Voltage Drop Calculator

This calculator estimates the voltage drop in electrical circuits based on wire parameters, current, and distance. Select a calculation method: NEC data, estimated resistance, or custom values.

Input Parameters

Voltage Drop Information

What is Voltage Drop?

Voltage drop is the reduction in electrical potential along the path of a current flowing in an electrical circuit. Voltage drop occurs in conductors (wires) due to their inherent resistance or impedance.

When electrical current flows through a wire, some of the electrical energy is converted to heat due to the resistance of the wire. This energy conversion results in a decrease in voltage along the length of the wire.

For example, if the voltage at the source is 120V but only 115V reaches the load, there is a 5V voltage drop in the circuit.

What is Voltage Drop?

For example, if the voltage at the source is 120V but only 115V reaches the load, there is a 5V voltage drop in the circuit.

How to Use This Voltage Drop Calculator

This calculator offers three methods to calculate voltage drop in electrical circuits:

Select a calculation method: NEC data, estimated resistance, or custom values.
Enter the required parameters such as wire material, size, distance, current, etc.
For custom calculations, enter your own resistance and reactance values.
Click the Calculate button to see the results.
Review the voltage drop value, percentage, and evaluation.

For most residential and commercial applications, voltage drop should be limited to 3% to ensure proper operation of electrical equipment.

Voltage Drop Calculation Formulas

Voltage drop calculations depend on the type of current (DC or AC) and phase configuration. The following formulas are used:

DC Voltage Drop Formula

$$V_{drop} = 2 \ imes I \ imes R \ imes L$$

For DC circuits: Vdrop = 2 × I × R × L Where: Vdrop = Voltage drop (V) I = Current (A) R = Resistance per unit length (Ω/length) L = One-way conductor length

AC Single-Phase Voltage Drop Formula

$$V_{drop} = 2 \ imes I \ imes (R \ imes PF + X_L \ imes \sin\ heta) \ imes L$$

For single-phase AC circuits: Vdrop = 2 × I × Z × L × PF Where: Vdrop = Voltage drop (V) I = Current (A) Z = Impedance per unit length (Ω/length) L = One-way conductor length PF = Power factor

AC Three-Phase Voltage Drop Formula

$$V_{drop} = \sqrt{3} \ imes I \ imes Z \ imes L \ imes PF$$

For three-phase AC circuits: Vdrop = √3 × I × Z × L × PF Where: Vdrop = Voltage drop (V) I = Current (A) Z = Impedance per unit length (Ω/length) L = One-way conductor length PF = Power factor

Impedance Calculation

$$Z = \sqrt{R^2 + X^2}$$

For AC circuits, impedance (Z) is calculated as: Z = √(R² + X²) Where: Z = Impedance (Ω) R = Resistance (Ω) X = Reactance (Ω)

Voltage Drop Percentage

$$V_{drop\%} = \rac{V_{drop}}{V_{source}} \ imes 100\%$$

Voltage drop percentage is calculated as: Vdrop% = (Vdrop / Vsource) × 100% Where: Vdrop% = Voltage drop percentage Vdrop = Voltage drop (V) Vsource = Source voltage (V)

Applications and Recommendations

Understanding and calculating voltage drop is critical in many electrical applications:

Residential Wiring

In homes, excessive voltage drop can cause dimming lights, reduced appliance performance, and even damage to sensitive electronics. The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total including feeder and branch circuit.

Commercial Buildings

Commercial buildings often have longer wire runs and higher loads, making voltage drop calculations essential for proper equipment operation and energy efficiency.

Industrial Applications

In industrial settings, voltage drop can affect motor performance, reduce equipment lifespan, and increase energy costs. Proper wire sizing based on voltage drop calculations is crucial.

Recommended Maximum Voltage Drop

Application	Recommended Max Drop
General Lighting and Receptacles	3%
Motors and Power Circuits	3%
Combined Load (Feeder + Branch) - Lighting	5%
Combined Load (Feeder + Branch) - Motors	5%

Frequently Asked Questions

Why is voltage drop important?

Voltage drop is important because excessive voltage drop can cause equipment malfunction, reduced efficiency, overheating, and potential damage to electrical devices. It can also lead to higher energy costs due to power losses in the conductors.

How can I reduce voltage drop in my circuit?

You can reduce voltage drop by using larger wire sizes (lower gauge number), using copper instead of aluminum conductors, reducing the circuit length, or reducing the load current. In some cases, increasing the system voltage may also be an option.

What is the difference between resistance and impedance?

Resistance is the opposition to direct current (DC) flow, while impedance is the total opposition to alternating current (AC) flow, which includes both resistance and reactance (inductive and capacitive effects).

How does the NEC (National Electrical Code) address voltage drop?

The NEC recommends (but does not require) that voltage drop be limited to 3% for branch circuits and 5% for the combined feeder and branch circuit. These recommendations are found in NEC 210.19(A)(1) Informational Note No. 4 and NEC 215.2(A)(1) Informational Note No. 2.

Does wire temperature affect voltage drop?

Yes, wire temperature does affect voltage drop because the resistance of conductors increases with temperature. Most voltage drop calculations are based on conductor temperatures of 75°C (167°F) for typical installations.

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