Voltage Drop Calculator

Voltage drop is one of the most practically important concepts in electrical installation work. It describes the reduction in voltage that occurs as electrical current travels through the resistance of a wire from the power source to the load. Even conductors with very low resistance experience some voltage drop, and in longer wire runs or high-current circuits, that drop can become significant enough to impair equipment performance or violate electrical code recommendations.

Understanding and calculating voltage drop allows electricians, engineers, and DIYers to select the correct wire gauge for a given circuit, ensuring that equipment receives adequate voltage and that installations comply with the National Electrical Code (NEC) or other applicable standards.

The Voltage Drop Formula

For single-phase AC circuits and DC circuits, the voltage drop is calculated using: Vdrop = 2 × I × R × L / 1000. In this formula, I is the load current in amperes, R is the conductor resistance in ohms per 1,000 feet (sourced from NEC Table 9), and L is the one-way wire run length in feet. The factor of 2 accounts for the round trip — current must travel to the load through the hot conductor and return via the neutral or ground conductor.

For a three-phase circuit, the formula becomes Vdrop = 1.732 × I × R × L / 1000, because the three phases share the return path more efficiently. This calculator focuses on single-phase circuits, which cover the majority of residential and light commercial branch circuits.

Wire Gauge and Resistance

The American Wire Gauge (AWG) system assigns numbers to wire sizes in inverse order: smaller AWG numbers indicate thicker wires with lower resistance. AWG 2 is considerably thicker than AWG 14. Resistance values used in this calculator come from NEC Table 9, which lists resistance per 1,000 feet for both copper and aluminum conductors at 75 degrees Celsius operating temperature.

Copper has lower resistivity than aluminum and is therefore preferred for most residential wiring. However, aluminum is lighter, less expensive, and commonly used for large-gauge service entrance conductors (typically AWG 4 and larger). When using aluminum, a larger gauge is typically required to achieve the same voltage drop performance as copper.

NEC Voltage Drop Recommendations

The National Electrical Code, in section 210.19(A) informational notes, recommends that voltage drop on any branch circuit not exceed 3% of the source voltage under maximum load conditions. For feeder circuits, NEC similarly recommends no more than 3%, with a combined feeder-and-branch-circuit drop not exceeding 5%. These are recommendations, not mandatory requirements under most jurisdictions, but following them is considered best practice for equipment longevity and energy efficiency.

At 120 V, a 3% drop represents 3.6 V, leaving 116.4 V at the load. At 240 V, 3% equals 7.2 V. Sensitive equipment such as motors, variable frequency drives, and electronic devices can be affected by voltages below their rated values, potentially shortening operating life or causing malfunction.

Practical Implications of Excessive Voltage Drop

When voltage drop exceeds recommended limits, the effects can range from minor to significant. Incandescent and halogen lamps will glow dimmer than intended. Motors may run hotter due to increased current draw at lower voltage, reducing their operational lifespan. Electronic devices with switching power supplies are generally more tolerant but may shut down or behave erratically under severe voltage drop conditions.

In commercial and industrial settings, excessive voltage drop can lead to increased energy costs because motors and other inductive loads draw higher currents at lower voltages to maintain power output. This higher current causes additional resistive losses in the wiring, creating a compounding effect.

How to Reduce Voltage Drop

The most straightforward way to reduce voltage drop is to increase the wire gauge (use a lower AWG number). Upgrading from AWG 12 to AWG 10 copper reduces resistance by approximately 37%. Another approach is to shorten the wire run by repositioning the panel or adding a subpanel closer to the load. For circuits with very long runs, this can be the most cost-effective solution.

Switching from aluminum to copper for the same gauge will also reduce resistance and thus voltage drop, though the cost difference may be significant for large conductors. In some cases, operating at a higher voltage (240 V instead of 120 V) can reduce drop since the same power is delivered at half the current.

This calculator uses resistance values from NEC Table 9 at 75 degrees Celsius. Actual resistance varies slightly with conductor temperature — a conductor running at full rated ampacity will be warmer and have slightly higher resistance than one at a lower load. For most practical calculations, the NEC Table 9 values provide a reliable baseline.

Wire Length Measurement

The wire run length used in the formula is the one-way distance from the panel or source to the load. The factor of 2 in the formula automatically accounts for the return path. Measure along the actual wire route — through conduit bends, around obstacles, and up or down walls — rather than the straight-line distance between panel and load.

For circuits with multiple loads at different distances (such as a lighting circuit with several fixtures), a conservative approach is to use the distance to the farthest load. A more precise analysis would calculate the voltage at each device individually, taking into account the cumulative current draw at each segment.