Wire Gauge Calculator
Find the minimum safe AWG wire gauge for an electrical circuit based on current load, wire run length, and system voltage. Use this when wiring new circuits, extending runs, or verifying existing installations.
About this calculator
Wire gauge selection balances two concerns: current-carrying capacity (ampacity) and voltage drop over distance. This calculator uses the formula: AWG = max(12, 18 − log₂((Current × Distance × 2) / (Voltage × 0.03))). The factor of 2 accounts for the full round-trip length of the circuit (hot and neutral/return conductors). The 0.03 term represents a 3% maximum allowable voltage drop, a common NEC guideline for branch circuits. The log₂ function maps the resistance requirement back to the AWG numbering scale, where each 3-gauge step roughly doubles the wire's cross-sectional area. The floor of AWG 12 enforces a practical minimum for safety. Always verify results against local electrical codes.
How to use
Suppose a 20 A circuit runs 50 feet from the panel on a 120 V system. Plug into the formula: AWG = max(12, 18 − log₂((20 × 50 × 2) / (120 × 0.03))). First compute the numerator: 20 × 50 × 2 = 2,000. Then the denominator: 120 × 0.03 = 3.6. Divide: 2,000 / 3.6 ≈ 555.6. Find log₂(555.6) ≈ 9.12. Subtract: 18 − 9.12 = 8.88, round up to AWG 9. Apply the floor: max(12, 9) = AWG 12. So 12 AWG wire is recommended for this run.
Frequently asked questions
How do I choose the right wire gauge for a long electrical run?
Longer wire runs increase resistance, which causes voltage to drop before it reaches the load. To compensate, you must use a thicker wire (lower AWG number). The key inputs are the current in amps, the one-way distance in feet, and your system voltage. A 3% voltage drop limit is the NEC guideline for branch circuits, while 5% may be acceptable for some feeder circuits. This calculator automates that calculation so you don't have to work through the resistance tables manually.
What is the difference between wire ampacity and voltage drop when sizing wire?
Ampacity is the maximum current a wire can safely carry without overheating, determined by the wire's cross-section and insulation rating. Voltage drop is the reduction in voltage along the wire due to its resistance, which increases with length and current. A wire may have sufficient ampacity for a given load but still cause problematic voltage drop over a long run. Best practice is to size wire for both constraints and use the larger (lower AWG) result. This calculator focuses on voltage drop, so always cross-check against ampacity tables in the NEC or your local code.
Why does AWG wire gauge get thicker as the number gets smaller?
The American Wire Gauge system is historically based on the number of drawing steps needed to produce a given wire diameter, so more steps produce thinner wire. Practically, AWG 4 is significantly thicker than AWG 14. Each decrease of 3 AWG numbers roughly doubles the wire's cross-sectional area and halves its resistance per foot. This counterintuitive numbering is a common source of confusion, but once understood it makes selecting heavier-duty wire straightforward: lower number always means more capacity.