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Electrical Wire Size Calculator

Determine the correct wire gauge for your electrical project based on amperage, distance, voltage, and NEC requirements.

Written & reviewed by Michael FairUpdated June 2026Based on U.S. codes & pricingHow we calculate

How to Select the Right Wire Size for Any Electrical Circuit

Choosing the correct wire gauge is critical for electrical safety and code compliance. Wire that is too small for the current it carries can overheat, damage insulation, and cause fires. The National Electrical Code (NEC) provides strict guidelines for wire sizing based on ampacity and voltage drop.

Wire size in North America is measured using the American Wire Gauge (AWG) system, where smaller numbers indicate larger wires. Common residential sizes include 14 AWG (15-amp circuits), 12 AWG (20-amp circuits), 10 AWG (30-amp circuits for dryers/AC), and 8 AWG (40-amp circuits for ranges).

Two factors determine minimum wire size: ampacity and voltage drop. Ampacity is the maximum current a wire can safely carry without overheating. The NEC requires that continuous loads (running 3+ hours) use only 80% of the wire's rated ampacity — so a 20-amp continuous load requires wire rated for 25 amps.

Voltage drop becomes critical for long wire runs. The NEC recommends no more than 3% voltage drop for branch circuits and 5% total from the service panel. A 3% drop on a 120V circuit means only 3.6V loss. On a 100-foot run, this might require upsizing from 12 AWG to 10 AWG.

Copper wire is the standard for most residential applications due to its superior conductivity. Aluminum wire costs less and is commonly used for service entrance cables and long feeder runs. When using aluminum, you must upsize by approximately 2 gauge numbers compared to copper for equivalent ampacity.

How to Use This Calculator

1. Enter the circuit amperage (check your breaker or appliance rating). 2. Select the voltage (120V for standard outlets, 240V for dryers/ranges, 208V/480V for commercial). 3. Enter the one-way distance from the panel to the load in feet. 4. Choose single-phase (residential) or three-phase (commercial). 5. Select copper or aluminum wire material. 6. Set max voltage drop % (3% is standard NEC recommendation). 7. Click Calculate to see the recommended wire gauge.

Understanding Your Result

Your result shows the minimum wire gauge (AWG number) needed for the circuit's amperage, plus the voltage drop percentage at your wire-run distance. If the voltage drop exceeds the NEC's 3% guideline for branch circuits, the calculator recommends upsizing to a larger gauge. A smaller AWG number means a thicker wire with lower resistance — 10 AWG is larger than 12 AWG.

Worked Example

A 20-amp kitchen circuit with a 75-foot one-way run from the breaker panel, 120V, copper wire.

  1. 1Standard wire for 20 amps: 12 AWG copper (rated for 20A per NEC Table 310.16).
  2. 2Check voltage drop at 75 ft: VD = (2 × 75 × 20 × 1.98) ÷ 1000 = 5.94 volts.
  3. 3Percentage: 5.94 ÷ 120 = 5.0% — exceeds the 3% NEC guideline.
  4. 4Upsize to 10 AWG: VD = (2 × 75 × 20 × 1.24) ÷ 1000 = 3.72 volts = 3.1% — at the guideline limit.

Use 10 AWG copper wire for this run. The standard 12 AWG carries 20 amps safely in terms of heat, but the 75-foot distance causes enough voltage drop to dim lights and underperform appliances. 10 AWG brings the drop to roughly 3%, which is the NEC's recommended maximum for branch circuits. 10 AWG THHN copper costs about $0.40–$0.60 per foot versus $0.25–$0.35 for 12 AWG — a modest upcharge for a long run.

Assumptions Behind This Estimate

  • Wire resistance values are for copper conductors at 75°C (NEC Chapter 9, Table 8): 14 AWG = 3.14 Ω/1000 ft, 12 AWG = 1.98 Ω/1000 ft, 10 AWG = 1.24 Ω/1000 ft, 8 AWG = 0.778 Ω/1000 ft.
  • Voltage drop formula: VD = (2 × one-way length × current × resistance per 1000 ft) ÷ 1000. The factor of 2 accounts for the round-trip (hot and neutral).
  • The NEC 3% guideline for branch circuits is a recommendation ("should"), not a hard requirement ("shall") — but exceeding it causes real performance problems.
  • Continuous loads (running 3+ hours) must use only 80% of the wire's rated ampacity — a 20-amp continuous load requires wire rated for 25 amps.
  • Aluminum wire has about 61% the conductivity of copper — upsize by roughly two gauge numbers (e.g., 10 AWG copper → 8 AWG aluminum) for equivalent performance.

Before You Buy: Practical Advice

  • Measure the actual wire-run distance along the path the cable will follow (through walls, across joists, down to the panel) — it is almost always longer than the straight-line distance.
  • For runs over 50 feet on 15- or 20-amp circuits, always calculate voltage drop before buying wire — upsizing one gauge costs a few extra dollars but prevents flickering lights and sluggish appliances.
  • Use THHN/THWN-2 wire rated for 90°C in conduit or NM-B (Romex) for residential in-wall runs — the insulation type affects the ampacity table you reference.
  • Label every circuit at the breaker panel with the wire gauge and what it serves — future electricians and inspectors will thank you, and it prevents someone from accidentally loading a 14 AWG circuit beyond 15 amps.
  • All electrical work requires a permit and inspection in most jurisdictions — even a single new circuit. Unpermitted wiring is a fire risk, an insurance liability, and a problem at resale.

Frequently Asked Questions

What size wire do I need for a 20-amp circuit?

12 AWG copper wire is required for a 20-amp circuit. For runs over 50 feet, consider upsizing to 10 AWG to minimize voltage drop. Always use 20-amp rated outlets and a 20-amp breaker with 12 AWG wire.

How do I calculate voltage drop?

Voltage drop = (2 × Length × Resistance × Current) / 1000 for single-phase circuits. Length is the one-way distance in feet, resistance is per 1000 feet of wire, and current is in amps. Keep total drop under 3% for branch circuits.

Can I use aluminum wire instead of copper?

Yes, but you must upsize the wire. Aluminum has about 60% the conductivity of copper, so where you would use 10 AWG copper, you would need 8 AWG aluminum. Aluminum requires special connectors rated for aluminum to prevent corrosion issues.

What wire size do I need for a 240V circuit?

It depends on the amperage, not just voltage. A 30-amp 240V circuit (dryer) needs 10 AWG copper. A 40-amp circuit (range) needs 8 AWG. A 50-amp circuit (large range/welder) needs 6 AWG. Always check NEC tables for your specific application.