Can I replace 12 gauge line to attic with 10 gauge when distributed wires will remain 12 gauge. Can I then increase Circuit breaker amps as each distributed line handles little amperage and only short main line to circuit board may have higher amperage. Changing wire from board to attic distribution point is easily done. Would this be Code? Would this endanger House insurance? Dan B.
You haven't said how you will manage the transition from 10 gauge to 12 gauge. If you are thinking of wire nuts in a junction box, then the answer is that you cannot do this safely. The 20A breaker's job is to protect the load wire connected to it. A 12 gauge wire needs a 20A breaker to protect it. You cannot guarantee that a device at the end of one of these runs will not draw in excess of 20A (the rated capacity for 12 gauge wire). Your larger breaker (30A, the maximum 10 gauge is rated for) will not trip in this situation and the 12 gauge wire may start a fire. In addition to being unsafe, it is also not to code. Code requires the breaker to be no larger than the ampacity of the wire and since you're using 12 gauge wire at some point that's your limiting factor.
However, if you are thinking of installing a sub-panel then you may be able to do this. It would be fine (if you follow all the rules for sub-panels) to run 10 gauge wire protected by a 30A breaker to a sub-panel and then run 12 gauge wires each protected by a 20A breaker from the sub-panel. However, this seems a little silly (if you're going to all this work you probably want to run more than 10 gauge to the sub-panel).
If you explain more about your goals you might get more useful advice about what approach to take.
You can do that. You must use a 20A breaker since there is any 12AWG in the circuit.
I gather that's not what you're after.
A single large load is typically 1440W, leaving 960W remaining on the 2400W breaker. Clearly the other loads are not small!
Since you do have an easy time running new cable from the service panel to the distribution point, run additional cables and use the distribution point to split the circuit into several circuits with additional breakers.
You were going to buy 10AWG and scrap 12AWG. I'd have you buy 12AWG and keep the other. Ideally run two 12AWG (for about the price of one 10) - wire and breakers are cheap, and now you have a legal 60A instead of illegal 30A.
Upgrading your wire is allowed.
But remember, the role of the breaker is to protect the wires connected to the breaker. Breakers are not protecting the loads, or whatever is connected to the outlets, breakers protect the wires.
So, that's simple: look at the wires which are downstream of the breaker, and size your breaker to protect those wires.