Wire Gauge Size Calculator (AWG)
Find the correct AWG wire size for a given current load and circuit length.
Checks both NEC ampacity and voltage drop limits.
What Is AWG (American Wire Gauge)? AWG is the standardized system used in North America to specify wire diameter. Counterintuitively, a lower AWG number means a thicker wire — AWG 4 is much thicker than AWG 14. Thicker wire carries more current and has lower resistance per foot.
The Two Selection Criteria Correct wire sizing must satisfy two independent requirements:
- Ampacity, the wire must safely carry the load current without overheating.
- Voltage drop, the wire must not drop too much voltage along its length.
Failing either test means the selected gauge is too small.
Ampacity (Current Capacity) The NEC (National Electrical Code) sets maximum current ratings for each AWG size. These values assume standard installation temperatures and insulation type. Aluminum wire has roughly 80% the ampacity of same-gauge copper wire, requiring one or two gauge steps up for equivalent capacity.
Voltage Drop Calculation Voltage drop is caused by the resistance of the wire itself: VD = 2 × L × I × R_per_foot Where L is the one-way circuit length, I is the current, and R_per_foot is the wire resistance in ohms per foot. The factor of 2 accounts for both the hot and neutral (or return) conductors. Resistance per foot values for copper (at 75°C): AWG 14 = 0.00321 Ω/ft, 12 = 0.00202 Ω/ft, 10 = 0.00127 Ω/ft, 8 = 0.000795 Ω/ft, 6 = 0.000510 Ω/ft, 4 = 0.000321 Ω/ft, 2 = 0.000202 Ω/ft, 1/0 = 0.000127 Ω/ft, 2/0 = 0.000101 Ω/ft, 3/0 = 0.0000797 Ω/ft, 4/0 = 0.0000626 Ω/ft. Aluminum resistance is approximately 1.64× copper for the same gauge.
NEC Voltage Drop Limits For branch circuits: maximum 3% voltage drop is recommended (NEC 210.19). For feeders: maximum 2% is recommended, with combined feeder + branch circuit under 5%. A 120V circuit with 3% max drop allows at most 3.6V drop (120 × 0.03).
Why Material and Application Matter Copper is preferred for most residential wiring — easier to terminate and less prone to oxidation. Aluminum is used for large feeders (200A service entrance, sub-panel feeds) to reduce cost and weight. Outdoor and direct-burial applications require cables with appropriate insulation ratings (USE-2, UF-B). Interior wiring uses NM-B (Romex) or THHN wire in conduit.
Practical Rule of Thumb For runs under 100 feet at typical household loads, ampacity usually governs the selection. For long runs (over 50 feet on 120V, over 100 feet on 240V), voltage drop often requires a larger gauge than ampacity alone demands. Always round UP to the next available gauge — never down.