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Old 04-02-2014, 09:54 AM   #1
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Connecting an inverter


Hey,
I got this question. I am wondering if it would be safe to have a 120v inverter output into a transfer switch by connecting the hot line to both of the inputs. The panel that the transfer switch feeds into doesn't have any 220v circuits nor will it. Would I be better off by putting all the circuits I am using on the same rail and not split the hot feed from the inverter. I made a little illustration to better explain if I wasn't clear.

You can see my ideas in the picture with method 1 and 2 roughly detailed.

Here are some specs.
Transfer switch rated for 50a.
Inverter supplies 3kw continuous.
The load on the circuits is the following
16 4ft led laps @ 18w each. About 2.3a
1 standard house fridge. Estimate about 8amps for when compressor is powering up and about 2.5a for when it is running
1 mini freezer estimate about 12 amps for when compressor kicks in and about 4a for when it is running.
For peak I am guessing a total draw of 22.5 amps if everything was kicking off at the start.
Also what gauge wire would you use for the connections? I was thinking a 10 gauge would work for this. I was planning on only using a 30a breaker for the connection from the main panel to this sub pannel. Now. From the inverter to the transfer switch would a 10 gauge also work there. Since the breaker is 50 amps but the inverter would shut off well before it even reached 30 amps? Should I use an in line fuse?
I know I should be getting an electrician but I want to get some information before I do anything. I had an electrician do all the work for an addition I did to my house and asked for 30amp circuits in a work room and I ended up getting 30amp breakers with 12 gauge wires. Also there wasn't a ground to the sub panel he added. So I am not so convinced with the work of my local electricians since after he installed it the utility company sent a guy here and he gave it the ok.
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Old 04-02-2014, 12:59 PM   #2
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Connecting an inverter


What is the make and model of the inverter ?
What is the make and model of the transfer switch ?

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Old 04-02-2014, 01:50 PM   #3
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Connecting an inverter


The make and model of the transfer switch is Progressive Dynamics (PD52V). The inverter has yet to be purchased. Probably be one in the $200-$300 price range so nothing super fancy. Just something that works. Will probably just have a 3 prong connector.
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Old 04-02-2014, 03:16 PM   #4
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Connecting an inverter


Are you working on a camper/rv/boat, or a house ?

Camper and RV equipment is tested under different UL standards than house electrical equipment.

Unless that transfer switch has been tested under both standards, it would not be code compliant to install it in a home.

You are not going to purchase a 3kw inverter for home use in $200-300 range.
A home inverter will be UL1741 listed. The car/rv/boat inverters are UL458.
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Old 04-02-2014, 03:34 PM   #5
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Connecting an inverter


Its a house. Any suggestions on an inverter then? And what would you guess the inverter price range would be? Am I looking for a grid tie inverter? Or something else?
edit:
I just read up on that standard. the UL1741 states that the inverter must stop supplying power in the event of a mains power outage. I want power on during a power outage so how would that work for me? So what kind of inverter is that?

Last edited by ericm115; 04-02-2014 at 03:44 PM.
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Old 04-02-2014, 04:43 PM   #6
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Connecting an inverter


You would need an off grid inverter.
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Old 04-02-2014, 06:27 PM   #7
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Connecting an inverter


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Originally Posted by beenthere View Post
you would need an off grid inverter.

why ?
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Old 04-02-2014, 07:37 PM   #8
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Connecting an inverter


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why ?
Because an on grid inverter won't put out power if it doesn't sense power coming from the grid. Its a safety feature.
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Old 04-02-2014, 07:58 PM   #9
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Connecting an inverter


Quote:
the UL1741 states that the inverter must stop supplying power in the event of a mains power outage.
No it doesn't. It says that it must stop supplying power to the mains, during a mains power outage.

They make a grid-tie inverter with battery backup that would allow you to sell power to the grid and then goes into stand alone mode on grid power outages.

But, I don't believe you are thinking that large of system or investment.

You need to spell out what you are trying to accomplish. Get thru 4 hours or 4 days of outages ? How often do they occur ? How are you going to charge the batteries, Battery charger with grid power, solar, wind ?

If you are not attempting to sell power to the grid, you are better off not connecting to the house. Put some batteries in a shed or garage and run cords to the fridge, freezer and a few lights.

Or, go with a generator.
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Old 04-03-2014, 03:54 AM   #10
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Connecting an inverter


Hey. All I need is about 24 amps of peak power and like 13 amps of continuous usage. I will dictate how long it will last based on the battery array thati use. Batteries would be charged by the main. Outages are at random intervals. I basically want a ups for a fridge, 288w worth of lighting, a small freezer and about 30watts to charge an iPad and 2 cell phones. The issue with running an extension cord is that it would look very ugly. And I thought of just using the wiring already available. I want to isolate it from the grid so that whenever the power does shut down I can keep those critical items up. I don't want to sell power to the grid. The meters here don't run backwards at all. Now I was thinking I could run a dedicated plug with 10gauge wire and put that on an outlet when I can hook my inverter into. Then have a transfer switch to make sure I never feed power into the grid. I would put a 25amp inline fuse in the inverter output so I would stop any overloading issues. The only thing tied directly into the box would be a transfer switch and I will look for one rated for home use. But if I put an outlet for the inverter and isolated it so that the only way it was fed into something was if the mains was disconnected would that be OK? Or would I still need to get some special hardware? Currently when the power is out most of my neighbors turn their main breaker off and plug their generators into a stove outlet that is 3 prong. I know that can't be up to code but even the electrician does that. Is it possible there is a deferent code in this jurisdiction? Because my house had no ground until I had it added. And I was looked at like an alien for asking for it to be installed. I will have to ask around. I from new York so instill somewhat expect things to be similar because I always thought that standards are standards. I am in Puerto Rico now and had to spend extra money on getting a ground to all my outlets after an electrician ran all the wires and hooked up the sub panel. My sub panel didn't even have a ground bus. And it was all to code. Maybe things are more relaxed. I'm going to call an electrician later and see what they tell me I can do. I will still do the work since electricians here like to overcharge and do in my opinion crap work.
Thanks for the help. I will see what I can do to make sure this is safe when installed.
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Old 04-03-2014, 08:05 AM   #11
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Connecting an inverter


Quote:
Originally Posted by beenthere View Post
Because an on grid inverter won't put out power if it doesn't sense power coming from the grid. Its a safety feature.
But he doesn't want to connect it to the grid "
He wants to use it like a genny, via a transfer switch
So it will never see the grid !
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Old 04-03-2014, 10:02 AM   #12
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Connecting an inverter


Method 1 will work BUT if you have any MWBC's you could possibly overload the neutral. Just treat is as you would any 120v generator.

I'd make up an adapter that connects to the inverter that ties the two hots together but WOULD NOT use the transfer cable for that purpose. Just in case at some point you do get a generator and "forget" to convert the transfer cable back.

You have other issues to resolve:
1. Batteries. 13 amps continuous at 120v is 1560 watts. At 12 volts that is 130 amps. How long do you want this to run?
2. The inverter. A "cheap" inverter produces a square wave. Motors, such as in your fridge, won't like it and can overheat. True sine wave inverters are available but pricey.

If it were me, I'd just get a generator.
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Old 04-03-2014, 12:58 PM   #13
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Connecting an inverter


your fridge compressor will trip a 3kw inverter, they are not made for high inrush peak like a compressor do (peak over 50 amps) and youll need a true sine wave one or you may kill appliances.
you need a small gen
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Old 04-03-2014, 04:55 PM   #14
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Connecting an inverter


Quote:
Originally Posted by dmxtothemax View Post
But he doesn't want to connect it to the grid "
He wants to use it like a genny, via a transfer switch
So it will never see the grid !

Thats why I said he needs an off grid inverter.
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Old 04-03-2014, 06:38 PM   #15
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Connecting an inverter


Quote:
Originally Posted by ericm115 View Post
Hey. All I need is about 24 amps of peak power and like 13 amps of continuous usage. I will dictate how long it will last based on the battery array thati use. Batteries would be charged by the main. Outages are at random intervals. I basically want a ups for a fridge, 288w worth of lighting, a small freezer and about 30watts to charge an iPad and 2 cell phones.
OK, so you have 1560W of continuous load, and 2880W maximum load, with some compressors. I'd say a 5kW inverter is a good match for that. Anything smaller risks not handling the compressor startups. A good 3kW that is made for high startup current might do it. How long do you want to run these things on battery? At 13A load on the output, you'd have a 160A+ load on a 12V battery bank. To avoid over-discharge, that will require a battery capacity of at least 300AH per hour of runtime you want. So that's two of these: http://www.batterystuff.com/batterie...lar/s-460.html for each hour of runtime.

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