
In Australia we call them "Suicide cables". I used one once but I worried one of the kids might get hold of it so I chopped a plug off as soon as I finished with it.

I also wondered whether I could have killed a linesman too but even non technical people said any power I produced would quickly be used up by the first fridge/light/fan/stove in the houses on the power distribution circuit. And they said the power would have dissipated after a few hundred meters of power-line anyway. That seems logical?


NOTE
I just did an online calculation of voltage drop (240V) over 1000 ft and it would only be about 8%.....I thought it would be a lot more.
Voltage drop depends on the current flowing and on the wires size and material. If there is no current, there is no voltage drop. The high voltage causes implicit low currents for the same power (P=V*I), and because the current is low, the losses by voltage drop are lower the higher the voltage is and the line can be thinner.

That is the reason why long distance power lines use high voltage.
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Old 30-01-2019, 06:07   #6
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Re: Plugging inverter into 120 outlet

Connecting a generator to the house wiring without a transfer switch or similar device is illegal. Technically if you open the house main breaker you are OK, but forget that step or flip the wrong breaker, you can power up the lines outside. This puts utility workers at risk and if the power comes back with the generator connected, it will be damaged.

Same precautions apply to an inverter on a boat. You need a transfer switch so you can power the 120V outlets with only one source at a time.
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Old 30-01-2019, 06:23   #7
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Re: Plugging inverter into 120 outlet

Not to put too fine a point on it as this horse is already dead but here is the reason for the Code requirement for a break before make transfer switch to select standby power:

Transformers are bi-directional. The primary side of a residential transformer is >2300VAC and the secondary is 240VAC feeding the panel in the garage. If the main disconnect in the panel is not opened and a suicide cable is used to feed the house loads from a genset then 120VAC will be on one leg of the 240VAC split phase service to the house which will than place about 1200VAC on the feed from the residential transformer.

A transfer switch is the only legal, safe and humane method for selecting a source of power for consumer use.
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Old 30-01-2019, 10:12   #8
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Re: Plugging inverter into 120 outlet

That is true, and there are inverter with built in transfer switch and also legal grid tied inverter, where you can feed excess energy (e.g. from Solar) back to the grid. Also some USV can be grid tied or have internal transfer switches.

It depends on your wiring, contract etc.

Of course, for most installations a transfer switch is a necessity.

Regarding the Honda Generators, they can be run in parallel to double the output by a dedicated sync cable, but cannot be not grid tied.

There are also generators, that can sync to external power and feed energy in or allow seamless switch over (e.g. in conjunction of a USV, that holds the house up for some minutes until the generator is warmed up and synced to take over).
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Old 30-01-2019, 10:18   #9
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Re: Plugging inverter into 120 outlet

You MUST turn off the main breaker so your generator doesn't feed back into the lines. It's criminal negligence and you may kill someone.
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Old 30-01-2019, 15:12   #10
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Re: Plugging inverter into 120 outlet

Quote:
Originally Posted by CatNewBee View Post
Voltage drop depends on the current flowing and on the wires size and material. If there is no current, there is no voltage drop. The high voltage causes implicit low currents for the same power (P=V*I), and because the current is low, the losses by voltage drop are lower the higher the voltage is and the line can be thinner.

That is the reason why long distance power lines use high voltage.

The voltage drop calculator I used was
Voltage Drop Calculator - for single and 3 phase ac systems and dc systems


And I think it took all that into account

Thanks

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Old 30-01-2019, 23:01   #11
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Re: Plugging inverter into 120 outlet

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Originally Posted by Cheechako View Post
You MUST turn off the main breaker so your generator doesn't feed back into the lines. It's criminal negligence and you may kill someone.
it's still illegal even if the main breaker is off.
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