Find the correct AWG conductor for any load, voltage, and run length using NEC 310.16 ampacity tables. Includes a voltage-drop check.
Choosing the correct wire size (AWG gauge) is one of the most important decisions in any electrical installation. An undersized conductor overheats, trips breakers, and poses a fire risk. An oversized wire wastes money on copper you do not need. The NEC (National Electrical Code) Table 310.16 provides the standard ampacity ratings that our calculator uses to recommend the right size for your load.
One of the most overlooked requirements in the NEC is the continuous load rule. Any load that runs for three hours or more (like a space heater, air conditioner, or EV charger) must be treated at 125% of its rated current. For example, a 20A continuous circuit actually requires wiring rated for 25A. Our calculator applies this rule automatically so you never miss it.
A common search is “what size wire for a 50 amp sub panel 150 feet.” At 240V with a 150-foot run, the NEC-compliant answer is typically #6 AWG copper or #4 AWG aluminum at 75°C. However, this depends on your actual distance and voltage drop tolerance. Use our voltage drop calculator to verify the run stays under the recommended 3% limit.
Copper has lower resistance per cross-sectional area, meaning a smaller copper wire can carry the same current as a larger aluminum one. However, aluminum is lighter and cheaper, making it popular for service entrance cables and long runs. Our calculator lets you switch between copper and aluminum with one click so you can compare both options side by side.
Wire ampacity is rated at specific temperature thresholds: 60°C, 75°C, and 90°C. The 75°C column is the most commonly used in residential and commercial work because most terminations (breakers, lugs, connectors) are rated for 75°C. Always match the wire insulation rating to your equipment terminations.
Conductors in free air dissipate heat more efficiently than those bundled in conduit. NEC allows a small ampacity boost for wires in free air. Our calculator accounts for this difference — select “in free air” for open tray or unsupported runs, and “in conduit” for enclosed conduit installations.
Even if your wire carries the current safely, excessive voltage drop over long distances can damage equipment and reduce efficiency. The NEC recommends no more than 3% voltage drop on branch circuits and 5% total (feeder + branch). After our calculator picks your wire size, it checks the voltage drop automatically and flags any issues. You can also run a detailed check with our dedicated voltage drop tool.
Once you know your wire size, you will also need the correct conduit size to house it. NEC limits how many wires can occupy a conduit based on fill percentage. Use our conduit fill calculator to verify your chosen conduit accommodates all the conductors in your circuit.
All recommendations from this tool are estimates based on NEC 310.16. Local codes may impose additional requirements. Always consult a licensed electrician for final verification of your installation.