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Switchboards are designed with busbars for fuses or circuit breakers that are used for protecting and supplying feeders and branch circuits. Panelboards are designed with a permanently installed main circuit breaker or single set of fuses for disconnecting the number of circuit breakers or fuseholders installed to protect and supply feeders or branch circuits.

SUPPORT AND ARRANGEMENT OF BUSBARS AND CONDUCTORS
408.3

Busbars shall be held firmly in place in the enclosure and shall be arranged in such a way to allow adequate space for conductors to be terminated. Busbars shall be sized with enough capacity to handle the normal currents in amps of the electrical system. In addition, they shall carry safely any type of short-circuit or ground-fault condition.

CONDUCTORS AND BUSBARS ON A SWITCHBOARD OR PANELBOARD
408.3(A)

Conductors and busbars on a switchboard, panelboard, or control board shall be so located as to be free from physical damage. Service sections of such boards shall be separated from the other electrical components. These components shall be properly secured in an approved manner. 

Note, conductors entering a certain section shall terminate in such section, except where it is not practical for control, interconnecting conductors, or other conditions of use.

OVERHEATING AND INDUCTIVE HEATING
408.3(B)

The arrangement of busbars and conductors shall be such as to avoid overheating due to inductive effects. The inductive effect of each conductor is neutralized when grouping all conductors together. To accomplish the minimizing of heating effects, the ungrounded (phase) conductors, grounded (neutral) conductors, and equipment grounding conductors, where used, shall be in the same enclosure, raceway, cable, cord, etc. (See Figure 7-1)