Subsection 21 “Location in Circuit” is by far the longest subsection in Art. 210 of the NEC. At nearly two and a half pages, it occupies about 85% of Part II. Of the remaining text, half a page consists of 240.24, which is titled “Location in or on Premises.” The title of Part II is “Location”; its other two subsections are about the grounded conductor. So with Part II literally telling us location, location, location we can see that where you install the overcurrent protection is of critical importance.
The two requirements presented by subsection 240.21 are that overcurrent protection be:
1) Provided in each ungrounded circuit conductor.
2) Located where the conductors receive their supply.
The second requirement has eight Exceptions [240.21(A) - (H)], and these pertain to specific applications. For example, feeder taps don’t need overcurrent protection at the tap if they meet the requirements of 240.21(B).
What about those grounded conductor requirements? You can’t put an overcurrent protection device in a grounded conductor unless it opens all the conductors (and no pole can operate independently) or is required (by 430.36 or 430.37) for motor overload protection.
The requirements of 240.24 include such common sense things as not exposing overcurrent protection devices to damage or locating them near easily ignitable material.