Voltage Drop Calculator
Calculate voltage drop in electrical circuits based on wire gauge, length, and load current per NEC standards.
Results
Voltage drop of 6.6% exceeds the NEC 3% recommendation. Consider using a larger wire gauge or shorter run.
How to Use This Calculator
Start by selecting your system voltage, such as 120V or 240V for residential circuits or 480V for commercial applications. Enter the load current in amps. For example, a 20A circuit is common for kitchen receptacles. Input the one way wire length in feet from the panel to the load. Select the wire gauge you plan to use, such as 12 AWG for a 20A circuit. Choose the conductor material, either copper or aluminum. Finally, select single phase or three phase depending on your system. The calculator instantly displays the voltage drop in both volts and percentage, the voltage at the load end, and whether the result meets NEC recommendations. For a typical 120V, 20A circuit using 12 AWG copper over 100 feet, you would see a voltage drop of approximately 3.96V or 3.3%.
Understanding the Concept
Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor. Every wire has resistance, and as current passes through that resistance, some of the electrical energy is converted to heat, resulting in lower voltage at the load end. This matters because equipment is designed to operate within a specific voltage range. Motors running on low voltage draw more current, overheat, and fail prematurely. Lighting on low voltage circuits appears dim and may flicker. The National Electrical Code addresses voltage drop in NEC 210.19(A) Informational Note No. 4 and NEC 215.2(A) Informational Note No. 2, recommending a maximum of 3% voltage drop for branch circuits and 5% total for the combination of feeder and branch circuit. While these are recommendations rather than mandatory requirements, most inspectors and experienced electricians treat them as practical limits. Keeping voltage drop within acceptable limits ensures equipment longevity, energy efficiency, and code compliance.
The Formula Explained
The voltage drop formula for single phase circuits is VD = (2 x I x R x L) / 1000, where VD is the voltage drop in volts, I is the current in amperes, R is the conductor resistance in ohms per 1000 feet, and L is the one way length of the circuit in feet. The factor of 2 accounts for both the outgoing and return conductors. For three phase circuits, replace the factor of 2 with the square root of 3 (approximately 1.732). Conductor resistance values come from NEC Chapter 9 Table 9 for AC resistance or Table 8 for DC resistance. The voltage drop percentage is calculated as (VD / Source Voltage) x 100. For example, using 12 AWG copper with a resistance of 1.98 ohms per 1000 feet, a 20A load over 100 feet yields VD = (2 x 20 x 1.98 x 100) / 1000 = 7.92V.
Frequently Asked Questions
What is the maximum allowable voltage drop per NEC?
The NEC does not set a mandatory maximum voltage drop. However, NEC 210.19(A) Informational Note No. 4 recommends that branch circuit voltage drop not exceed 3%, and the total voltage drop for both feeder and branch circuit combined should not exceed 5%. Most jurisdictions and electricians follow these recommendations as best practice for ensuring proper equipment operation and energy efficiency.
How do I reduce voltage drop in a long wire run?
You can reduce voltage drop by increasing the wire gauge to a larger size, which lowers the resistance per foot. For example, upgrading from 12 AWG to 10 AWG nearly cuts the resistance in half. Other options include shortening the wire run by relocating the panel closer to the load, reducing the load current, or increasing the system voltage. For very long runs, some electricians install a sub panel closer to the load.
Does voltage drop differ between copper and aluminum wire?
Yes. Aluminum has a higher resistance than copper for the same wire gauge. For example, 12 AWG copper has a resistance of 1.98 ohms per 1000 feet, while 12 AWG aluminum has 3.25 ohms per 1000 feet. To achieve the same voltage drop performance as copper, you typically need to go up two wire gauge sizes when using aluminum. This is why aluminum conductors are most common in larger feeder and service entrance cables where the cost savings offset the larger conduit requirements.
What NEC table do I use for wire resistance values?
Use NEC Chapter 9 Table 9 for AC resistance and reactance values of conductors in conduit. This table provides resistance values in ohms per 1000 feet for both copper and aluminum conductors at 75 degrees Celsius. For DC circuits, use NEC Chapter 9 Table 8 instead. The values in Table 9 account for skin effect and are the appropriate choice for standard AC power circuits.
Is voltage drop calculated differently for three phase circuits?
Yes. For three phase circuits, the multiplier in the formula changes from 2 to the square root of 3, which is approximately 1.732. This is because three phase systems use three conductors with currents that are 120 degrees apart, and the effective voltage drop is determined by the line to neutral relationship. The formula becomes VD = (1.732 x I x R x L) / 1000. This means three phase circuits have slightly less voltage drop than single phase circuits of the same wire size and length.