🧮 OmniCalc

Voltage Drop Calculator

Calculate the voltage drop across a conductor or component using Ohm's Law. Enter the current flowing through the circuit and the resistance to find the voltage drop in volts.

Voltage Drop5 V
Power Dissipated
50 W

Voltage drop is calculated using Ohm's Law: V = I x R. This assumes a purely resistive load (DC or unity power factor). For AC circuits with reactive components, use impedance instead of resistance.

What the Voltage Drop Calculator Does

This tool calculates the voltage drop across a resistive load or conductor when you know the current flowing through it and its resistance. Enter the current in amperes (A) and the resistance in ohms (Ω), and the calculator returns the voltage drop in volts (V).

It is useful for electricians, electronics hobbyists, students, and anyone troubleshooting a DC circuit. Typical uses include checking the drop across a length of wire, a resistor, a heating element, or a connector, so you can confirm enough voltage actually reaches the device at the end of the run.

How It Works: The Voltage Drop Formula

The calculator uses Ohm's law for a resistive element:

V = I × R

where V is the voltage drop in volts, I is the current in amperes, and R is the resistance in ohms. The result is the difference in electrical potential between the two ends of the resistance, measured along the direction of current flow.

This relationship assumes a purely resistive load. It does not account for the reactance found in AC circuits with motors, transformers, or capacitors, where impedance replaces simple resistance.

Worked Example With Real Numbers

Suppose you run a 12 V DC circuit drawing 8 A through copper wire that has a total resistance of 0.15 Ω over the round-trip length.

V = I × R = 8 A × 0.15 Ω = 1.2 V

The wire drops 1.2 V, so the load receives 12 V − 1.2 V = 10.8 V. That is a 10% drop, which is high for most applications. As a quick second example, a 2 A current through a 3.3 Ω resistor drops V = 2 × 3.3 = 6.6 V.

Factors That Affect Voltage Drop

Several physical factors change the resistance, and therefore the drop, in real wiring:

  • Conductor length: longer runs mean more resistance and a larger drop, since resistance is proportional to length.
  • Wire gauge (cross-sectional area): thicker wire (lower AWG number) has less resistance and less drop.
  • Material: copper has lower resistivity than aluminum, so aluminum drops more for the same size.
  • Temperature: copper resistance rises roughly 0.4% per degree Celsius, so hot conductors drop slightly more.
  • Current: drop scales directly with current, so heavier loads increase the drop.

Practical Tips and Common Mistakes

Use the round-trip conductor length, not the one-way distance. Current flows out to the load and back, so a 25 ft cable run involves 50 ft of conductor when you calculate total resistance.

Many guidelines recommend keeping voltage drop at or below 3% for branch circuits and 5% for the combined feeder and branch path. Compare your result against the supply voltage to see the percentage.

Keep your units consistent: amperes and ohms in, volts out. Mixing milliamps with amps or using one-way length are the most frequent errors. For AC circuits with motors or other reactive loads, this resistive formula will underestimate the real drop.

Frequently asked questions

How is voltage drop calculated?

Voltage drop is found with Ohm's Law: voltage drop equals current (in amperes) multiplied by resistance (in ohms). For example, 10 A through 0.5 ohms gives a 5 V drop.

Why does voltage drop matter?

Excessive voltage drop reduces the voltage delivered to a load, which can cause poor performance, dimming lights, overheating, and inefficient operation. Electrical codes often limit drop to about 3-5% of the supply voltage.

Does this work for AC circuits?

This calculator uses pure resistance, which is exact for DC and for AC with a unity power factor (purely resistive loads). For AC with significant inductance or capacitance, substitute the circuit impedance for resistance.

How can I reduce voltage drop?

Use a larger conductor (lower resistance), shorten the cable run, reduce the current, or raise the supply voltage so the same power is delivered at lower current.