Panel Interlock and Generator.

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Old 09-23-18, 09:57 AM
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Panel Interlock and Generator.

So we are having our electrical panel replaced in our house for a number of reasons, but I am looking to have the the electrician install an interlock so I can hook up a small generator to feed a few circuits in the event of a power failure. I've got a few questions though.

​​​​​A bit of background, we live in a city with very strict noise ordenace laws so I am looking at doing a very small generator that could supply a couple of circuits to run the couple key appliances and a few lights. Nothing major. We don't have outages very often so this is just more of a convenience thing.

So my questions.....
1) are there any smaller generators (4000w-ish) that have 230V output that don't cost a fortune? I'm trying to fond one but am having difficulty.
2) If there are not, can I use some sort of transformer to set up a 115V to 230V?
3) if not 1 or 2, could I have the electrician install 2 15A breakers instead of one 30A Amp breaker and feed each bus bar in the breaker box with its own feed from the outlets of the generator.

Thanks!
 
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Old 09-23-18, 12:21 PM
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I#3 will definitely not work. If you connect one receptacle of the 115 or so volt only generator to each side of the breaker box you will get zero volts between the sides, not 230 or so. Also you will run into other problems such as an overloaded neutral in certain circuits (120/240 volt aka multiwire branch circuits) which problem requires too many words to describe here.

Motor driven equipment such as water pumps, sump pumps, and refrigerators draw extra power to get started. The 4000 watt generator is small enough that giving power to start something may leave little power to keep smaller things like lights going without tripping the generator master breaker. You will have to do the math to see what combination of equipment you have will work on just 4000 watts total.
 
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Old 09-23-18, 02:18 PM
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What is it that you plan on running that needs 240V? Or is it you want a gen that has 240V output to power both 120V legs of the panel.
 
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Old 09-23-18, 02:20 PM
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4000W generator will not be any quieter than 5000 or 6000W generator in my experience. 4000W still is a pretty big generator. You should look into generator's muffler design instead of wattage. Ones with full size mufflers are generally more quieter.
If you need them to be much quieter, you should look into inverter generators. Downside is they are significantly more expensive (more than double).

It is possible to increase voltage with a transformer, but you will have problem if what you are running is 120V/240V equipment instead of 240V only.
120V/240V equipment will be expecting 120V between one of the hot and neutral. If you use a transformer, you only have 1 hot and the other pole will be neutral. 120V circuit will get 0V or 240V depending on which hot it taps into.
If you use isolation transformer, it is possible to make it work, but then you have to isolate neutral in the panel.

#3 is only possible if your generator is designed to run that way and most generators are not. They have to be 180 degrees out of phase to each other for it to work.
This is near impossible to do with regular generator, but can be done with inverter generator. It still needs special circuits do so, therefore generally they don't support it. it is different from parallel hookup, but same idea. Parallel hookup make 2 generators run in sync instead of out of sync.
 
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Old 09-23-18, 02:27 PM
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Thabks all and yes I am look for 240 to power both legs of the panel. I hadn't even thought that the two 120 outputs wouldn't be 180 out of phase, but I guess that makes sense. I guess if I never ran anything that used 240 it'd probably work, but not a smart move.

I assumed a generator inverter was in my future. I'll have to keep looking for something that is really quiet has 240v and doesn't require a second mortgage.

Any recommendations?
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Old 09-23-18, 03:30 PM
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If all you want is feed both legs and will not be using 240V, then you can feed both legs from 120V without problem. 240V equipment will not work, but 120V equipment will work fine.
Just feed both poles of 2 pole breaker for back feed with same hot wire from your generator.
The down side is it is real easy to overload your generator. Installing a watt or current meter next to your panel is recommended.

Outputs from 2 different generator will be out of phase, but there is no way to tell by how much because they run independently. The phase is determined by stator and winding position, which cannot be controlled in regular generator.
Inverter generators can be synced because they convert DC to AC using electronics.
 
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