Basic 220V breaker question

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  #1  
Old 03-05-07, 08:25 AM
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Basic 220V breaker question

I have a Square D panel and the 220V breakers are "double wide" obviously. I have often wondered what the breaker Amp rating really means. If I have a 220V circuit with a "30 Amp" breaker, does that mean that each pole has a 15 amp or a 30 amp rating? In the "old" days I remember seeing double or 220V breakers with a metal bar that connected the two switches. Nowadays most 220V breakers, I see, have a single switch.

I've been wondering this as I am installing a transfer switch for a generator. The instructions specify a 60A breaker. I know the generator is rated to 30A (7,000W). I am confused.
 
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Old 03-05-07, 09:10 AM
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Re: If I have a 220V circuit with a "30 Amp" breaker, does that mean that each pole has a 15 amp or a 30 amp rating? -

Each pole has a 30 amp rating, or the same rating as the overall breaker. It just has two separate breakers to handle two 110 volt circuits of opposite phase, 2 x 110 = 220 volt.


Re: In the "old" days I remember seeing double or 220V breakers with a metal bar that connected the two switches. Nowadays most 220V breakers, I see, have a single switch.

Nowadays, there are still two switches (breakers) within the 220V breaker, but there is just 1 handle to actuate. Maybe it's a safety thing so they can't come apart? Maybe it's just cheaper to make that way?

60A breaker for a 30A generator might be to handle some high surge loads without tripping or damaging the breaker contacts. In that current range, I believe the Square D breakers cost the same. I bring my 4KW (18.2 Amp) generator in through a 10 gage, 30A breaker setup.
 
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Old 03-05-07, 09:14 AM
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In your example of the 2 pole 30, each leg could draw up to 30 amps. You do not add up the ampacity of each leg, ie the 2 pole 30 does not make a 60 amp breaker.
 
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Old 03-05-07, 10:06 AM
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Then I find it strange that the instructions recommend a 60 amp breaker for a 30A/7000W peak generator transfer switch. They recommend 10 gauge wire. I don't think 10 gauge can handle 60 amps. Am I wrong? Could the spike loads be double the rating? Is it assumed that 10 gauge can handle 60A for a short spike?
 
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Old 03-05-07, 10:42 AM
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The ampacity (how much current it can carry) of wire is not merely a function of the size of the wire. It is also a function of the application. The National Electrical Code generally allows manufacturers to specify the wire requirements of their equipment. This highlights the problems with the simple ampacity charts you see in home centers.
 
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Old 03-06-07, 04:29 AM
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A transfer switch is a type of sub panel. The breaker that it feeds it from the main panel is sized based on the specifics of the transfer switch and the size of the wires used to feed the sub panel. Generally speaking, it is always larger than the amount of current that the generator is rated for.
 
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