best way to switch between a MSW inverter and home AC mains

MarkNewYork · July 2020

I want to use this MSW inverter as backup power for my home well pump:
https://theinverterstore.com/product/5000-watt-modified-sine-inverter-12-vdc-to-240-vac

Its manual says that the AC neutral is bonded to ground so it I assume it will not "short out" if installed with its neutral connected to ground. I've read on here that, generally, MSW inverters should NOT have their AC grounds bonded with neutral.

From the manual:
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The neutral (common) conductor of the power inverter AC output circuit is connected to the chassis ground. Therefore, when the chassis is connected to ground, the neutral conductor will also be grounded.
This conforms to national electrical code requirements that separately derived AC sources (such as inverters and generators) have their neutral tied to ground in the same way that the neutral conductor from the utility line is tied to ground at the AC breaker panel.
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So, it seems like I can tie together the house ground, the inverter ground, and the well pump ground... and use a DPDT switch or relay to switch the 2 legs of 240v power between mains power and inverter power. Does this seem true?

The water pump is not presently wired with a neutral, just the 2 hot legs and a ground.

I'm using it with a 12v solar MPPT controller and 100 ah battery.

Thanks for any opinions!

MichaelK · July 2020

Please do NOT even attempt to run your well pump with that inverter. Firstly, a modified sine wave is a poor choice for any electrical motor device. You want a clean PureSineWave inverter for it. Secondly, well pumps are high wattage units. What size pump do you have? My pump is 1hp, at 240VAC, and requires 9000W at startup, and ~2000W to run. A 100AH battery is not going to supply the amount of current you need for most pumps, and pumps are VERY expensive to replace.

Before anything else, buy a clamp meter that can measure inrush current and determine what your power needs are for the pump. A 24V system with a 400AH battery might be able to start smaller pumps, but you really want 48V for bigger pumps.

You haven't mentioned anything about how many watts of solar your system has? I'm going to assume it's inadequate based on the other components you have. My rule of thumb is to have 2X the number of watts of solar for your single biggest load. Say your pump draws 1000W while running. Don't try to power that with less than 2000W of solar. And, 2000W of solar is really too big for a 12V system.

Don't dive into this blind! Determine what your loads are before you start plugging things in.

mike95490 · July 2020

Slow Down !
First, a MSW inverter will cause your pump to run hot and waste about 20% more power as heat.

Second, say it's a half HP pump, that would actually consume about 1,000watts. At 12V, that's 83 amps and will need more battery and very heavy wire.

edit:
Third How often will this pump run off a battery ? how long each cycle ? Can you consider using four 6v batteries in series to get 24VDC and that takes your amps down to 40A which would be much more manageable.

Fourth. A MSW inverter will put half output voltage on one of the battery terminals, so your battery and solar panels could end up floating at 120VAC

MarkNewYork · July 2020

Michael and Mike thanks for your comments. I thought I had gotten the sizing/capacity settled but your questions are making me re-examine things.

My 12v solar system is already in place. It's small - with 2 100 watt panels, a 10 amp MPPT controller and a Vmax 100ah battery.
I use it primarily to drive 12v internet equipment as a backup. BUT - I also have a 24amp DC charger that I could run off a generator if my solar can't keep up. That's what I think I'd end up doing for any extended use of the inverter. I don't presently have any inverter hooked into the system.

As far as the pump - how often it would run and the cycle - the cycle is about 40 seconds and I think i'd be very conservative with water and it would run maybe 5-10 times a day. Just enough for a quick shower and hand/dish washing. It's a very small cottage with 2 people. The pump draws 1000 watts at 240v as measured by my wattmeter. And I actually have a 2nd 100ah battery that I could hook to the inverter if the first one runs out and is being charged.

But the battery itself - you're saying that a 100ah capacity is too low to realistically drive the pump at all. I did ask this question to the folks at theinverterstore and their answer based on my above anticipated usage is that it "should work fine" - bad advice? Also they're marketing this inverter as a specifically good choice to drive a well pump. Their description:
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https://theinverterstore.com/product/5000-watt-modified-sine-inverter-12-vdc-to-240-vac/
The AIMS Power 5000 Watt inverter with 240 volts AC output is a great choice for an off grid well pump that requires 240VAC to operate. Whether for a well pump or a 240 volt motor that needs back up power, the AIMS Power 5000 Watt 240VAC inverter is a great option.
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They're also specifying 4/0 awg battery cables and a 500 amp fuse for the DC. What's your opinion on that?

Also your comments on the inverter floating 120v to my solar, I think I would then disconnect the solar when I fire up the inverter so that a good tip, thanks.

I appreciate your comments and warnings - and again - this is absolutely for the occasional power outage. I could run the pump with my generator directly but when I wake up at 5 in the morning and the power's out I don't have time to drag out the gen and hook it up. I'd rather just switch on the inverter.

Thanks! Further input appreciated!

mike95490 · July 2020

I've given you my opinion on the pump & inverter. I guess you will have to discover for yourself, if it will work in your application, with your battery.

How long are your inverter power cables to the battery ?

1kw running load, is likely 5kw for starting the motor, so thats a 450A load on the batteries and you will want cables rated for at least 200A continuous, or the voltage sag will shut the inverter down. if more than a couple feet cable length, you have to go even larger. And you can bet the chassis wires inside the 5kw inverter, are nowhere near the the required gauge.

best way to switch between a MSW inverter and home AC mains

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