Voltage Drop Calculator
Calculate voltage drop in electrical circuits using NEC-compliant formulas. Supports copper and aluminum conductors, single-phase and three-phase systems. Get voltage drop, percentage drop, and recommended wire size for your project.
What Is Voltage Drop?
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. It occurs because conductors have resistance, and when current flows through them, some voltage is lost as heat. Excessive voltage drop can cause motors to overheat, lighting to dim, and sensitive equipment to malfunction.
The NEC (National Electrical Code) recommends limiting voltage drop to 3% for branch circuits and 5% total (feeder + branch) for reasonable efficiency.
How the Voltage Drop Calculator Works
The calculator uses the standard NEC formulas:
Single-Phase AC / DC: Vdrop = (2 × K × I × D) / CM
Three-Phase AC: Vdrop = (1.732 × K × I × D) / CM
Where:
K = Resistivity constant: 12.9 for copper, 21.2 for aluminum (at 75°C)
I = Current in amperes
D = One-way distance in feet
CM = Circular mils (cross-sectional area of the conductor)
1.732 = √3 (the three-phase factor)
Percentage Drop = (Vdrop ÷ Source Voltage) × 100
Voltage at Load = Source Voltage − Vdrop
The calculator also recommends a minimum wire size based on the calculated voltage drop, helping you select the right conductor for your application.
NEC Voltage Drop Recommendations
- Branch Circuits: Maximum 3% voltage drop
- Feeders + Branch Circuits: Maximum 5% total voltage drop
- Sensitive Electronic Equipment: 1.5% for feeders, 2.5% for branches
- Fire Pumps: Additional maximum voltage drop requirements (NEC Section 695)
While these are FPN (Fine Print Note) recommendations rather than strict code requirements, following them ensures efficient operation and prevents equipment problems.
Why Use This Voltage Drop Calculator?
- NEC Compliant: Uses standard formulas from NEC Chapter 9, Tables 8 and 9.
- Multiple Configurations: Supports single-phase and three-phase circuits.
- Copper & Aluminum: Choose between copper and aluminum conductors.
- Wire Size Recommendation: Get the recommended minimum wire gauge.
- Free & Private: No registration, no data storage.
❓ Voltage Drop Calculator FAQ
What is voltage drop?
Voltage drop is the reduction in voltage in an electrical circuit between the source and the load. It occurs because conductors have resistance, and when current flows through them, some voltage is lost as heat.
What is the NEC recommended maximum voltage drop?
The NEC recommends limiting voltage drop to 3% for branch circuits and 5% total (feeder + branch) for reasonable efficiency.
What is the K factor in voltage drop calculations?
The K factor is the resistivity constant for conductor materials. K = 12.9 for copper and K = 21.2 for aluminum at 75°C. These values are based on NEC Chapter 9, Table 8.
What is the difference between single-phase and three-phase voltage drop?
Single-phase uses a factor of 2: Vdrop = (2 × K × I × D) / CM. Three-phase uses a factor of √3 (1.732): Vdrop = (1.732 × K × I × D) / CM.
What are circular mils (CM)?
Circular mils (CM) is a unit of area used to measure the cross-sectional area of a wire. One circular mil is the area of a circle with a diameter of one mil (0.001 inch). The CM values for standard AWG sizes are listed in NEC Chapter 9, Table 8.
How does distance affect voltage drop?
Voltage drop is directly proportional to distance. Doubling the distance doubles the voltage drop. Longer runs require larger conductors to keep voltage drop within acceptable limits.
How does current affect voltage drop?
Voltage drop is directly proportional to current. Higher current means higher voltage drop. This is why heavy loads require larger conductors.
How does wire gauge affect voltage drop?
Larger wire gauges (lower AWG numbers) have larger cross-sectional areas and lower resistance, resulting in lower voltage drop. Smaller gauges (higher AWG numbers) have higher resistance and greater voltage drop.
What is the difference between copper and aluminum for voltage drop?
Copper has lower resistivity (K=12.9) than aluminum (K=21.2). For the same gauge and length, copper conductors have about 40% less voltage drop than aluminum conductors.
When does voltage drop become a problem?
Voltage drop becomes a concern when it exceeds 3% on a branch circuit or 5% combined (feeder + branch). Symptoms include motors running hot, reduced torque, dim or flickering lights, and nuisance tripping of VFDs and PLCs.
How do I calculate the voltage at the load?
Voltage at Load = Source Voltage − Voltage Drop. For example, if the source is 240V and the drop is 5.76V, the voltage at the load is 234.24V.
What is the power factor in voltage drop calculations?
Power factor affects voltage drop in AC circuits because the current and voltage are out of phase. This calculator uses a power factor adjustment for more accurate AC calculations. The default is 1.0 for DC or unity power factor.
How does conductor temperature affect voltage drop?
Conductor resistance increases with temperature. The standard K values (12.9 for copper, 21.2 for aluminum) are based on 75°C. Higher temperatures increase resistance and voltage drop.
What is the recommended wire size for a given voltage drop?
The calculator finds the smallest standard AWG size that keeps the voltage drop within the NEC recommended 3% for branch circuits (or 5% for feeders). It considers your specific circuit parameters.
Can I use this calculator for DC circuits?
Yes. The single-phase formula with a power factor of 1.0 gives accurate results for DC circuits.
What is the difference between voltage drop and voltage regulation?
Voltage drop is the absolute voltage loss in a circuit. Voltage regulation is the percentage change in voltage from no-load to full-load, often used to describe power supply and transformer performance.
How do I reduce voltage drop in a circuit?
You can reduce voltage drop by: using larger conductors (lower AWG), reducing circuit length, using copper instead of aluminum, increasing the source voltage, or reducing the load current.