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Location: California | United States

I'd like to better understand the roles of main breakers and sub-breakers. Why are there multiple levels of breakers? I suspect that sub-breakers are intended to regulate/control electricity for specific sections of the house while allowing other sections to still be powered?

-M

P.S. I'm SURE I'm misusing the term sub-breakers so I apologize to the electricians here. Hopefuly my question still makes sense. =)

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up vote 11 down vote accepted

The main breaker is sized to trip before the total current draw of the panel exceeds either a) the current carrying capacity of the feed from the electric company, or b) the current carrying capacity of the breaker panel bus bar, whichever is less. So if you order a 100 amp service and your breaker panel is rated for 120 amps, then your main panel should have a 100-amp breaker at the top.

The breakers for the branch circuits are rated to prevent the branch circuit from drawing more power than the branch circuit wiring can handle.

If you look at your panel and do the math, it is very possible that the branch breakers have a total capacity greater than the rating of the main breaker. This is fine because you will typically never have everything drawing it's maximum rated current.

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Also, the main breaker allows you to turn off power to the entire panel - if you are adding new breakers for example (although electricians will do this when they are live, personally I'd rather not take the chance). –  Eric Petroelje Nov 8 '10 at 18:02
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To rephrase: a breaker has two purposes: to protect installed wires from overcurrent, and to function as a manual disconnect. Note that protecting plugged-in devices is not part of the purpose of a breaker - that's up to the user to get right. –  Jay Bazuzi Dec 29 '11 at 18:37
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To add to what @JayBazuzi said. Breakers provide short-circuit protection, overcurrent protection, as well as overload protection of the circuit wires, while also functioning as a means of disconnect. Some may even provide ground fault, and/or arc fault protection. –  Tester101 Feb 14 '13 at 16:40
    
Don't forget that it's also there to protect the public system from you overdrawing current. If the wire from the transformer to your home is rated at 100 A, and you draw 200, you could melt the wire and or blow the transformer (unlikely). The main breaker protects the electric company from you. –  Chris Cudmore Feb 14 '13 at 16:54
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That's what I said: "the current carrying capacity of the feed from the electric company" –  longneck Feb 14 '13 at 17:07
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