Wire Size / Ampacity Calculator

The Dangers of Undersized Wire

Electricity is incredibly dangerous because you cannot see it. When an electric current flows through a copper wire, the friction of the moving electrons generates heat.

Every wire has an Ampacity Limit—the absolute maximum amount of electrical current (Amps) the wire can safely carry before the copper gets so hot that it physically melts the plastic insulation, exposing bare live metal and starting a catastrophic house fire.

If you want to run a massive 50-Amp electric stove, you absolutely cannot use standard thin wire. You must use massive, heavy-gauge wire that contains enough physical copper mass to handle the heat.

However, Ampacity is not the only rule. You must also account for Voltage Drop. If you run a wire 300 feet to a detached garage, the wire might be thick enough to survive the heat (Ampacity), but it might be so long that the voltage drops too low, destroying your appliances. You must size the wire based on BOTH Ampacity limits and Voltage Drop limits.

The AWG System (American Wire Gauge)

In North America, wire thickness is measured using the American Wire Gauge (AWG) system. The AWG system is highly counter-intuitive: The smaller the number, the thicker the wire.

• 14 AWG: Very thin. (Max 15 Amps). Used for standard lighting circuits.
• 12 AWG: Standard. (Max 20 Amps). Used for kitchen and bathroom outlets.
• 10 AWG: Thick. (Max 30 Amps). Used for electric water heaters and clothes dryers.
• 8 AWG / 6 AWG: Very thick. (Max 40 to 55 Amps). Used for electric ovens and HVAC units.

How to Calculate Wire Size

To accurately size a wire, you must first check the NEC Ampacity chart to find the absolute minimum wire gauge required for safety. Then, you must run the Voltage Drop formula to see if the distance requires you to "upsize" the wire even further.

Step 1: Base Ampacity

1. Determine the maximum Amperage (Amps) your appliance will draw.
2. If it is a "Continuous Load" (an appliance that runs for more than 3 hours straight, like an EV charger), you must multiply the Amperage by 1.25.
3. Check the NEC Ampacity table. If your load is 20 Amps, the table says you must use a minimum of 12 AWG wire.

Step 2: The Voltage Drop Upsize

1. Determine the one-way Length of the wire run in feet.
2. Determine the System Voltage (usually 120V or 240V).
3. Determine the Maximum Drop Allowed (The NEC mandates a maximum 3% drop for branch circuits).
4. Use the engineering formula to find the required Circular Mils (CM) of copper area: **