240V circuit breaker amperage question
I was wondering how the amperage of 240V circuits works. I need to add a few new 240V circuits in my garage for tools and was not sure how CB labeled amperage differs from total circuit amperage and the amperage marked on the tool.
First, if you have a 240V double pole CB, for instance a CB that is labeled 20A, that means 20A on EACH of the hot legs, so the total circuit amperage would actually be 40A. Is that correct? I understand that folks say when voltage doubles, amperage is halved. Is that because you're dealing with two legs instead of only one?
The reason I ask about total circuit amperage is I am trying to determine what wire size I need to use and the chart I am looking at goes by length, voltage, conductor material and total circuit amperage, so I need to know if they is different from the amperage labeled on the CB.
Next, one of the tools is an air compressor, and it is labeled 15A on the motor. Is there any rule of thumb for sizing a CB other than match the label on the load? So in other words, if the motor is labeled 15A, I use a 15A CB?