Wire Size Calculator

Step-by-Step Guide

1. 1Enter load current: Type the full-load current in amperes. For motor circuits, use the nameplate full-load amp (FLA) rating. For general branch circuits, use the circuit breaker amperage. Always size for the maximum expected load, not the average.
2. 2Set voltage and system type: Select your supply voltage (120V, 240V, 380V, 415V, 480V) and choose single-phase or three-phase. Three-phase systems require less conductor material for the same power — the calculator applies the correct √3 factor automatically.
3. 3Enter cable length: Enter the one-way run distance from the source to the load in meters or feet. For conduit runs with multiple bends, add 10–15% to the straight-line distance to account for the actual routed path.
4. 4Choose conductor material: Select copper or aluminum. Copper offers higher ampacity per mm² and is standard for most residential and commercial work. Aluminum requires a larger cross-section for the same current capacity but is cost-effective for large feeder runs.
5. 5Set maximum voltage drop: Enter your maximum allowable voltage drop percentage — typically 3% for NEC branch circuits, 2% for feeders. Use 1–2% for sensitive equipment. The calculator finds the smallest wire that satisfies both the ampacity and voltage drop constraints simultaneously.
6. 6Read and apply the result: The calculator returns the recommended wire size in both mm² and AWG, the actual voltage drop at that size, power loss in watts, and the next-size-up as a conservative alternative. Copy the result or export a text report for your project documentation.

What This Tool Provides

• ✓Recommended wire size in mm² and AWG
• ✓Actual voltage drop at selected size
• ✓Voltage drop percentage vs your limit
• ✓Power loss in watts for the cable run
• ✓Next-size-up conservative alternative
• ✓Single-phase and three-phase support
• ✓Copper and aluminum conductor options
• ✓Multiple voltage presets (120V–480V)
• ✓Calculation history (last 10 entries)
• ✓Export results as text report
• ✓Copy result to clipboard
• ✓100% browser-based — no data sent to server
• ✓No registration required

Pro Tips

• 💡For motor circuits, multiply the nameplate FLA by 1.25 before entering it into the calculator. NEC 430.22 requires branch circuit conductors to be rated at 125% of the motor's full-load current for continuous-duty motors — not 100%. A 10A motor needs a 12.5A conductor rating minimum.
• 💡Always use the one-way distance to the load, not the total round-trip length. The voltage drop formula already accounts for both conductors (the factor of 2 in single-phase calculations). Entering the round-trip distance doubles the result and will oversize your wire unnecessarily.
• 💡If the calculator recommends a size larger than you expected, check whether switching from single-phase to three-phase power is feasible. The √3 factor in three-phase circuits reduces voltage drop for the same conductor, often allowing you to drop one or two wire sizes on long runs to motors.
• 💡For cable runs buried directly in the ground or pulled through conduit with more than three current-carrying conductors, derate the ampacity. NEC Table 310.15(B)(3)(a) requires 80% capacity for four cables, 70% for five or six, 50% for seven to nine. Multiply your actual current by the inverse of the derating factor before entering it — this gives the effective ampacity the wire must supply.
• 💡Size for future load growth when possible. Running an extra wire size larger (e.g., 4 mm² instead of 2.5 mm²) on a long home run costs relatively little in material versus the labor cost of pulling new wire later. This is especially true for sub-panel feeders and service entrance conductors.
• 💡Verify that your selected wire size is compatible with the breaker. A 2.5 mm² (14 AWG) copper wire must be protected by a maximum 20A breaker (15A is safer). Connecting 2.5 mm² to a 30A breaker is a NEC violation — the overcurrent protection must match or be smaller than the wire's ampacity rating.

Common Mistakes to Avoid

• ✕Don't size wire for ampacity alone and ignore voltage drop. A 20A circuit at 120V over 50 meters technically has enough ampacity with 2.5 mm² (14 AWG), but the voltage drop is 5.6% — almost double the 3% limit. Lights will flicker, motor torque will drop, and equipment may fault. Always run the voltage drop calculation for any run longer than 15 meters.
• ✕Don't use the 'rule of thumb' AWG shortcuts without checking your specific voltage and distance. Common shortcuts assume 120V, 30°C, and short runs. At 48V DC or 24V control circuits, the same current over the same distance produces far higher percentage voltage drops — the rule of thumb breaks down entirely.
• ✕Don't forget that aluminum requires anti-oxidation compound and aluminum-rated lugs at every termination point. Aluminum conductors form an oxide layer that increases contact resistance over time. Without proper compound and connectors, the connection heats up, and the insulation eventually fails — a documented cause of residential fires.
• ✕Don't assume the same wire size works for both the hot and neutral conductors in a multi-wire branch circuit. A shared neutral serving two 20A ungrounded conductors on opposite phases carries up to 20A of unbalanced current. The neutral must be sized identically to the hots — it is not a 'lighter' conductor just because it doesn't carry current under balanced load.
• ✕Don't size wire for the continuous load without applying the 125% continuous load factor where required. NEC 210.19 requires branch circuit conductors to be sized at 125% of the continuous load (a load expected to operate for 3 or more hours). Failing to apply this factor means the wire runs at 100% of its thermal rating continuously — shortening insulation life significantly.