Why does my circuit breaker accept #6 Cu, and#4 Al?

Question

I have a Square D, QO250GFI breaker. I'd like to use #4 Stranded Cu with it. But it only goes to #6 for Cu? Why couldn't I just install #4 Cu if it will fit?

Answer 1

You can install the #4. Skipjack is right, it has a higher capability then #6. The issue is make sure the amp draw to whatever device you power is limited by the breaker.
#4 copper can carry between 70 and 85 amps , but for no more then 75 feet. The wire may be oversized because of the length it travels.

For the most part, 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 match. Should be a label somewhere on the equipment that says " maximum overcurrent protection". Thats the number you want.

Answer 2

Because the larger diameter of the wire would not allow the screw assembly to torque down enough turns to safely encapsulate the wire. This could cause excessive heat build-up at the breaker potentially damaging the breaker shell.

Answer 3

You can install the largest size that will fit. It is always safe to use a larger size, but if you use Al. you must use a larger size than if you use Copper. This is what they are telling you. And in a house Al.

Answer 4

Because copper can carry more current than aluminum without getting too hot.