When you install a fuse or circuit breaker, you must install it at the point where the conductors receive their supply [240.21]. Because this rule exists, we define a circuit by its breaker [110.22]. But there are exceptions to this, and those are listed in Sec. 240.21(A) through (H). This list takes up nearly three pages, so bears some explanation.
Where Do Breakers Go?
We define a circuit by its breaker [110.22], but there are exceptions to this.