Sub panel confusion

Hi,
I'm confused about the very essense of a sub panel: is it just a glorified junction box or can it actually increase the amount of current that enters your house? For example, if I want to increase the amount of current from 100amp to 150amps can I accomplish that with a sub panel?
Thanks.
Aaron
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no.
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"buffalobill's" short and sweet answer is correct.
What it does,basically, is gives you extra spaces for more breakers,so you can run additional circuits. More or less an extension off of the main panel. The only way to increase the amount of current that enters your house , or better said, the amount of current available to use in your house, is a service up grade. New main panel, larger wire going to and coming from the electric meter, and often times larger wire from the power company's transformer. Tony
wrote:

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Anthony Diodati wrote:

I don't think it is quite that simple. Suppose you have 200 amp service, and a panel that will accommodate only eight breakers, each of which is occupied by a 15 amp breaker. Then the only way to access the other 80 amps of capacity you already "have" is via a subpanel. It doesn't really increase the capacity you have available, but it allows you to access it. Adjust the numbers if you like, and argue about how mis-designed my hypothetical situation might be, but the principle is there.

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That's a good point CJT, but is there really a lot or any 200 Amp mains, that will only hold eight breakers? Still your point is valid. Tony
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Anthony Diodati wrote:

First off adding up the ratings of the installed branch circuit Over Current Protective Devices is not how you calculate how much of an existing services ampacity is in use.
Second there are a hell of a lot of main lug only panels that have only eight slots in them but the smallest breaker would have to be thirty ampere for the panel to pass electrical inspection as service equipment.
It's important to realize that "sub panel is not a term that you will find in the US NEC. A panel can be supplied by service entry conductors, a feeder, the secondary conductors of a transformer, or a local power source. Since the term itself is undefined it is difficult to give you a good answer. I have converted multiple panel installations to a larger service size by using a new main lug only panel installed as service equipment to control the feeders that supply each lighting and appliance or power panel but I doubt that is what you had in mind.
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A panel is just a box to hold circuit breakers (overcurrent protection devices, or OCPDs). The panel that holds the main shutoff for the electrical service is called the main panel, and other panels are called subpanels.
You can have all your breakers in the main panel if that is convenient and you have enough space. Or you can use a subpanel if you need more space for the breakers or it is more convenient. For example, if you have 8 parallel circuits all running to your kitchen at the opposite end of the house, it might be more convenient to have a subpanel near the kitchen, run a single larger feeder circuit to the subpanel, and then originate your kitchen circuits at the subpanel.
The other thing about panels is that the calculated demand of the supplied loads from the panel should be less than the rating of the feeder supplying the panel. So in the case of the main panel of a house, the total demand of the house should be less than the rating of your service conductors. Nothing you put downstream of the service conductors can increase the rating of those service conductors; the presence or absence of subpanels is immaterial.
Lastly, note that it is perfectly fine for the sum of the ratings of the breakers in a panel to exceed the rating of the feeder for the panel. Each of these ratings is a maximum and will seldom be achieved.
For example, if you have 6 circuits rated at 20 amps in a panel supplied by a 100 amp feeder, then you could run 5 of those circuits at their full load of 20 amps as long as you like. If all 6 circuits were at full load, then eventually the 100 amp breaker protecting the feeder should trip (it won't be immediate). But in practice almost never will all 6 of those circuits be at their full load--you might be able to service a whole house with 40 circuits from a single 100 amp feeder.
Yours, Wayne
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On Mon, 26 May 2008 22:10:53 GMT, Wayne Whitney

Expanding on Whayne's point, a good use of a sub panel; is to reduce voltage drop problems. If you have a big house it might be worthwhile to run a feeder to the far end and serve those loads from another panel. If you are not running all the circuits at a level stressing the feeder your voltage drop will be minimal. "Diversity" usually assures that does not happen.
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