Wire Gauge Calculator
Pick the thinnest wire whose ampacity covers your load and whose voltage drop stays under 3%. Voltage drop = 2 × resistance per foot × one-way distance × amps (1.732 instead of 2 for three-phase).
Safety: This is an estimate based on NEC ampacity and voltage drop. Electrical work can cause fire or injury. Local code, conduit fill, temperature derating and the breaker and terminal ratings also apply. Have a licensed electrician check any installation.
How to use this wire gauge calculator
- Enter the load in amps. If you only know watts, divide watts by volts.
- Pick the circuit voltage and whether it is DC or single-phase AC, or three-phase.
- Enter the one-way distance from the source to the load, following the route the wire will take. The calculator accounts for the return path.
- Choose copper or aluminum, the insulation temperature column and the voltage drop limit you want. Tick the continuous load option for loads that run three hours or more.
- Read the recommended AWG size, then check the comparison table to see why smaller sizes failed.
What size wire do I need for my amps?
On a short run (10 ft at 240 V, 75°C column) the minimum size is set by ampacity alone:
| Amps | Copper | Aluminum |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG |
| 50 A | 8 AWG | 6 AWG |
| 60 A | 6 AWG | 4 AWG |
| 100 A | 3 AWG | 1 AWG |
Longer runs often need a bigger size because of voltage drop. At 120 V, copper:
| Load | 25 ft | 50 ft | 100 ft | 150 ft |
|---|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 8 AWG | 8 AWG |
| 20 A | 12 AWG | 10 AWG | 8 AWG | 6 AWG |
| 30 A | 10 AWG | 8 AWG | 6 AWG | 4 AWG |
NEC ampacity table for copper and aluminum
Allowable current from NEC Table 310.16, up to three current-carrying conductors in a raceway at 30°C ambient. Small sizes are limited by NEC 240.4(D): 14, 12 and 10 AWG copper at 15, 20 and 30 amps, and 12 and 10 AWG aluminum at 15 and 25 amps.
| AWG | Copper 60°C | Copper 75°C | Copper 90°C | Aluminum 60°C | Aluminum 75°C | Aluminum 90°C |
|---|---|---|---|---|---|---|
| 14 | 15 | 15 | 25 | – | – | – |
| 12 | 20 | 20 | 30 | 15 | 15 | 25 |
| 10 | 30 | 30 | 40 | 25 | 25 | 35 |
| 8 | 40 | 50 | 55 | 30 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Source: NEC (NFPA 70) Table 310.16. Always confirm against the edition adopted where you live. Higher 90°C values are useful for derating, but the circuit is normally limited to the 60°C or 75°C rating of the terminals.
The voltage drop formula
For DC and single-phase circuits: voltage drop = 2 × R × L × I ÷ 1,000, where R is resistance in ohms per 1,000 ft, L is the one-way length in feet and I is the current in amps. For three-phase circuits replace the 2 with 1.732. Percent drop = voltage drop ÷ source voltage × 100. Resistance values are the NEC Chapter 9, Table 8 figures for uncoated stranded copper and for aluminum at 75°C.
The NEC recommends in an informational note that branch circuit voltage drop stay at about 3% or less, and about 5% or less for the feeder and branch circuit combined. These are recommendations rather than requirements for most circuits, but they keep equipment running properly.
Worked example: 40 amp EV charger at 75 feet
An EV charger that draws 40 amps for hours is a continuous load, so size for 40 × 1.25 = 50 amps. At 240 V and 75 feet, the calculator picks 8 AWG copper (ampacity 50 A, voltage drop 1.9%). The ampacity requirement sets the size here, and drop is well under 3%.
Worked example: 100 amp subpanel 150 feet away
At 240 V and 150 feet, 3 AWG copper has 100 amps of ampacity at 75°C but drops too much voltage. The calculator recommends 2 AWG copper (2.4% drop). In aluminum the answer is 1/0 AWG (2.5% drop). Subpanel feeders have extra requirements such as a ground wire and a neutral, so confirm the design with an electrician.
Copper or aluminum wire?
Copper carries more current and has lower resistance for a given size. Aluminum is lighter and cheaper, so it is common for large feeders and service cables. Aluminum needs connectors and anti-oxidant compound rated for it, and it usually has to be one or two sizes larger than copper for the same load. Do not mix them without approved connectors.
What this calculator does not account for
It checks two things: ampacity and voltage drop. A real installation has more. Hot attics and conduit in the sun reduce how much current a wire can safely carry, and so does bundling more than three current-carrying conductors together. Motor circuits follow their own sizing rules. Terminals and breakers are often rated 60°C or 75°C, which limits the column you may use even if the wire insulation is rated higher. Enter the one-way distance only, since the calculator already allows for the return path. Your local code and inspector have the final word.
Key terms
- AWG
- American Wire Gauge, a size standard. A lower number means a thicker wire.
- Ampacity
- The current a conductor can carry continuously without exceeding its temperature rating.
- Voltage drop
- Voltage lost along a conductor due to resistance.
- NEC
- The National Electrical Code (NFPA 70), the US standard for electrical installations.
- Derating
- Reducing the allowed current because of heat or bundled conductors.
Wire Gauge Calculator FAQ
What size wire do I need for a 20 amp circuit?
12 AWG copper is the standard for a 20 amp circuit, and 10 AWG aluminum is the usual aluminum size. For long runs, voltage drop may require a larger size.
What wire gauge for 50 amps?
8 AWG copper (50 A at 75°C) or 6 AWG aluminum is the usual minimum for 50 amps on a short run, and many ranges and EV chargers use 6 AWG copper. Longer runs and continuous loads can require larger wire.
What is voltage drop and why does it matter?
Voltage drop is the voltage lost along a wire because of its resistance. Too much makes lights dim and motors run hot and wastes energy. About 3% or less on a branch circuit is a common target.
Does a smaller AWG number mean thicker wire?
Yes. 10 AWG is thicker than 14 AWG, and 1/0 (one-aught) is thicker than 1 AWG.
Why does the calculator double the distance?
Current flows out on one wire and back on the other, so the resistance that matters is for twice the one-way length. For three-phase circuits the factor is 1.732 (the square root of 3) instead of 2.
Can I use aluminum wire?
Yes, in sizes where it is allowed, but aluminum carries less current than copper of the same size and has higher resistance, so it usually needs to be one or two sizes larger. Use connectors rated for aluminum.
What is the 125% rule for continuous loads?
A load that runs for 3 hours or more is treated as continuous and the circuit is sized for 125% of the load current. EV chargers are a common example.
What temperature column should I use?
Use the lowest temperature rating among the wire insulation, the breaker and the terminals. Many residential devices are rated 60°C or 75°C, so 75°C is common for larger conductors.
Related calculators
Last updated: November 11, 2026 · How we check this calculator