Electrical calculators
Voltage Drop Calculator
Estimate voltage loss along a copper or aluminum cable using system voltage, load current, one-way length, conductor size, material, and circuit type.
Measurement System
Switch units without changing the underlying calculation.
Circuit path
V
Length × current × resistance
V − drop
Longer runs, higher current, and smaller conductors increase voltage loss. Check the result against the project design and applicable code.
Input Dimensions
Calculation Results
Planning estimate only
Results are based on the values and assumptions entered here. Confirm quantities with current product information and suppliers; use qualified local professionals for structural, safety-critical, permitted, or code-regulated work.
Read the calculator terms and limits →How to Use the Voltage Drop Calculator
Enter the nominal system voltage and the current drawn by the load. Then enter the one-way distance from the source to the load. Select the conductor size, choose copper or aluminum, and identify whether the circuit is single-phase/DC or three-phase. The calculator uses common conductor resistance values to estimate the loss along the run.
Choose a design target of 3% or 5% to receive a simple planning check. A result above the selected target does not automatically mean the installation is unsafe or noncompliant, and a result below it does not complete the electrical design. Actual conductor selection also depends on ampacity, insulation temperature, installation method, ambient temperature, bundling, overcurrent protection, equipment instructions, and local code.
Understanding Voltage Drop
Voltage drop is the reduction in voltage that occurs as current travels through conductor resistance. Longer cable runs, larger loads, and smaller conductors generally create more voltage loss. Sensitive equipment, motors, lighting, controls, and low-voltage systems may respond poorly when the voltage at the load is lower than expected.
For single-phase or DC circuits, the estimate includes the outgoing and return path. For three-phase circuits, the calculator uses the standard square-root-of-three relationship for a balanced run. These are planning formulas, not a substitute for an engineered design or a manufacturer’s installation instructions.
Safety and Limitations
This calculator is a preliminary planning tool. It does not approve a wire size, breaker, service, installation method, or code compliance. Electrical work can cause shock, fire, equipment damage, or death when improperly designed or installed. Have the final design reviewed and the work performed or inspected by a qualified electrical professional under the applicable local requirements.
Frequently Asked Questions
How is voltage drop calculated?
This calculator estimates voltage drop from circuit current, one-way cable length, conductor resistance, and the circuit factor: 2 for single-phase or DC and √3 for three-phase.
What voltage drop percentage should I target?
Three percent is a common design target for a branch circuit, while five percent is often used as a total-circuit planning target. The applicable electrical code, equipment instructions, and project design control.
Does cable length mean one-way or total length?
Enter the one-way distance from the source to the load. The calculator applies the return-path factor for single-phase or DC circuits.
Can this calculator select the final wire size?
No. It is a planning estimate using common resistance values. Final conductor sizing must also consider ampacity, installation method, temperature, bundling, protection, code, and equipment requirements.