What is the difference between copper and aluminum wire?

Copper wire has lower resistance (about 10.4 ohms per 1,000 feet for 12 AWG), making it more efficient for the same gauge. Aluminum wire has higher resistance (about 17.0 ohms per 1,000 feet for 12 AWG) and requires a larger gauge to carry the same current with the same voltage drop. Aluminum is cheaper and lighter, so it is commonly used for large-gauge service entrance cables and feeder circuits.

AWG stands for American Wire Gauge. It is the standard system used in the United States to measure wire diameter. The AWG scale is inversely proportional: a smaller AWG number means a larger, thicker wire. For example, 4 AWG is thicker and can carry more current than 14 AWG. Gauges above 1 AWG use the designation 1/0 (one-aught), 2/0, 3/0, and 4/0 for the largest common sizes.

How does one-way distance affect wire gauge calculations?

Wire gauge calculations use the total conductor length, which is twice the one-way distance — the current travels out through one wire and returns through the other. Doubling the one-way distance gives the round-trip length used to calculate resistance and voltage drop. This is why long runs (such as outbuildings or solar installations) require larger wire gauges than short runs carrying the same load.

What is NEC ampacity and how is it different from the calculated current?

NEC ampacity is the maximum current a wire can safely carry in a conduit under standard conditions (30°C ambient, no more than three current-carrying conductors), as defined by the National Electrical Code. It is a thermal rating — the wire must not overheat. Voltage drop is a separate electrical performance concern. A wire can be within its ampacity rating but still have excessive voltage drop on a long run, or vice versa. Always size wire to satisfy both criteria.

Wire Gauge Calculator

Find the correct AWG wire size for any circuit. Enter load and run length to get recommended gauge, voltage drop, and ampacity instantly.

Current (Amps)

One-Way Distance (ft)

System Voltage

Max Voltage Drop

Wire Material

Recommended AWG Gauge

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Voltage Drop

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Power Loss

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in the wire run

Wire Resistance

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round-trip

NEC Ampacity

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75°C rating

AWG Reference Table

Copper and aluminum resistance at 20°C; NEC ampacity for conductors in conduit (75°C rating, 30°C ambient, 3 conductors max).

AWG	Diameter (in)	Cu Ω/1000ft	Al Ω/1000ft	Cu Ampacity	Al Ampacity

How to Use the Wire Gauge Calculator

Choosing the right wire size is one of the most important steps in any electrical project. Use too thin a wire and you risk excessive heat, wasted energy, and fire hazard. Use too thick a wire and you waste money on materials. This calculator takes four inputs — load current, one-way run length, system voltage, and acceptable voltage drop — and tells you the minimum AWG size that keeps voltage drop within your target, then confirms the wire can also handle the current thermally according to NEC ampacity tables.

Step 1: Enter Your Load Current

Enter the maximum continuous current the circuit will carry in amperes. For a single appliance, use its nameplate current or calculate it from wattage: I = W / V. For branch circuits, the NEC requires that the wire be rated for at least 125% of the continuous load, so enter the derated value (multiply by 1.25 before entering).

Step 2: Enter the One-Way Distance

Measure the distance from the power source to the load in feet. The calculator automatically doubles this to account for the complete circuit — current travels out through one conductor and returns through the other. Long runs are the most common reason a larger-than-expected wire gauge is required. A 20 A, 120 V circuit that needs only 12 AWG for a 25-foot run may need 10 AWG at 75 feet to stay within a 3% voltage drop limit.

Step 3: Choose System Voltage and Acceptable Voltage Drop

Select the system voltage (12 V, 24 V, 120 V, or 240 V). Low-voltage DC systems (12 V and 24 V) are far more sensitive to voltage drop because any drop is a larger percentage of the total available voltage. The NEC recommends no more than 3% voltage drop on a branch circuit, with a combined maximum of 5% from the service panel to the load. For sensitive electronics and motor loads, 2% is a better target. For non-critical lighting or heating loads, 5% is generally acceptable.

How the Calculation Works

The calculator uses the standard voltage drop formula: V_drop = (2 × L × I × ρ) / 1000, where L is the one-way distance in feet, I is the current in amps, and ρ is the resistivity of the conductor in ohms per 1,000 feet. It steps through the AWG table from the smallest (14 AWG) to the largest (4/0 AWG) and selects the smallest gauge whose voltage drop is within your target, then checks that gauge against the NEC ampacity limit. If the ampacity limit requires a larger gauge than the voltage drop calculation, the larger gauge wins.

Copper vs. Aluminum Wire

Copper has lower resistivity than aluminum, which means a copper wire of a given gauge has less resistance and lower voltage drop than the same aluminum gauge. Aluminum wire is roughly 1.6 times less conductive than copper by cross-sectional area, so you typically need to go up two AWG sizes (for example, 2 AWG aluminum where 4 AWG copper would suffice). Aluminum is commonly used for residential service entrance cables (the large cables from the utility meter to the main panel) and for large feeder circuits where its lighter weight and lower cost outweigh the need for a larger gauge. Aluminum wiring for branch circuits (outlets, lighting) was used in the 1960s-1970s and is no longer recommended due to connection reliability issues.

NEC Ampacity Ratings

NEC ampacity ratings specify the maximum continuous current a conductor can carry without exceeding its insulation temperature rating. The ratings in this calculator use the 75°C column from NEC Table 310.15(B)(16), which applies to most THHN, THWN, and similar conductors in conduit with up to three current-carrying conductors and an ambient temperature of 30°C (86°F). If your installation has more than three conductors in a conduit, or operates in a higher-temperature environment (such as an attic or near HVAC equipment), you must apply derating factors that reduce the allowable current. Consult a licensed electrician for installations subject to these conditions.

FAQ

Voltage drop is the reduction in voltage that occurs as current flows through a wire due to the wire's resistance. It matters because excessive voltage drop causes motors and appliances to run inefficiently, overheat, or fail. The NEC recommends keeping voltage drop to 3% or less for branch circuits and 5% or less for the total from the service panel to the load.

Copper wire has lower resistance than aluminum wire of the same gauge, making it more efficient. Aluminum wire is cheaper and lighter but requires a larger gauge to carry the same current with the same voltage drop. Aluminum is commonly used for large service entrance and feeder cables but is not recommended for residential branch circuits.

AWG stands for American Wire Gauge. It is the standard wire sizing system in the United States. The scale is inversely proportional: a smaller number means a thicker wire. For example, 4 AWG is thicker than 14 AWG. Gauges larger than 1 AWG are designated 1/0, 2/0, 3/0, and 4/0 (read as "one-aught" through "four-aught").

Entering one-way distance is the most intuitive input — it is the physical length you measure from source to load. The calculator automatically doubles it internally to account for the complete circuit path: current flows out on the positive (or hot) conductor and returns on the neutral or negative conductor. Both conductors contribute to total resistance and voltage drop.

NEC ampacity is the maximum current a wire can safely carry without overheating, as defined in NEC Table 310.15(B)(16). It is a thermal limit. Voltage drop is a separate electrical performance concern. A wire can be within its ampacity limit yet still have excessive voltage drop on a long run. This calculator checks both constraints and recommends the gauge that satisfies whichever is more restrictive.