Electrical
Voltage Drop Calculator
Voltage drop is the reduction in voltage across a resistance when current flows through it. This calculator determines the voltage drop, percent voltage drop, and the resulting voltage at the load. It supports both direct circuit resistance and resistance-per-length modes. It does not perform code-compliant cable sizing.
TypeInteractive calculator — separate from unit conversions
Calculator
Result
Voltage drop2 V
Percent drop8.33333%
Load voltage22 V
Formulas
Voltage drop (direct resistance)
V_drop = I × R
Voltage drop (per-length mode)
V_drop = I × r × L
Percent voltage drop
% drop = V_drop / V_supply × 100
Load voltage
V_load = V_supply − V_drop
Diagram
Worked example
A 24 V supply feeds a load through a cable with total circuit resistance of 0.4 Ω. The load draws 5 A. What is the voltage drop?
- 01V_drop = I × R = 5 × 0.4 = 2 V
- 02% drop = 2 / 24 × 100 = 8.333%
- 03V_load = 24 − 2 = 22 V
Result
The voltage drop is 2 V (8.33%). The voltage at the load is 22 V.
FAQ
Is this a code-compliant cable sizing calculator?
No. This calculator determines voltage drop from user-supplied resistance. It does not select conductor sizes or apply code-mandated derating. For code compliance, consult NEC, IEC, or your local standard.
Does this select the conductor size for me?
No. You must supply the circuit resistance (or resistance per length and cable run length). No conductor sizing table is implemented.
Does this apply temperature or installation derating?
No. Resistance is used as entered. Temperature correction and installation method derating are not applied.
What is "total circuit length"?
It is the total conductor length for both the supply and return conductors. For a single-phase cable run of 50 m one-way, the total circuit length is 100 m (50 m out + 50 m return).