Why is voltage drop important in electrical circuits?

Excessive voltage drop can lead to poor performance of electrical equipment, such as dimming lights, inefficient motors, and malfunctioning electronics. It also causes energy to be wasted as heat in the conductor, which can be a safety hazard. Electrical codes mandate limits on voltage drop (typically 3% to 5%) to ensure safety and efficiency.

How do you calculate voltage drop for single-phase and three-phase AC circuits?

AC circuit calculations are similar but must also account for the power factor (cosφ). For single-phase, the approximate formula is ΔV = (2 × L × I × ρ × cosφ) / A. For balanced three-phase circuits, the formula is ΔV = (√3 × L × I × ρ × cosφ) / A. More precise calculations also include the effect of reactance (X).

What percentage voltage drop is acceptable?

As a general rule, a voltage drop of 3% is recommended for branch circuits (serving the final load), and a total drop of 5% is acceptable for the entire system (from the service entrance to the load). Adhering to these limits ensures equipment runs efficiently and safely.

How do conductor material and size affect voltage drop?

Conductor material and size are the primary factors you can control. Copper has lower resistivity than aluminum, so it will have less voltage drop for the same size wire. A larger conductor size (cross-sectional area) reduces resistance, which in turn reduces voltage drop. Using a thicker wire is the most common way to solve a voltage drop problem.

Voltage Drop Calculator — DC, Single & Three Phase Circuits

Voltage Drop Calculator

Select Circuit Type

System Type

DC Single-Phase AC Three-Phase AC

Circuit Parameters

Source Voltage (V)

Load Current (A)

One-Way Conductor Length

Conductor Size

Conductor Material

Copper Aluminum

Power Factor (cos φ)

Results

What Is Voltage Drop?

Voltage drop is the decrease in electrical potential along the path of a current-carrying wire. Every wire has some resistance, and when current flows through it, a portion of the voltage is 'lost' as it overcomes this resistance. This phenomenon is a fundamental aspect of all electrical circuits. While a small drop is normal and expected, an excessive drop can prevent equipment from functioning correctly and waste energy as heat.

Voltage Drop Formula for DC and AC Circuits

The method for calculating voltage drop differs based on the circuit type. This calculator uses the following standard formulas (simplified for clarity):

DC Circuits: The calculation is based on Ohm's Law. The formula for the round-trip voltage drop is:
$ΔV = (2 \times L \times I \times ρ) / A$
Single-Phase AC Circuits: This accounts for the power factor (cosφ), which represents the phase angle between voltage and current. The approximate formula is:
$ΔV = (2 \times L \times I \times ρ \times \cos(φ)) / A$
Three-Phase AC Circuits: For balanced loads, the calculation includes the square root of 3 (≈1.732) to account for the phase relationships. The formula is:
$ΔV = (\sqrt{3} \times L \times I \times ρ \times \cos(φ)) / A$

In these formulas, L is the one-way length, I is the current, ρ is the material's resistivity, A is the conductor's cross-sectional area, and cos(φ) is the power factor.

Why Voltage Drop Matters in Wiring & Cable Selection

Proper cable selection is critical for safety and efficiency. If a wire is too small for the length and load, the voltage drop will be high, leading to several problems: Inefficient Operation (motors may overheat or fail to start), Dimming Lights, Damage to Electronics (sensitive equipment may shut down or get damaged by low voltage), and Wasted Energy (energy lost as heat in the wire). By calculating the voltage drop beforehand, you can select a conductor size that ensures the load receives adequate voltage.

Recommended Voltage Drop Limits (3% & 5% Rules)

Most electrical codes, including the National Electrical Code (NEC), provide guidelines for acceptable voltage drop. The most common rules of thumb are:

3% Rule: The voltage drop for a final branch circuit (the part of the circuit that directly powers the appliance or outlet) should not exceed 3% of the source voltage.
5% Rule: The total voltage drop from the main service panel to the final load should not exceed 5%. This allows for some drop in the feeder wires and some in the final branch circuit.

Staying within these limits ensures that your electrical system operates efficiently and safely, prolonging the life of your appliances and preventing potential hazards.

How Conductor Size and Material Affect Voltage Drop

Two primary factors you can control to manage voltage drop are the conductor's material and its size.

Material: Copper and aluminum are the most common conductors. Copper has lower resistivity than aluminum, making it a more efficient conductor. For the same size, a copper wire will have less voltage drop.
Size (Area): The cross-sectional area of the conductor is inversely proportional to its resistance. A larger wire (greater mm² or smaller AWG number) has less resistance and therefore less voltage drop over the same distance. This is the most common method used to correct an excessive voltage drop issue.

Frequently Asked Questions

What is resistivity (ρ)? Resistivity is an intrinsic property of a material that measures how strongly it resists electric current. Materials with low resistivity, like copper, are excellent conductors.
What is power factor? In AC circuits, the power factor describes the efficiency of power usage. A purely resistive load (like a heater) has a power factor of 1.0, while motors have a lower power factor. A lower power factor can increase voltage drop.
What happens if my voltage drop is too high? You should use a larger conductor (wire). This calculator will suggest a maximum run length for the selected wire to stay within a 3% drop, helping you decide if you need to upgrade.

Disclaimer: This calculator provides estimates for planning purposes only. Final cable selection must comply with local electrical codes and be verified by a qualified professional.