AC Voltage Drop Calculator

Electrical AC Voltage Drop Calculator

AC phase

Source voltage, V

Load current, A

One-way circuit length

Length unit

Conductor material

Conductor size

Custom circular mils

Enter 0 to use the selected size.

Parallel runs per phase

Power factor

Power factor type

Conductor temperature, °C

Reactance estimate

Custom X, Ω / 1000 ft

Allowed voltage drop, %

Formula Used

Single-phase: Vd = 2 × I × L × (R cosθ ± X sinθ) / 1000

Three-phase: Vd = √3 × I × L × (R cosθ ± X sinθ) / 1000

Here, I is current in amperes. L is one-way length in feet. R is effective conductor resistance in ohms per 1000 feet. X is effective reactance in ohms per 1000 feet. The plus sign is used for lagging power factor. The minus sign is used for leading power factor.

The resistance is adjusted by material, conductor area, temperature, and parallel runs. The calculator then divides voltage drop by source voltage to find the voltage drop percentage.

How to Use This Calculator

Select single-phase or three-phase AC.
Enter the source voltage, load current, and one-way circuit length.
Choose copper or aluminum and select a conductor size.
Enter custom circular mils only when your conductor is not listed.
Add power factor, temperature, reactance, and parallel run details.
Press calculate to view voltage drop, percent loss, power loss, and suggested size.
Use CSV or PDF buttons to save the same calculation.

Example Data Table

Case	Phase	Voltage	Current	Length	Material	Conductor	Power Factor	Typical Result
Branch circuit	Single	120 V	16 A	75 ft	Copper	12 AWG	0.95 lagging	Moderate drop
Small feeder	Single	240 V	40 A	100 ft	Copper	8 AWG	0.90 lagging	Often near target
Motor feeder	Three	480 V	85 A	220 ft	Aluminum	1/0 AWG	0.88 lagging	Check larger size

AC Voltage Drop Planning Guide

Why Voltage Drop Matters

Voltage drop happens when current moves through real conductors. Every conductor has resistance. AC circuits also have reactance. These values reduce the voltage available at the load. A small drop is normal. A large drop can cause dim lights, weak motors, warm conductors, nuisance trips, and poor equipment performance.

Important Inputs

This calculator uses phase, current, source voltage, one-way distance, conductor material, conductor area, temperature, power factor, and reactance. Each input changes the final result. Longer runs increase drop. Higher current increases drop. Aluminum usually drops more voltage than copper for the same size. More parallel runs reduce effective impedance.

Single-Phase and Three-Phase Circuits

Single-phase calculations use a round-trip conductor factor of two. Three-phase calculations use the square root of three. This is why a three-phase result may look different from a single-phase result with the same current and length. The calculator treats the entered length as one-way length only.

Power Factor and Reactance

AC voltage drop is not only a resistance problem. Motor loads and transformer loads can have lagging power factor. That makes reactance more important. The calculator includes a reactance estimate and a custom reactance option. Leading power factor can reduce the reactive part of the drop.

Using the Result

The result shows volts dropped, percent drop, receiving voltage, power loss, effective impedance, and a suggested conductor size. Many designers use three percent for branch circuits and five percent for combined feeder and branch runs. These are common planning targets. They do not replace code rules. Always verify ampacity, insulation rating, terminals, ambient correction, conduit fill, grounding, and local authority requirements before installation.

Frequently Asked Questions

1. What is AC voltage drop?

AC voltage drop is the voltage lost between the source and load. It is caused by conductor resistance and reactance. Long runs, high current, small conductors, and poor power factor usually increase the drop.

2. Should I enter one-way or round-trip length?

Enter one-way length only. The calculator applies the correct phase factor. Single-phase uses a two-conductor path. Three-phase uses the square root of three factor.

3. What voltage drop percentage is acceptable?

Many designs target about three percent for branch circuits. Some use five percent for combined feeder and branch paths. Your project may need stricter limits for motors, electronics, or local rules.

4. Does this calculator check ampacity?

No. It estimates voltage drop only. Ampacity depends on insulation, terminals, ambient temperature, number of conductors, installation method, and code tables. Check those separately before choosing a final conductor.

5. Why does power factor matter?

AC circuits include resistance and reactance. Power factor decides how much each part affects the drop. Lagging loads, such as motors, often increase the reactive component.

6. Why include conductor temperature?

Conductor resistance changes with temperature. Hotter conductors have higher resistance. Higher resistance causes more voltage drop and more watt loss along the run.

7. Can I use custom conductor area?

Yes. Enter circular mils in the custom field. A value greater than zero overrides the selected conductor size. This helps with unusual cables or manufacturer-specific data.

8. What do CSV and PDF downloads include?

They include the main inputs, voltage drop, percent drop, receiving voltage, power loss, impedance values, suggested conductor size, and target status for record keeping.