Voltage Drop is a measurement of the amount of voltage dropped through the length of a circuit given the circuit conductor size and material and the conditions under which the circuit is constructed and operated. The voltage drop calculated is the percent voltage loss from the originating source voltage. The U.S. National Electric Code establishes a maximum percentage loss acceptable for certain circuit uses.
Where VD is the percent voltage drop, Kp is a constant for the circuit phase (single, 3-phase, etc.), L is the length of the run, Km is a constant for the conductor material (i.e., copper, aluminum, etc.), I is the circuit load current, CM is the size of the conductor in circular mils, and E is the source voltage.
The following tables are used for the constant values (defaults in bold):
Note that the approximate K constants used here are for uncoated copper (Cu) and compact aluminum (Al). The formula used to derive the exact value is:
Where R is the wire resistance in ohms per 1000 feet and CM is the wire size in circular mils. Both of these values are taken from NFPA 70 Table 8.