Voltage Drop Formula
By John Houdek, Power Quality Editor
Voltage drop formula defines the mathematical relationship between current, resistance, conductor length, and impedance used to calculate voltage drop using V = I × R, including single-phase and three-phase electrical equations expressed in standard electrical units.
Voltage drop formula expresses how electrical variables are combined to determine voltage drop values using current, resistance, conductor length, and impedance. These equations define how to calculate voltage drop using standard electrical variables.
Voltage drop is expressed in volts and is calculated using standard electrical equations relating current and resistance. These formulas provide the direct mathematical expression used to calculate voltage drop values.
Voltage Drop Formula Equations
The fundamental voltage drop relationship is:
V = I × R
Where resistance represents the total conductor resistance over the circuit length.
Voltage drop may also be expressed in terms of conductor length and resistance per unit length.
Single Phase Voltage Drop Formula
For single-phase circuits:
Vdrop = 2 × I × L × R
Where resistance is expressed per unit length of conductor.
Three Phase Voltage Drop Formula
For three-phase circuits:
Vdrop = √3 × I × L × R
Voltage Drop Formula Variables
Each variable in the voltage drop formula represents a specific electrical quantity:
Vdrop = Voltage drop (volts)
I = Current (amperes)
R = Resistance per unit length (ohms per meter or ohms per 1000 feet)
L = One-way conductor length (meters or feet)
Z = Impedance (ohms), used when reactance is included
When impedance is used:
Vdrop = I × Z
Resistance and Impedance Representation
Resistance is expressed per unit length and is combined with the conductor length to determine the total resistance.
Impedance is expressed in ohms and includes resistance and reactance components used in electrical equations.
Unit Conventions
Voltage drop is expressed in volts.
Current is expressed in amperes.
Resistance and impedance are expressed in ohms.
Conductor length is expressed in meters or feet.
Consistent units must be used when applying the voltage drop formula.
Related Electrical Relationships
Voltage drop formula is based on standard electrical relationships between current, resistance, and impedance used in electrical equations.
For a broader definition of the concept, see Voltage Drop.
For DC-specific calculation procedures, see DC Voltage Drop Calculation.
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