Wire Size & Voltage Drop Calculator
Enter your circuit parameters below to determine the correct wire gauge per NEC standards. The calculator accounts for ampacity requirements and voltage drop limits to recommend the smallest compliant conductor.
Enter your circuit specifications above and click Calculate Wire Size to see the recommended wire gauge, voltage drop, and NEC compliance status. The calculator checks both ampacity and voltage drop requirements.
Pre-Computed Wire Size Examples
Below are five common wiring scenarios with pre-calculated results. These examples use copper conductors at 75°C termination rating, which is the most common configuration in modern residential and light-commercial installations. Click any example to see the full details, or use the calculator above for custom parameters.
20A 120V at 50 ft (Kitchen Outlet)
20A, 120V, 50 ft, copper, 3% max drop
40A 240V at 100 ft (Electric Range)
40A, 240V, 100 ft, copper, 3% max drop
50A 240V at 75 ft (EV Charger)
50A, 240V, 75 ft, copper, 3% max drop
100A 240V at 150 ft (Subpanel)
100A, 240V, 150 ft, copper, 5% max drop
200A 240V at 50 ft (Service Entrance)
200A, 240V, 50 ft, copper, 3% max drop
Tools and Training for Electricians
Quality resources for your next wiring project.
How Voltage Drop Works
Every electrical conductor has resistance, measured in ohms per unit length. When current flows through a conductor, a portion of the supply voltage is consumed by this resistance and dissipated as heat. The resulting voltage reduction at the load end of the circuit is called voltage drop. The amount of voltage drop depends on four factors: the wire gauge (which determines resistance per foot), the length of the conductor run, the current flowing through the wire, and whether the circuit is single-phase or three-phase.
For single-phase circuits, voltage drop is calculated using the formula: Vd = (2 × R × D × I) / 1000, where R is the conductor resistance in ohms per 1000 feet (from NEC Chapter 9, Table 8), D is the one-way distance in feet, and I is the current in amperes. The factor of 2 accounts for the current traveling out on the hot conductor and returning on the neutral or second hot conductor. For three-phase circuits, the factor changes to 1.732 (the square root of 3) because the return path is shared among three conductors: Vd = (1.732 × R × D × I) / 1000.
The voltage drop percentage is the ratio of the voltage drop to the supply voltage, expressed as a percentage. For example, a 3.6-volt drop on a 120-volt circuit equals a 3% voltage drop. This percentage is the key metric used to determine whether a wire size is adequate for the run distance. Lower voltage drop percentages mean more efficient power delivery and better performance of connected equipment.
It is important to understand that voltage drop and ampacity are two separate requirements that must both be satisfied. A wire may have sufficient ampacity to safely carry the current without overheating, but the voltage drop over a long run may be excessive. In such cases, you must increase the wire size beyond what ampacity alone requires. This is why voltage drop calculations are especially critical for long runs to detached garages, outbuildings, well pumps, and similar installations where the distance from the panel is significant.
NEC Wire Sizing Requirements
The National Electrical Code (NEC), also known as NFPA 70, establishes the minimum wire sizing requirements for electrical installations in the United States. Wire sizing involves two primary considerations: ampacity and voltage drop. Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating, as specified in NEC Table 310.16. Voltage drop is addressed in NEC 210.19(A) Informational Note No. 4 and NEC 215.2(A)(1) Informational Note No. 2, which recommend limits of 3% for branch circuits and 5% for the combined feeder and branch circuit.
NEC Table 310.16 is the primary ampacity table for conductors rated 0 through 2000 volts in raceways, cables, or earth. It provides three ampacity columns for each conductor material (copper and aluminum): 60°C, 75°C, and 90°C. The column you use must match the lowest temperature rating of any component in the circuit, including the wire insulation, the breaker terminals, and the device terminals. Most residential breakers are rated 60°C or 75°C, so even when using 90°C wire (like THHN), you typically size based on the 75°C column.
For continuous loads (those expected to operate for 3 hours or more), the NEC requires that conductors be sized at 125% of the continuous load current. This means a 16-amp continuous load requires conductors rated for at least 20 amps. This derating factor ensures that the conductor and its terminations do not overheat during sustained operation. The circuit breaker must also be rated at 125% of the continuous load unless it is specifically listed for 100% continuous operation.
Conductor resistance values used for voltage drop calculations come from NEC Chapter 9, Table 8, which provides DC resistance for uncoated copper and aluminum conductors at 75°C. These values are used in the standard voltage drop formulas. For AC circuits, the impedance values in Chapter 9, Table 9 can be used for more precise calculations, as they account for the skin effect and the reactance of the conductor, but for most residential and commercial applications, the simpler DC resistance method produces sufficiently accurate results.
Copper vs Aluminum Wire
The choice between copper and aluminum conductors involves trade-offs in cost, performance, weight, and installation requirements. Copper has been the traditional conductor material for electrical wiring due to its excellent conductivity, durability, and ease of termination. Aluminum costs significantly less per foot and weighs roughly one-third as much as copper, making it attractive for large feeders and service entrance conductors where material cost is a major factor.
In terms of electrical performance, copper has approximately 61% of the resistance of aluminum for the same conductor size. This means aluminum requires a larger gauge to carry the same current safely. As a general rule, you must increase the aluminum conductor by two AWG sizes compared to copper. For example, where 6 AWG copper is adequate, you would need 4 AWG aluminum. The ampacity tables in NEC Table 310.16 reflect these differences with separate columns for each material.
Aluminum wiring requires specific installation practices to ensure safe, reliable connections. Aluminum expands and contracts more than copper with temperature changes, which can cause connections to loosen over time. All terminations must be torqued to manufacturer specifications, and anti-oxidant compound should be applied to prevent the formation of aluminum oxide at connection points. All devices, lugs, and breakers used with aluminum conductors must be rated for aluminum (marked AL-CU or CO/ALR for switches and receptacles).
For residential branch circuits (15 and 20 amp), copper is almost exclusively used. Aluminum is most commonly found in service entrance conductors (100-400 amp), large feeder circuits to subpanels, and commercial or industrial power distribution. The NEC does not prohibit aluminum for branch circuits, but practical considerations and local codes often favor copper for smaller circuits. When calculating voltage drop for aluminum conductors, the higher resistance values mean longer runs may require significantly larger wire sizes compared to copper.
Frequently Asked Questions
What is voltage drop and why does it matter for wire sizing?
Voltage drop is the reduction in voltage that occurs as electrical current flows through a conductor due to the conductor's inherent resistance. Every wire has some resistance, and as current passes through it, a portion of the voltage is lost as heat. This means the device at the end of the wire receives less voltage than what was supplied at the panel. Excessive voltage drop causes lights to dim, motors to overheat and run inefficiently, and sensitive electronics to malfunction. The NEC recommends limiting voltage drop to 3% on branch circuits and 5% total for the combined feeder and branch circuit.
What is the NEC recommended maximum voltage drop?
The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits and 5% for the combined total of feeder and branch circuit conductors. While these are recommendations (Informational Notes) rather than mandatory requirements, they represent accepted industry practice and are enforced by most inspectors. On a 120-volt circuit, 3% equals 3.6 volts, meaning the voltage at the outlet should not drop below 116.4 volts. On a 240-volt circuit, 3% equals 7.2 volts, and the delivered voltage should remain above 232.8 volts.
How do I choose between copper and aluminum wire?
Copper wire has lower resistance and higher ampacity than aluminum for the same gauge size, making it the preferred choice for most residential branch circuits. However, aluminum wire costs significantly less and weighs less per foot, making it a practical choice for large feeders, service entrance conductors, and long runs where the cost savings are substantial. When using aluminum, you must increase the wire gauge by typically two sizes (for example, use 4 AWG aluminum where 6 AWG copper would suffice) to achieve equivalent ampacity. Aluminum also requires special anti-oxidant compound at connections and terminations rated for aluminum use.
What temperature rating should I use for wire sizing?
The temperature rating you should use depends on the temperature rating of the terminations (breakers, outlets, switches) in the circuit, not just the wire itself. Most residential breakers and devices are rated for 60 degrees C or 75 degrees C. Even if you install 90 degrees C rated wire (like THHN), you must size it based on the lowest temperature rating in the circuit, which is typically the termination. The 90 degrees C column is primarily used for ampacity adjustment when applying derating factors for bundled conductors or high ambient temperatures.
Does wire size affect electrical safety?
Yes, wire size directly affects electrical safety. An undersized wire carrying too much current will overheat, potentially melting the insulation and causing an electrical fire. The NEC ampacity tables in Table 310.16 specify the maximum current each wire size can safely carry at each temperature rating. The circuit breaker provides a second layer of protection by tripping if current exceeds the wire's rated ampacity. Using the correct wire size ensures that the breaker trips before the wire overheats, maintaining the safety chain. Voltage drop is a performance concern, but ampacity is a life-safety requirement.
Wire Sizes Reference
Browse detailed specifications and voltage drop tables for each wire gauge. Each page includes ampacity ratings at all temperature levels, resistance values for copper and aluminum, common applications, and pre-computed voltage drop tables at various distances.
Common Applications
Find the recommended wire size for specific electrical applications. Each page provides NEC code references, installation tips, and voltage drop tables at multiple distances.