You're planning an electrical circuit for a new outlet, a subpanel, or a dedicated appliance line, and you need to know: what gauge wire do I actually need for this distance and amperage? Choose too small a gauge and the wire overheats; choose too large and you waste money. A wire size calculator gives you the correct answer based on the National Electrical Code (NEC).
What This Calculator Does
This wire size calculator determines the minimum wire gauge (AWG) required for your electrical circuit based on three factors: the circuit amperage (how much current it must carry), the distance from the breaker panel to the load, and the acceptable voltage drop (typically 3% maximum per NEC guidelines). It uses standard ampacity tables from the NEC to ensure the wire is rated for safe continuous current and accounts for the resistance that builds up over longer distances.
How to Use This Calculator
First, determine your circuit amperage. This is the rating of the breaker protecting the circuit (15A, 20A, 30A, 40A, 50A, etc.). If you're wiring an outlet on a 20-amp breaker, use 20 amps. If you're running a 240V circuit for a dryer (typically 30 amps) or a range (typically 40-50 amps), use that amperage.
Next, measure the distance from the breaker panel to the farthest point on that circuit (for a room outlet, it's the distance from the panel to the outlet; for a subpanel, it's the distance from the main panel to the subpanel). This is one-way distance, not round-trip.
Enter these values, and the calculator applies NEC ampacity tables and voltage drop formulas to recommend the minimum wire gauge. For most residential circuits, you'll see recommendations like 14 AWG for 15A circuits under 50 feet, or 10 AWG for 30A circuits at distance.
The Formula Behind the Math
Wire resistance per foot varies by gauge. NEC tables provide ampacity (safe current capacity) for each gauge and insulation type.
Voltage drop = (2 × Current × Distance × Resistance per foot) / Voltage
For example, a 20-amp 120V circuit at 50 feet distance:
A 14 AWG wire has 2.49 ohms per 1,000 feet (0.00249 per foot):
Result: Use 12 AWG copper for this circuit.
Our calculator does all of this instantly-but now you understand exactly what it's computing.
Standard Residential 120V Branch Circuits (15-20 Amps)
Most household outlets and lights run on 15 or 20 amp circuits. A 15A circuit typically uses 14 AWG wire, while a 20A circuit uses 12 AWG. These are usually short runs (under 50 feet from the panel to the farthest outlet), so the wire gauge doesn't change much. Always use the larger gauge (12 AWG) for a 20A circuit, as 14 AWG can overheat at 20 amps over longer distances.
240V Circuits for Large Appliances (30-50 Amps)
Dryers, ranges, water heaters, and AC units typically run 30-50 amp 240V circuits. A 30A circuit uses 10 AWG copper, while 40-50A circuits use 8 AWG or larger. These are usually single runs from the panel directly to the appliance, so distance is less of a concern. However, if your appliance is far from the panel (example: a detached building or distant pool equipment), distance becomes significant, and you may need a larger gauge.
240V Subpanel from Main Panel
A subpanel in a detached garage, workshop, or addition needs its own large wire run. A 100A subpanel typically uses 4 AWG copper (or 2 AWG for longer runs over 100 feet). If the subpanel is only 60A, 6 AWG suffices for shorter distances. The distance from main to subpanel is critical-get it wrong and the subpanel doesn't get full power, or the wire overheats.
Overhead vs. Underground Runs
If wire runs overhead (aerial), it's exposed to temperature changes and UV, so you might choose a slightly larger gauge for durability. Underground wire must be in conduit and is protected from temperature swings, but conduit friction can make pulling difficult-larger conductors are physically harder to pull. Most installers stick to NEC minimums for underground runs but may upsize slightly for overhead or surface runs for longevity.
Tips and Things to Watch Out For
Always consult the National Electrical Code (NEC) or a licensed electrician for actual installation. This calculator provides guidance, but actual installations require professional design and inspection, especially for circuits over 20 amps, subpanels, or anything larger than a simple branch circuit.
Measure distance one-way from breaker to load, not round-trip. The wire runs out one way; voltage drop is based on that single distance, not the return. A panel 50 feet away has 50 feet of distance, not 100 feet.
Copper wire is more conductive than aluminum, so aluminum typically needs one or two gauge sizes larger. The NEC provides separate ampacity tables for copper and aluminum. Copper 12 AWG at 20A is equivalent to roughly aluminum 10 AWG. Copper is preferred for most residential work.
Account for conduit fill if you're pulling multiple wires in the same conduit. NEC rules limit how many wires fit in a conduit size. Tighter packing increases heat and may require derating (using a lower ampacity). A professional electrician accounts for this; calculators usually assume single-wire runs.
Never downsize wire because "it's just for a short distance." Voltage drop still happens over short distances if amperage is high. A 50A circuit at 30 feet needs 8 AWG even though it's nearby.
Always order 10-15% more material than your calculation to account for waste, cuts, and breakage. Wire gets damaged during pulls, spliced incorrectly, or wasted on connections. Order 10-20% extra to cover unknowns.
Don't mix wire gauges in a circuit. Once you've sized the wire, all wire in that circuit must be the same gauge. If you use 12 AWG for the first half of a run and 14 AWG for the second, the 14 AWG is a bottleneck and can overheat.
Verify breaker sizing matches wire gauge. A 12 AWG wire (rated for 20A at 60°C) should have a 20A breaker, not a 25A or 30A. Oversizing a breaker for undersized wire creates a fire hazard.
Frequently Asked Questions
What does AWG mean?
AWG stands for American Wire Gauge. It's a standard measurement system for wire diameter and cross-sectional area. Lower numbers are thicker (bigger) wires. 14 AWG is thinner than 12 AWG, which is thinner than 10 AWG. Thicker wire (lower number) carries more current safely.
What's the difference between 14 AWG and 12 AWG?
14 AWG is smaller (0.064" diameter) and rated for 15A at 60°C per NEC. 12 AWG is larger (0.081" diameter) and rated for 20A at 60°C. For a 20A circuit, you must use 12 AWG or larger. 14 AWG on a 20A circuit will overheat.
Can I use a wire one size smaller if it's just a short run?
No. A 50-foot run at 20A still needs 12 AWG (or larger); voltage drop and ampacity requirements don't waive for distance. The calculator accounts for distance, but there's a minimum gauge for every amperage regardless of how close the load is.
What happens if I use wire that's too small?
The wire overheats due to excessive resistance, which can melt insulation, cause shorts, or ignite nearby materials. Undersized wire also violates the NEC and your local electrical code, causing failed inspections and potential liability.
When would I use 10 AWG vs. 12 AWG?
Use 10 AWG for 30A circuits, longer distances on 20A circuits, or 240V circuits. 12 AWG is fine for 20A branch circuits under ~50 feet. The calculator tells you which is required for your specific distance and amperage.
What's voltage drop and why does it matter?
Voltage drop is the slight reduction in voltage as electricity travels through wire resistance. If your panel supplies 120V but the wire drops 5 volts, the outlet gets 115V. NEC allows 3% drop per branch circuit or 5% combined (main + branch). Excessive voltage drop dims lights, slows motors, or reduces appliance performance.
Do I need a subpanel, or can I just extend from the main panel?
You can extend circuits from the main panel up to practical limits. Subpanels are used when you need many circuits in a distant location (detached building, large addition) or when you're running out of breaker space in the main panel. The calculator works for both main-to-outlet and main-to-subpanel scenarios.
What wire should I use for a 30A dryer circuit?
A 30A 240V circuit (typical for electric dryers) uses 10 AWG copper. The run from breaker to dryer is usually short (under 50 feet) and doesn't vary much. Always use 10 AWG or larger-never 12 AWG for 30A.
Should I hire an electrician or can I do this myself?
For simple branch circuits (15-20A outlets), some homeowners with electrical knowledge can handle the work with proper permits and inspection. For anything over 20A, subpanels, or complex layouts, hire a licensed electrician. Electrical code violations can cause fires, shocks, or permit failures.
Related Calculators
For HVAC circuits, use our HVAC sizing calculator to determine if your electrical service has capacity for a new unit. Our material cost estimator helps budget wire, conduit, breakers, and installation labor for the full project.