I have 6AWG wiring with a 50amp breaker; no device is yet connected to the circuit. I need to swap the 50amp breaker for a 40amp breaker. I have popped the breaker loose from the bus bar in the panel. Are the red and black wires I need to transfer to the 40amp breaker actually live wires? I'm not going to touch them but rather just wanted to know for peace of mind.
The metal bar that the breaker connects to is hot. The breaker provides power to the connected wires, and cuts power to them when it is off. So, if everything else is working correctly, the wires connected to the breaker do not have any power.
While it's possible to not have a main breaker in that panel to cut off all power, there's normally a switch or a breaker in another box that will do it. The bus bars inside a panel are large and easy to touch, so its best to turn off power to the whole panel when swapping breakers. Cutting off all power also keeps you safe from wiring mistakes that could result in power being fed back into the panel from another, seemingly unrelated breaker.
If you rock the breaker out of its snapped down position, the breaker disengages from the bus bars and should be cold. I frequently change CH breakers this way.
However, if the breaker is back-fed from a generator or mains, then all bets are off. However back fed breakers are supposed to bolted down!
All that said... there is a much easier way to de-energize a wire. Move the breaker's onboard switch to the position marked "Off".
On Cutler Hammer CH and BR types, as well as CL types, that should be away from the middle. When 2 breakers are connected side by side, for both to be on, they must be toward each other. If a piece of metal the right size was there, they couldn't both be on at once. That is a very cheap way to do a generator interlock, and they make factory authorized kits that cost as little as $25. These kits also typically strap the breakers to each other, to satisfy the bolt-down requirement.
Do not always think power is off,because breaker is off.That breaker could have been fed from another panel.If that was the case,the breaker is off but the conductors still have power.You would need to shut the other breaker down. Always double check for live power,and use a meter to test it. This could save your life. And if you are in doubt shut the main off .
The code allows using wire larger than required. It is actually required when wire runs are longer than normal, and has the advantage of lower voltage and power loss when loaded heavily. The code expects up to 3% voltage drop, which represents about 3% wasted power, and stresses AC motors. Motors tend to supply constant power, which requires extra current at the reduced load voltage. There can be problems if the wire doesn't fit inside the wire hole of the device, but they will usually accommodate at least a single up-step. If not, a short pigtail is needed before the device.