WIRE GAUGE (AWG)
About this calculator
This wire gauge calculator returns the minimum AWG (American Wire Gauge) for a copper or aluminum conductor based on circuit amperage and run length. Sizing comes from NEC Table 310.16 at 75°C terminations (the standard for residential and commercial breakers, lugs, and panelboards). The calculator also runs a 3% voltage-drop check on the chosen size and bumps up if a long run pushes drop past code-recommended limits, since NEC 210.19(A) Informational Note 4 caps branch circuits at 3%. ESTIMATE ONLY — final wire size and install must be verified by a licensed electrician and the local AHJ before any work goes in.
How to use this calculator
Enter the circuit's current load in amps. Apply the 125% continuous-load factor first if applicable: dryers, EV chargers, electric heat, and any load running ≥3 hours need amps × 1.25 entered (e.g., 12A continuous → enter 15). System voltage is 120V for standard outlets and lights, 240V for dryers and AC, 208V for commercial 3-phase. One-way run length is the panel-to-load distance.
Pick copper (the common residential default) or aluminum (used for service entrance and large feeders to save cost on heavy gauges). The calculator returns the minimum AWG per NEC 310.16 ampacity tables, then runs a 3% voltage-drop check and bumps up to the next size if the run is too long for the picked gauge.
Worked example
For a 20A 240V circuit running 100 ft on copper:
NEC 310.16 ampacity: 12 AWG copper handles 20A → start at 12 AWG. Voltage-drop check: VD = (2 × 12.9 × 20 × 100) ÷ 6,530 ÷ 240 × 100 = 3.29% — over the 3% cap.
Bumped to 10 AWG: VD = (2 × 12.9 × 20 × 100) ÷ 10,380 ÷ 240 × 100 = 2.07% — passes. Final spec: 10 AWG copper (one size larger than ampacity-only would dictate), 20A breaker.
For a 50A EV charger 80 ft from the panel on copper: 8 AWG handles 50A ampacity. Drop check: 1.94% — passes without a bump. Final: 8 AWG copper, 50A breaker.
For a 200A service feeder 50 ft to a sub-panel on aluminum: 4/0 aluminum at 200A, drop ~1.2% — passes. Saves significant material cost vs 3/0 copper.
Common mistakes & waste factors
Forgetting the 125% continuous-load factor. NEC requires sizing wire and breaker for continuous loads (≥3 hr) at 125% of rated load. A 32A EV charger running for hours needs to be sized as if it's 40A.
Using residential 60°C terminations on commercial gear. Most modern panelboards have 75°C terminations (the calculator assumes this). Older or budget gear with 60°C terminations needs a larger wire for the same amperage.
Sizing for ampacity but not drop. Ampacity keeps the wire from melting; voltage drop keeps motors and lights running properly. Long runs almost always need the next size up beyond ampacity minimum.
Mixing aluminum and copper at terminations. Aluminum-to-copper junctions need anti-oxidant compound and listed connectors (CO/ALR or AL-CU rated). Otherwise corrosion creates resistive joints that overheat and burn.
Rules of thumb
Copper ampacity (NEC 310.16, 75°C): 14 AWG = 15A · 12 = 20A · 10 = 30A · 8 = 50A · 6 = 65A · 4 = 85A · 2 = 115A · 1/0 = 150A.
Aluminum runs at about 80% of copper ampacity for the same AWG — usually needs the next size larger.
Continuous loads (≥3 hr): apply ×1.25 to amps before sizing.
Voltage drop bump: roughly add one wire size for every 50–100 ft beyond what ampacity dictates.
Service entrance: 4/0 aluminum or 2/0 copper handles 200A. Common residential service today.
Common questions
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