Fine Print Note No. 4 discusses voltage drop. Although the NEC doesn't consider voltage drop to be a safety issue, except for fire pumps (695.7), size branch-circuit conductors to prevent a voltage drop not to exceed 3%. This will ensure reasonable efficiency of operation of electrical equipment. In addition, don't allow the maximum total voltage drop on feeders and branch circuits to exceed 5%.

Conductor voltage drop can be determined by multiplying the current flowing through the circuit by the resistance of the circuit conductors: Voltage drop = I x R. Where (I) is equal to the load in amperes and (R) is the resistance of the conductor [Chapter 9, Table 8 for direct current circuits, or Chapter 9, Table 9 for alternating-current circuits]. For three-phase circuits, simply adjust the single-phase voltage drop value by a multiplier of 0.866.

Branch-circuit conductors that supply more than one receptacle for cord-and-plug connected portable loads must have an ampacity no less than the rating of the circuit [210.3 and 240.4(D)].

Branch-circuit conductors that supply household ranges, wall-mounted ovens, or counter-mounted cooking units must have an ampacity no less than the rating of the branch circuit and no less than the maximum load to be served. For ranges of 8.75kW or more rating, the minimum branch-circuit ampere rating must be 40A. Note the Code does offer a few exceptions to this rule.

Branch-circuit conductors must have an ampacity sufficient for the loads served and must not be smaller than 14 AWG. An exception notes that tap conductors must have an ampacity no less than 15A for circuits rated less than 40A and no less than 20A for circuits rated at 40A or 50A for the following loads: individual outlets, other than receptacle outlets, with taps no more than 18 inches in length. Note that branch-circuit tap conductors aren't permitted for receptacle outlets.