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Thread: Embarrassing circuit breaker/generator question

  1. #1
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    Embarrassing circuit breaker/generator question

    My house’s breaker box is actually a sub panel supplied from my daughters house 30 feet away. In their cellar, there’s a 125A 240v breaker. We don’t draw anywhere near that much power. Most of the time we draw we draw under a kilowatt. I have an energy monitor to work out our share of the monthly bill. As I write this, our house is pretty quiet, drawing 338w. The fridge is running at 130w.

    If I multiply volts x amps, 120x125 is 15,000 watts. So if I wanted to supply my house with a generator at that 125A. Would I need a 15K generator or twice that. Put another way, do I use the amperage on the breaker or multiply by two legs?

    I’m sure this has been explained to me many times and for some reason it refuses to stick. I remember all sorts of useless stuff but not this. Maybe I should write it down someplace..

    Sheepishly,
    Roger

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    With every fan in the house going, plus a portable fan, the fridge, a computer, and batteries charging for tools and cameras, I’m pulling 0.5 kWh.

    FCAFCB6E-44D4-4CD6-A818-7976B83B4457.jpg

    Edit: I’m cross eyed this morning. It’s 0.3, not 0.5.

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    Why would you need any more than a smaller gen set, say a 5,000 watt one?

    edit: maybe I am reading too much into this. Are you trying to replace your feed from the other house or looking for emergency power.

    To supply your sub with the same power you have from the other panel you need 30,000 watts.
    To feed your panel for your needs in an emergency you can get by with much less.
    Last edited by Charlie Velasquez; 07-03-2021 at 10:05 AM.
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    …sorry, my point was your numbers seem high. 125A would be running all breakers at capacity and you would never be doing that. You’re running now at 0.338kW.

  5. #5
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    I know I would need less. I think I muddied the waters.

    I want to know if a 120A double pole breaker is the equivalent of two single pole 120A breakers or two 60A single pole breakers.

    This question is hypothetical only. I don’t have tied single pole breakers.

    (the dryer is running now and I’m still under 1KW)

  6. #6
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    Quote Originally Posted by Malcolm Schweizer View Post
    …sorry, my point was your numbers seem high. 125A would be running all breakers at capacity and you would never be doing that. You’re running now at 0.338kW.
    Malcom, you are right that 10KW is a lot. I’m looking into having an electrician put in a single transfer switch on our house and running a cable out a window well to a portable generator in the driveway. I think I can easily supply my house and have enough leftover to power my daughters fridge and another fridge in the garage. I’ll just stay out of my shop during the outage.

    I know I can get a smaller generator but the difference isn’t that much.

  7. #7
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    125A double pole breaker is 125 amps on each leg of the service. You can provide a max of 125A to 240 volt loads, or 250 A to 120 V loads, or any combination of 120 and 240 volt loads that don't exceed 125 Amps on either leg. So you have a theoretical maximum power of 125A x 240V or 30,000 watts (30 Kilowatts). It would be equally valid to calculate this as (125A x 120V) + (125A x 120V) which gives the exact same result.
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  8. #8
    Quote Originally Posted by Roger Feeley View Post
    Malcom, you are right that 10KW is a lot. I’m looking into having an electrician put in a single transfer switch on our house and running a cable out a window well to a portable generator in the driveway. I think I can easily supply my house and have enough leftover to power my daughters fridge and another fridge in the garage. I’ll just stay out of my shop during the outage.

    I know I can get a smaller generator but the difference isn’t that much.
    If you don't have a lot of power outages, when you do have an outage, put the generator in a reasonable place and run long extension cords to the appliances you need to keep going. Putting in a transfer switch is somewhat expensive.

    Mike
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    Quote Originally Posted by Mike Henderson View Post
    If you don't have a lot of power outages, when you do have an outage, put the generator in a reasonable place and run long extension cords to the appliances you need to keep going. Putting in a transfer switch is somewhat expensive.

    Mike
    You would also need a tranfer switch for whatever you plan to power in your daughter`s house.

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    Are you looking for a standby generator or a portable one? I ask because a 10kw standby is not half the price of a 20kW. Since your house is tied to your daughter's house I would see if they wanted a generator too. I say this because you could tie it into their main panel to feed both houses. To figure out how big you need go through your house and determine what you will need to run. Things like a furnace and fridge/ freezer you would want to power. Things like well pumps can be controlled if needed to only come on when other loads are not required. By doing both houses together it reduce the amount of "extra" you would need. There's several on-line calculators to help figure out what size generator is needed.

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    Quote Originally Posted by Lee Schierer View Post
    You would also need a tranfer switch for whatever you plan to power in your daughter`s house.
    I’m not sending that kind of money on her house. We’ll run extension cords to the fridge.

  12. #12
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    I would suspect a 10,000 watt portable would handle both loads for most events. A transfer switch would work, but an interlock is cheaper and more versatile. Run the power to the kids' house.
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  13. #13
    Speaking of generators, I have a question-

    I'm considering picking up 2 HF 9500 watt Predator generators, which will supply 15,200 running watts when parallel-connected. ($4000 total for both generators is a good price IMO, I have a 3500 version and it works great!)

    My question is, can the 120v output of each generator be connected to a breaker box's separate incoming legs so the 240 breakers would be usable? Or will the AC be out of phase, or?
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  14. #14
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    Two years ago, we had a 21 KW Generac generator and transfer switch installed. It was about $6500 and well worth it. It can power the well, AC and everything else in the house.

  15. #15
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    Quote Originally Posted by Kev Williams View Post
    Speaking of generators, I have a question-

    I'm considering picking up 2 HF 9500 watt Predator generators, which will supply 15,200 running watts when parallel-connected. ($4000 total for both generators is a good price IMO, I have a 3500 version and it works great!)

    My question is, can the 120v output of each generator be connected to a breaker box's separate incoming legs so the 240 breakers would be usable? Or will the AC be out of phase, or?
    Running two 120 volt gen sets will not give you 240 volts. But I don’t know of any 9500 watt units that can not produce 240 volts.
    Comments made here are my own and, according to my children, do not reflect the opinions of any other person... anywhere, anytime.

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