Wire Size Calculator

Introduction

Choosing the right wire size is one of the most important steps in any electrical project. If you use wire that is too small, it can overheat and cause a fire. If you use wire that is too big, you waste money. This wire size calculator helps you find the correct wire gauge for your project based on key factors like the length of the wire run, the current (amps) the circuit will carry, and the voltage of your system. It also accounts for acceptable voltage drop so your lights, motors, and other devices work the way they should. Whether you are wiring a new home, running power to a shed, or setting up a circuit in a commercial building, this tool takes the guesswork out of picking the right wire. Just enter your project details, and the calculator will recommend the proper American Wire Gauge (AWG) size for a safe and efficient installation. For a focused look at voltage loss over long runs, you can also try our Voltage Drop Calculator.

How to Use Our Wire Size Calculator

Enter a few details about your electrical setup below, and this calculator will tell you the right wire size (gauge) you need for your project.

Voltage: Enter the voltage of your electrical system. Common values are 120V or 240V for homes and 480V for commercial buildings.

Single Phase or Three Phase: Pick the phase type of your circuit. Most homes use single phase. Larger buildings and heavy equipment often use three phase.

Amperage (Amps): Enter the total amps your circuit needs to carry. You can find this on the nameplate of your equipment or breaker. If you need help relating voltage, current, and resistance, our Ohm's Law Calculator is a useful companion tool.

One-Way Wire Length (Feet): Enter the distance in feet from your electrical panel to the device or load. Measure just one way, not the round trip.

Acceptable Voltage Drop (%): Enter the maximum voltage drop you will allow. The National Electrical Code (NEC) recommends no more than 3% for branch circuits and 5% total for feeder and branch circuits combined.

Conductor Material: Choose copper or aluminum. Copper carries more current in a smaller size but costs more. Aluminum is lighter and cheaper but requires a larger gauge.

What Is Wire Size and Why Does It Matter?

Wire size refers to the thickness of an electrical conductor, measured in AWG (American Wire Gauge) in the United States. Picking the right wire size is one of the most important steps in any electrical project. If the wire is too small, it can overheat, waste energy, or even start a fire. If the wire is too big, you spend more money than you need to. The goal is to find a wire that can safely carry your electrical load over the required distance without losing too much voltage along the way.

How Wire Gauge Numbers Work

AWG uses a numbering system that can seem backwards at first. Smaller numbers mean thicker wire. For example, 14 AWG is a thin wire used for light-duty circuits like bedroom outlets, while 4/0 AWG is a very thick cable used for heavy-duty applications like main service panels. Once you go past 4/0, wire sizes are measured in kcmil (thousands of circular mils), such as 250, 300, or 500 kcmil. Each step up in wire size roughly doubles the cross-sectional area of the conductor.

Understanding Voltage Drop

Every wire has some resistance, and that resistance causes a loss of voltage as electricity travels through it. This loss is called voltage drop. The longer the wire run and the more current it carries, the greater the voltage drop. The National Electrical Code (NEC) recommends keeping voltage drop at or below 3% for branch circuits and 5% total when you combine the feeder and branch circuit together. Too much voltage drop can cause lights to dim, motors to run hot, and equipment to malfunction. For a detailed analysis of voltage loss on a specific circuit, use our dedicated Voltage Drop Calculator.

Key Factors That Determine Wire Size

• Current (Amps): The amount of electrical current the circuit must carry. Higher current needs thicker wire.
• Distance: Longer wire runs create more resistance, which increases voltage drop. This is the number one reason people need to upsize their wire beyond the minimum required for ampacity alone.
• Voltage: Lower-voltage systems (like 12V DC) are much more sensitive to voltage drop than higher-voltage systems (like 240V AC). This is why low-voltage landscape lighting and solar installations often require surprisingly large wire.
• System Type: Single-phase AC, three-phase AC, and DC circuits each use a different formula to calculate voltage drop. Three-phase systems use a factor of √3 (about 1.73) instead of 2, which gives them a slight advantage.
• Conductor Material: Copper has about 61% less resistance than aluminum, so it can carry more current in a smaller size. Aluminum is lighter and cheaper but requires a larger gauge for the same job.
• Insulation Temperature Rating: Wire insulation rated for higher temperatures (like THHN at 90°C) allows the conductor to carry more current than insulation rated for lower temperatures (like TW at 60°C).

Ampacity and Derating

Ampacity is the maximum current a wire can safely carry without overheating. The NEC publishes ampacity tables based on conductor size, material, insulation type, and installation method. However, real-world conditions often reduce a wire's ampacity through a process called derating:

• Ambient temperature: If the surrounding air is hotter than 30°C (86°F), the wire cannot dissipate heat as easily, so its ampacity must be reduced.
• Conduit fill: When more than three current-carrying conductors are bundled together in a conduit, heat builds up and the ampacity of each conductor is reduced. For example, 4 to 6 conductors in a conduit derate ampacity to 80%. Our Conduit Fill Calculator can help you determine the correct conduit size for the number and type of wires you need to pull.
• Continuous loads: The NEC requires that circuits serving continuous loads (running 3 hours or more) be sized at 125% of the load current. This provides a built-in safety margin.

Copper vs. Aluminum

Copper is the most common choice for residential and commercial wiring because it conducts electricity better, is easier to work with, and fits in smaller spaces. Aluminum costs less and weighs about one-third as much, making it a popular choice for larger feeder cables and utility service entrances. When using aluminum, you typically need to go one or two wire sizes larger than you would with copper to achieve the same performance. Aluminum also requires special connectors rated for aluminum to prevent corrosion and loose connections.

Practical Tips

Always measure the one-way distance from the panel to the load — the calculator accounts for the full round-trip length of the circuit internally. When in doubt, it is always safer to go one size larger than the minimum. A slightly oversized wire costs a little more upfront but reduces energy waste, runs cooler, and gives you room to add load in the future. If you are sizing a backup power source for your project, our Generator Sizing Calculator can help you match your generator to your load. For projects that involve running circuits through walls, you may also find our Stud Calculator and Framing Calculator helpful when planning your rough-in layout. To estimate your ongoing energy costs once the circuit is live, check out our Electricity Cost Calculator. For any project involving electrical wiring, check your local building codes and consult a licensed electrician to make sure your installation is safe and legal.

Frequently Asked Questions

What wire size do I need for a 30 amp circuit?

For a 30 amp circuit using copper wire, you typically need 10 AWG wire for short runs. If the wire run is long (over 100 feet), you may need to go up to 8 AWG to keep voltage drop within the recommended 3%. Enter your exact distance and voltage in the calculator to get a precise answer.

What does AWG mean?

AWG stands for American Wire Gauge. It is the standard system used in the United States to measure wire thickness. The smaller the AWG number, the thicker the wire. For example, 10 AWG is thicker than 14 AWG.

Why does the calculator ask for one-way distance instead of total wire length?

You only need to enter the distance from your electrical panel to the device or load. The calculator automatically doubles this distance (or uses the √3 factor for three-phase) to account for the full round-trip the electricity travels through the wire.

What is the NEC 125% rule for continuous loads?

The National Electrical Code says that if a load runs for 3 hours or more without stopping, you must size the wire to handle 125% of the load current. Check the "Continuous load" box in the calculator, and it will multiply your current by 1.25 automatically.

What is the difference between single-phase and three-phase?

Single-phase power is what most homes use. It has two wires (hot and neutral) carrying current. Three-phase power is used in commercial buildings and for large motors. It has three hot wires and uses a different formula (with a √3 multiplier) for voltage drop, which makes it more efficient for heavy loads.

What does the insulation temperature rating mean?

The insulation rating tells you the maximum temperature the wire can handle safely. Common ratings are 60°C (TW), 75°C (THW/THWN), and 90°C (THHN/XHHW). Higher-rated insulation allows the wire to carry more current because it can tolerate more heat.

Why does conduit vs. free air matter for wire sizing?

Wire installed in free air can cool itself more easily, so it is allowed to carry more current. Wire inside a conduit or raceway traps heat, which lowers its ampacity. The calculator adjusts the ampacity rating based on your selection.

What is derating and when do I need it?

Derating means reducing the ampacity of a wire because of real-world conditions. You need to derate when the ambient temperature is above 30°C (86°F) or when more than 3 current-carrying conductors are bundled together in the same conduit. The Advanced Settings section of this calculator handles both of these factors.

What is power factor and when should I change it?

Power factor measures how efficiently an AC circuit uses electricity. A value of 1.0 means all the power is used (resistive loads like heaters). Motors, transformers, and fluorescent lights typically have a power factor of 0.8 to 0.9. For DC circuits, power factor does not apply. You can adjust it in the Advanced Settings.

Can I use this calculator for 12V or 24V DC systems?

Yes. Select "Single-Phase DC" as the system type and enter your DC voltage (12V, 24V, 48V, etc.). Low-voltage DC systems are very sensitive to voltage drop, so you will often need much thicker wire than you might expect, especially on longer runs.

What does the safety factor setting do?

The safety factor multiplies your design current by the value you enter. A value of 1.0 adds no extra margin. A value of 1.2 adds a 20% buffer. This is useful if you plan to add more load in the future or want extra peace of mind.

How do I read the wire gauge comparison table?

The table shows every AWG size and how each one performs with your inputs. It lists the resistance, ampacity, voltage drop, and power loss for each gauge. A green ✅ Pass means the wire meets both ampacity and voltage drop requirements. The recommended wire is highlighted with a ★ symbol.

What if the calculator says no wire size passes?

If even the largest wire in the table fails, your circuit may need parallel conductors (two or more wires run side by side to share the load). You could also reduce the circuit distance, increase the system voltage, or consult a licensed electrician for a custom solution.

How much voltage drop is too much?

The NEC recommends no more than 3% voltage drop on a branch circuit and no more than 5% total when you add the feeder and branch circuit together. More than 5% can cause lights to dim, motors to overheat, and sensitive electronics to malfunction.

What is the total wire length shown in the results?

The "Wire Needed (Total)" result shows the full length of wire you need to buy. For single-phase and DC circuits, this is twice the one-way distance (hot wire + neutral or return wire). For three-phase circuits, it is three times the one-way distance (three hot wires).

Does this calculator replace a licensed electrician?

No. This calculator is a helpful planning tool, but it does not replace professional judgment. Always check your local building codes and have a licensed electrician review your plans before installing any wiring. Permits and inspections may be required in your area.