Find Voltage Drop Calculator

Calculate the voltage drop in an electrical circuit. Our Voltage Drop Calculator helps you determine the voltage loss across a wire based on material, size, current, and length, ensuring your electrical systems operate efficiently and safely.

Voltage Drop Calculator

What is a Voltage Drop Calculator?

A Voltage Drop Calculator is a tool used to determine the amount of voltage lost along the length of a wire or cable in an electrical circuit. When electric current flows through a conductor, there is always some resistance, which causes a drop in voltage from the source to the load. This phenomenon is known as voltage drop. The Voltage Drop Calculator takes into account factors like wire material (copper or aluminum), wire size (AWG or kcmil), length of the wire, the current flowing through it, the source voltage, and the phase (single or three phase) to estimate this loss.

Electricians, engineers, and designers use a Voltage Drop Calculator to ensure that the voltage at the load (the device using the electricity) remains within acceptable limits. Excessive voltage drop can lead to inefficient operation of equipment, dimming lights, overheating of wires, and even damage to sensitive electronics. National and local electrical codes often specify maximum allowable voltage drops for different types of circuits (e.g., feeders, branch circuits). Our Voltage Drop Calculator helps in selecting the appropriate wire size to minimize these issues.

Common misconceptions are that voltage drop is negligible or only matters for very long distances. However, even moderate distances with high currents or undersized wires can lead to significant voltage drop, impacting performance. Using a Voltage Drop Calculator is crucial for proper circuit design.

Voltage Drop Calculator Formula and Mathematical Explanation

The voltage drop (VD) in a circuit can be calculated using Ohm's Law (V = IR) and considering the resistance of the conductors. The formulas used by the Voltage Drop Calculator depend on whether the circuit is single-phase or three-phase:

For Single-Phase circuits:

VD = 2 * K * I * L / CM

For Three-Phase circuits:

VD = √3 * K * I * L / CM (where √3 ≈ 1.732)

Where:

• VD is the Voltage Drop in volts.
• K is the resistivity of the conductor material in ohm-circular mils per foot (ohm-cmil/ft). It's approximately 12.9 for copper and 21.2 for aluminum at 75°C (167°F).
• I is the current flowing through the conductor in amperes (A).
• L is the one-way length of the conductor in feet (ft).
• CM is the circular mil area of the conductor, which is derived from the wire size (AWG or kcmil).
• The '2' in the single-phase formula accounts for the total length of the wire (to the load and back). The '√3' in the three-phase formula is used for line-to-line voltage drop calculations in balanced three-phase systems.

Variable	Meaning	Unit	Typical Range
VD	Voltage Drop	Volts (V)	0 – Source Voltage
K	Resistivity	ohm-cmil/ft	12.9 (Cu), 21.2 (Al) at 75°C
I	Current	Amperes (A)	0 – Wire Ampacity
L	One-way Length	Feet (ft) or Meters (m)	0 – Several thousand
CM	Circular Mil Area	Circular Mils	1620 (18AWG) – 500000+ (500kcmil)

Variables used in the Voltage Drop Calculator formulas.

The Voltage Drop Calculator first determines the circular mil area based on the selected wire size and then applies the appropriate formula. We also calculate the percentage voltage drop: %VD = (VD / Source Voltage) * 100%.

Practical Examples (Real-World Use Cases)

Let's see how the Voltage Drop Calculator works with some examples.

Example 1: Outdoor Lighting Circuit

You are installing outdoor lighting 150 feet away from the panel. The lights draw a total of 5 amps, and the source is a 120V single-phase circuit. You are considering using 14 AWG copper wire.

• Material: Copper
• Wire Size: 14 AWG (CM = 4110)
• Voltage: 120V
• Current: 5A
• Distance: 150 feet
• Phase: Single

Using the Voltage Drop Calculator or formula: VD = (2 * 12.9 * 5 * 150) / 4110 ≈ 4.71 volts. Percentage drop = (4.71 / 120) * 100% ≈ 3.93%. This is acceptable for many branch circuits (often a 3-5% limit).

Example 2: Subpanel Feeder

You are feeding a subpanel in a workshop 200 feet from the main 240V single-phase service, with an expected load of 40 amps. You are planning to use 4 AWG aluminum wire.

• Material: Aluminum
• Wire Size: 4 AWG (CM = 41740)
• Voltage: 240V
• Current: 40A
• Distance: 200 feet
• Phase: Single

Using the Voltage Drop Calculator or formula: VD = (2 * 21.2 * 40 * 200) / 41740 ≈ 8.13 volts. Percentage drop = (8.13 / 240) * 100% ≈ 3.39%. This is within typical limits for a feeder.

How to Use This Voltage Drop Calculator

1. Select Conductor Material: Choose either 'Copper' or 'Aluminum' from the dropdown.
2. Select Wire Size: Choose the appropriate wire gauge (AWG) or size (kcmil) from the list.
3. Enter Source Voltage: Input the voltage at the beginning of the circuit (e.g., 120V, 240V).
4. Enter Current: Input the maximum expected current (in Amps) that will flow through the wire.
5. Enter One-way Distance: Input the length of the wire from the source to the load, and select 'Feet' or 'Meters'. The Voltage Drop Calculator converts meters to feet internally if needed (1 meter ≈ 3.28084 feet).
6. Select Number of Phases: Choose 'Single Phase' or 'Three Phase' based on your electrical system.
7. Calculate: The Voltage Drop Calculator will automatically update the results as you change the inputs, or you can click "Calculate".
8. Review Results: The primary result shows the voltage drop in volts and percentage. Intermediate results show the end voltage and total wire resistance. The table and chart provide further insights.

The results help you decide if the selected wire size is adequate. If the percentage voltage drop is too high (e.g., above 3% for branch circuits or 5% for feeders and branch combined, as per NEC recommendations), you should consider using a larger wire size (smaller AWG number or larger kcmil). Our Wire Size Calculator can also assist here.

Key Factors That Affect Voltage Drop Calculator Results

• Wire Material: Copper has lower resistivity than aluminum, resulting in less voltage drop for the same size and length. Our Voltage Drop Calculator uses different 'K' factors.
• Wire Size (Cross-sectional Area): Larger wires (smaller AWG number or larger kcmil) have lower resistance and thus less voltage drop. The Voltage Drop Calculator uses the Circular Mil (CM) area.
• Wire Length: The longer the wire, the greater the resistance and voltage drop.
• Current (Load): Higher current flow through the wire leads to a greater voltage drop (V=IR).
• Source Voltage: While it doesn't directly affect the voltage drop in volts for a given current and wire, it does affect the *percentage* voltage drop. A 3V drop is more significant on a 120V circuit than on a 240V circuit. Check out our Ohm's Law Calculator for basics.
• Temperature: Conductor resistance increases with temperature. While our basic Voltage Drop Calculator assumes a standard temperature (75°C for K values), in real-world scenarios, ambient temperature and load can affect wire temperature and resistance.
• Phase: Three-phase systems are more efficient for power transmission over long distances and generally have less voltage drop compared to single-phase for the same power, but the formula differs as used by the Voltage Drop Calculator.

Frequently Asked Questions (FAQ)

Why is voltage drop important?

Excessive voltage drop can cause lights to dim, motors to run hot or with less power, and electronic equipment to malfunction or fail. It also represents wasted energy. Using a Voltage Drop Calculator helps prevent these issues.

What is a safe percentage for voltage drop?

The National Electrical Code (NEC) recommends a maximum of 3% voltage drop for branch circuits and 5% total for the feeder and branch circuit combined. However, some equipment may have more stringent requirements. Always consult local codes and equipment specifications.

How do I reduce voltage drop?

You can reduce voltage drop by using a larger wire size (increasing CM), using copper instead of aluminum, shortening the wire length if possible, or reducing the current load on the circuit.

Does the Voltage Drop Calculator account for temperature?

This Voltage Drop Calculator uses standard K values for 75°C. For very precise calculations involving different operating temperatures, temperature correction factors for resistivity would be needed.

What is AWG and kcmil?

AWG (American Wire Gauge) is a standard for wire sizes, where smaller numbers mean larger wires. kcmil (kilo circular mils, formerly MCM) is used for very large wires (above 4/0 AWG), representing the cross-sectional area in thousands of circular mils.

Can I use this Voltage Drop Calculator for DC circuits?

Yes, the single-phase formula used by the Voltage Drop Calculator is also applicable to DC circuits as it's based on resistance, current, and length.

What if my wire size is not listed?

If your wire size is not in the dropdown, you would need to find its Circular Mil (CM) area and use the formula directly or find a more advanced Voltage Drop Calculator that allows CM input.

Does conduit type affect voltage drop?

The conduit type itself doesn't directly affect the DC resistance and thus the voltage drop calculated here. However, it can affect heat dissipation, which influences wire temperature and resistance, especially in AC circuits with reactance (not covered by this basic calculator). For more complex scenarios, see our AC Power Calculator.