2-pole breakers

[Praedatus1]

Okay, I'm sure this has been covered, but I can't seem to locate a thread for it:
On 2-pole breakers, is the rated trip amperage triggered by over-current on either legs AND combined?
For example: 200 main breaker on a residence. Can BOTH legs carry 199 amps at the same time? Or does the main trip when the combined current of both legs pass 200? It just seems interesting to me that a 200 amp service is capable of 400 amps. Please clarify this for me!
Thanks,
Tim

 

[tallgirl]

The breaker trips when either leg exceeds the breaker's rating. So an overload or fault on either leg will trip the breaker, even if the other leg is okay.

A 200A service is 200A @ 240v (assuming a typical residential 120/240 service). That's how you can wind up with 400A @ 120v from it -- each leg is 200A @ 120v.

 

[iwire]

For example: 200 main breaker on a residence. Can BOTH legs carry 199 amps at the same time?

With a standard 200 amp breaker you could code compliantly run a 200 amps through both legs for 3 hours or 160 amps through both legs forever.

That 200 amp load could be made up of a combination of 240 volt loads and 120 volt loads with good balancing.