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01 Electrical DeWalt 8/29/05 4:20 PM Page 13 VOLTAGE DROP FORMULAS The NEC® recommends a maximum 3% voltage drop for either the branch circuit or the feeder. Single-Phase: 2xRxIxL VD = ____________ CM Three-Phase: 1.732 x R x I x L VD = ________________ CM VD = Volts (voltage drop of the circuit) R = 12.9 Ohms/Copper or 21.2 Ohms/Aluminum (resistance constants for a conductor that is 1 circular mil in diameter and 1 foot long at an operating temperature of 75O C.) I = Amps (load at 100 percent) L = Feet (length of circuit from load to power supply) CM = Circular-Mils (conductor wire size) 2 = Single-Phase Constant 1.732 = Three-Phase Constant CONDUCTOR LENGTH/VOLTAGE DROP Voltage drop can be reduced by limiting the length of the conductors. Single-Phase: L= CM x VD ____________ 2xRxI Three-Phase: CM x VD L = ________________ 1.732 x R x I CONDUCTOR SIZE/VOLTAGE DROP Increase the size of the conductor to decrease the voltage drop of circuit (reduce its resistance). Single-Phase: 2xRxIxL CM = ____________ VD Three-Phase: 1.732 x R x I x L CM = ________________ VD