Voltage Drop Explained
Voltage drop is the loss of voltage across a conductor as current flows through its resistance. Learn the formula, percent drop, and the difference between calculation and cable sizing.
Definition
Voltage drop is the reduction in voltage across a conductor or cable as current flows through its resistance. Every real conductor has finite resistance, so some of the supply voltage is 'lost' driving current through the cable rather than being delivered to the load. The voltage available at the load equals the supply voltage minus the total voltage drop across the cable run.
Why it matters
Excessive voltage drop can cause instruments to malfunction, motors to underperform, and heaters to deliver less power than specified. In instrumentation, a long cable run to a 4-20 mA transmitter may reduce the available voltage below the minimum required for the transmitter to operate. Understanding voltage drop lets engineers verify that the load receives adequate voltage before selecting or approving a cable run.
Formula
Units involved
- •V (volts) — voltage at source and load
- •I (amperes) — current flowing through the cable
- •R (ohms) — total resistance of the cable run (outgoing + return conductors)
- •% — percent voltage drop relative to supply voltage
Concept diagram
Worked example
A 24 V supply feeds a transmitter through a cable with total round-trip resistance of 50 Ω. The loop current is 20 mA (0.020 A). What voltage reaches the transmitter?
- 01V_supply = 24 V, I = 0.020 A, R_cable = 50 Ω
- 02V_drop = I x R_cable
- 03V_drop = 0.020 x 50 = 1.0 V
- 04% drop = (1.0 / 24) x 100 = 4.17%
- 05V_load = 24 - 1.0 = 23.0 V
The voltage at the transmitter is 23.0 V — a 4.17% drop. The transmitter should operate normally if its minimum supply requirement is below 23 V.
Common mistakes
- •Forgetting to include both outgoing and return conductors. The total cable resistance is the round-trip resistance, not just one conductor leg.
- •Using resistance per unit length without multiplying by both the cable length and the number of conductors (typically 2 for a simple loop).
- •Confusing voltage-drop calculation with code-compliant cable sizing. This calculation finds the voltage lost across a known resistance — it does not select a cable size or check compliance with NEC, IEC, or any electrical code.
- •Assuming cable resistance is constant. Resistance increases with temperature, so a cable in a hot environment has higher resistance than its rated value at 20 °C.
When to use the calculator
Use the Voltage Drop calculator to find the voltage drop, percent drop, or load voltage for a given current and cable resistance. Enter any combination of supply voltage, current, and resistance to see the result.