Hooking up a GFI to Jacuzzi 220 v olt 40 amp, why can't I use a Circuit breaker in it's place?
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A hot tub is required by the National Electric Code to be protected by a ground fault protection device.

TK has given you te correct but brief answer, it's the law. The law, the code was created to protect people from electric shock and death in wet areas. GFCI in wet areas and ARC faults in bedrooms, it's the law made to keep you alive. If you go to Home Depot or Menards they sell a special package deal for hot tubs, everything you need in one box.

Circuit breakers only work with overloaded circuits. GFCI on the other hand have no relation to overload, they react to imbalance, particularly noted with a shortcoming in the ground part of the system, and trip. Saving money by using a breaker alone is foolhardy when it comes to water.

Do the installation instructions require it?
If so... NEC 110.3(B) requires it as well or the owner will accept
All liability for damage or injury.

A circuit breaker is designed to "Open" in the event of an overcurrent (high amperage) on the branch circuit.

A "GFCI" gevice is designed to protect people from getting themselves killed. It reacts when it detects a .6 difference in amperage between the hot and neutral of the connected circuit.

You can get a "GFCI" breaker, but that means that any fault detection that trips the GFCI circuit inside the breaker will make you walk to the breaker to reset it.

Clear as mud?