Why does my circuit breaker adopt #6 Cu, and#4 Al? I have a Square D, QO250GFI breaker. I'd similar to to...

I have a Square D, QO250GFI breaker. I'd similar to to use #4 Stranded Cu with it. But it one and only goes to #6 for Cu? Why couldn't I only install #4 Cu if it will fit?
Ive get a flouresent muted box that be...
Answers:    You can install the #4. Skipjack is right, it has a better capability after #6. The issue is make sure the amp draw to doesn`t matter what device you power is limited by the breaker.
#4 copper can fetch between 70 and 85 amps , but for no more then 75 foot. The wire may be oversized because of the length it travels.

For the most bit, if you're replacing a 60amp breaker .. with a 60amp breaker you should be fine. Just be sure the equipment your running and the breakers amp draw ratings contest. Should be a label somewhere on the equipment that say " maximum overcurrent protection". Thats the number you want.
I am looking for a fresh Entry door...
Because copper can carry more current than aluminum lacking getting too hot.