sizing breaker boxes


Old 08-17-06, 09:01 PM
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Join Date: Aug 2006
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Question sizing breaker boxes

At work we were discussing how to determine what size generator you would need to run a house, office etc., and when we went to our breaker box, we got very confused. I have a 200 amp service at home, but when you add up all the breakers, it comes to 715 amps. I have 9 20 amp, 9 15 amp, 1 60 amp 220, 1 50 amp 220, and 3 30 amp 220 breakers. How does this work? How is a breaker box sized for a house?

Thanks for any help,

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Old 08-17-06, 10:40 PM
Join Date: Jul 2006
Location: Bozeman
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The electrical service and breaker box's size is determined from calculations in Article 220 of the NEC.

The reason all of your breakers add up to such a high amount is because you don't use all of these items at the same time. For example, out of those 9-20 amp breakers, 7 of them could be receptacle circuits with nothing plugged into them.

To size your generator, you need to add up the wattage of everything you want the generator to control during an outage. To do this find the electrical nameplate on each appliance you want to operate off of the generator. Most times you will only find an amperage rating. To convert amps to watts, the formula is: volts x amps = watts.

You also need to triple the numbers for all motors to allow for starting. This is a "rule of thumb". I'm sure if an engineer is monitoring this they could give you an exact formula.

Once you get this number then you need to determine which generator manufacturer you will purchase your generator from and get their deration numbers. Most generators are derated for altitude and temperature.

So, let's say you determine that you need a 15kW (15,000 watts) generator to operate everything you want during an outage. Here in Montana, due to cold temperatures and high altitude, our generators are typically derated at least 20 percent. So a 15kW generator is going to be too small. However, in Florida, a 15kW may work just fine due to minimum deration.

Clear as mud right? Leave the sizing to the professionals. A qualified electrician or the generator sales person can size the generator for you. It is helpful if you make a list of everything that you want the generator to control during an outage before you call the electrician or generator saleperson.
Old 08-18-06, 01:43 PM
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One additional comment is to make certain that you have a transfer switch controlling everything, so you don't send power beyond your house and fry the linesman trying to repair downed wires.
Old 08-18-06, 01:54 PM
Join Date: Aug 2005
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Originally Posted by gilchrist-elect
You also need to triple the numbers for all motors to allow for starting...
Unless you need to start all the motors at the same time (unlikely,) you only need to do this for the largest motor, not all of them.
Old 08-22-06, 07:23 PM
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Join Date: Aug 2006
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thanks for the info / advice. i added up all the stuff plugged into my house, and if i turned everything i have on, it still wouldn't be 200 amps, so to size a generator, we'll just count up everything we plan to power with it.

Thanks again for your help!
Old 08-22-06, 07:36 PM
Join Date: Aug 2005
Location: USA
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You aren't planning on the ability to power all 200A from a generator, are you? That's a BIG generator for a homeowner.

You should be able to get by with less than a fourth of that, if that much.
Old 08-22-06, 08:16 PM
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no, we were kicking around trying to keep the computers up at work. we have a 'computer room' that has most of the important servers in it, and our discussion went from work to home and then to how do you size panels? our needs don't get close to 200 amps (unless they start talking about the air conditioners)(and we are steering them away from that). We have power outages about every other month for 2-4 hours at a time. the power company hasn't been too much help, and so we've been thinking about a generator for a while.
Old 08-22-06, 08:24 PM
Join Date: Nov 2004
Location: USA
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Do you already have a decent sized UPS unit in place? Depending on how may servers you are talking about, something along the lines of an APC Matrix-UPS can keep a small server room going for a pretty good while.

If you do have a good sized UPS, and you put in a generator to keep things going longer (generator powers UPS powers servers), make sure your generator produces clean enough power. UPS' are pretty finicky...if they do not like the quality of the power they are fed, they switch to battery and thus, the generator becomes worthless. We have that problem at one site I am associated with where the guy who bought the generator went cheap and we've been paying for it ever since.
Old 08-22-06, 08:53 PM
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each of the machines in the server room has a 1500 va ups on it, and usually they stay up for around 20 minutes. The windows boxes can shut down on their own, but most of the machines are unix/linux machines, and we've never taken the time to see if they communicate with the ups's. maybe we should spend the time to do that. we have a client that recently put in a generator and had to buy new ups's because the old ones were too sensitive to frequency and/or (depending on brand) voltage fluctuations. the old ones were switching back and forth between battery and line too often, and they had about a 12:1 charge/discharge ratio. so for every minute they supplied power, it would take 12 to get the charge back up, and they finally would get behind the curve.

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