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Old 09-27-2008, 08:10 AM   #1
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2p breakers, ratings, and amp capacity


Hey folks,

I've been lurking here for a while, but this is my first post. I apologize if these are stupid questions.

I have a 30amp 2 pole breaker in my main panel that feeds a small 2 slot sub panel right next to the main. I'm considering replacing the existing sub panel with a 100 amp, main disconnect type panel. I'm a little confused about the amp capacity of a 2 pole breaker.

Does the 30 amp 2 pole breaker provide 30 amps at 220v or 60 amps at 110v?
If used to feed a sub panel, do you have to worry about balancing the loads on the 2 legs to not exceed 30amp@110v on either leg?
Is 100amp service 100amps@220v, 200amps @110v?

I have a bryant main panel. What's the most economical breaker to use to feed a 100amp sub panel? What wire size do I need to run from the main to the sub (the subs right next to the main, maybe 3 feet of wire total)? I'm pretty sure they don't sell NM by the foot, so what kind of wire/conduit is best for this app?

Thanks for the help.

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Old 09-27-2008, 08:37 AM   #2
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2p breakers, ratings, and amp capacity


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Originally Posted by reverend_maynard View Post
Hey folks,

I've been lurking here for a while, but this is my first post. I apologize if these are stupid questions....Thanks for the help.
FWIW: There is no such thing as a "stupid Question" on this site.

....Once in a while there may be a "stupid answer".

All members, especially new ones, are encouraged to just "ask-away"....

Welcome to the site.


Last edited by AtlanticWBConst.; 09-27-2008 at 09:31 AM.
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Old 09-27-2008, 08:58 AM   #3
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2p breakers, ratings, and amp capacity


Hi, and welcome to the forum. You will always find answeres here.

My first question is why do you want a sub panel?
Have you run out of breaker slots in the main?
For 30A, I would just put the breaker in the main panel and forget the sub.

That said;
The total current through each pole of the breaker cannot exceed the breaker's rating.
If you had all 110V loads, you could pull 100A from each leg, thus a total of 200A @ 110V.
If you had all 220V loads, you could pull 100A total.

As far as the sub panel goes, I am curious as to why you want a 100A sub-panel off a 100A main panel?
It would make sense if you had a 40 or 50A sub, or maybe a 100A sub off a 200A main.

As for wire size, you must use the size of wire that matches the breaker you are using in the main panel.
For 30A, you would need #10. Since you have 30A double pole breaker, there is potential to draw up to 60A total from both legs, so you do need to watch the balance between legs so as not to overload the neutral wire.
Alternatively, you could run a heavier wire so you don't have to worry about the balance, but it's always best to balance your loads between legs.

It is essential to the POCO that people do this in their homes, since if there was a huge imbalance, there would be excessive current in the neutral (and on one half of the transformer) on the street, and usually that wire is not as heavy as the two hot legs (at least here in the US, and on my street).

FW
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Old 09-27-2008, 09:03 AM   #4
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2p breakers, ratings, and amp capacity


Correction:
In my last post, I said that you would need to watch the balance between legs in the sub panel to avoid overloading the neutral wire. That was incorrect.
If the breaker is rated at 30A and is double pole, each leg cannot carry more than 30A, which means that the current in the neutral will never be higher than 30A.
Neutral current will drop as the legs are balanced out.

Sorry about the goof.

FW
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Old 09-27-2008, 09:25 AM   #5
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2p breakers, ratings, and amp capacity


Thanks for the answers, that's pretty much what I thought.

The reason I want to add a sub panel is because my main is only a 12/20 slot, already has 2 tandem breakers in it, and has no more room on the neutral/ground buses. I don't have any large loads, so 100amp service is plenty, but I would like more room for more breakers. I figure if I use a larger than needed sub panel then if I did need the extra capacity and wanted to upgrade the service, I could upgrade the main panel without worrying about moving the stuff that's connected to the sub. I have the 30amp breaker now, but I would probably replace that with at least a 50amp. I should be able to easily replace the breaker and the feeder wire to increase the capacity of the sub later if needed.
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Old 09-27-2008, 10:50 AM   #6
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2p breakers, ratings, and amp capacity


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Originally Posted by reverend_maynard View Post
....I should be able to easily replace the breaker and the feeder wire to increase the capacity of the sub later if needed.
Up to the ampacity rating of the sub-panel.
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Old 09-27-2008, 12:41 PM   #7
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Up to the ampacity rating of the sub-panel.
Which is why I want to use a 100amp panel now.

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