Wire Gauge Calculator
Find the right wire gauge (AWG) for any current and run length. Enter load current, one-way distance, and voltage drop limit — get the required AWG with ampacity verification.
AWG
Enter your values above to see the results.
Tips & Notes
- ✓AWG is counterintuitive — lower gauge numbers are larger wires. AWG 4 is thicker than AWG 12. Every 6 AWG numbers doubles the cross-sectional area; every 3 AWG numbers multiplies diameter by √2 ≈ 1.41.
- ✓Two separate sizing checks are required: (1) ampacity — the wire must handle the continuous current without overheating; (2) voltage drop — the wire must keep drop within 3% (NEC recommendation) for the run length.
- ✓Conduit fill affects ampacity: wires in conduit with multiple conductors are derated because they cannot dissipate heat as easily as single conductors in open air. NEC Table 310.15 provides adjustment factors.
- ✓Temperature rating matters: 60°C rated wire (older installations) has lower ampacity than 75°C or 90°C rated wire. Most modern THWN-2 wire is 90°C rated in dry locations, 75°C in wet. Always use the lower of the termination and wire temperature ratings.
- ✓For aluminum conductors: never use aluminum for circuits smaller than AWG 8, and always use anti-oxidant compound at connections. Aluminum expands and contracts more than copper with temperature, causing connection loosening over time.
Common Mistakes
- ✗Sizing wire for ampacity only without checking voltage drop — in long runs (over 15 m at typical residential currents), voltage drop often requires a larger wire than ampacity alone dictates. Always check both criteria.
- ✗Ignoring the 80% continuous load rule — NEC requires circuit breakers to be loaded to no more than 80% of their rating for continuous loads (3 hours or more). A 20 A breaker should not supply more than 16 A continuous. Size wire for the breaker rating, not just the expected load.
- ✗Using the wrong temperature column in ampacity tables — terminations (breakers, lugs) are typically rated at 75°C even if the wire is 90°C rated. Use the 75°C ampacity column for most branch circuit applications.
- ✗Confusing AWG with metric wire sizes — AWG 12 = 3.31 mm², AWG 10 = 5.26 mm². Metric uses mm² directly: 2.5 mm² is common in Europe (close to AWG 13), 4 mm² is close to AWG 11. Never mix AWG and mm² without converting.
- ✗Undersizing wire for parallel conductors — when two conductors are run in parallel (for high-current loads), each conductor must be the same length, material, and cross-section, and each must individually be sized to carry the full load in an emergency.
Wire Gauge Calculator Overview
Wire gauge selection combines two engineering constraints: thermal capacity (ampacity — the wire must not overheat at the operating current) and electrical performance (voltage drop — the wire must not waste too much voltage on the way to the load). In short runs, ampacity governs; in long runs, voltage drop governs.
AWG diameter and area formula:
d (mm) = 0.127 × 92^((36−AWG)/39) | A (mm²) = π × (d/2)² | Resistance (Ω/km) = ρ × 1000 / A
EX: AWG 12 → d = 0.127 × 92^(24/39) = 0.127 × 16.16 = 2.053 mm → A = π × (1.026)² = 3.31 mm² → R = 1.68×10⁻⁸ × 10⁶ / 3.31 = 5.08 Ω/km = 0.508 Ω per 100 mWire sizing for voltage drop:
A_required (mm²) = 2 × I × ρ × L(m) / V_drop_max | Then select next larger standard AWG
EX: 25 A load, 30 m run, limit 3% of 240 V = 7.2 V → A = 2 × 25 × 1.68×10⁻⁸ × 30 / 7.2 = 3.5×10⁻⁶ m² = 3.5 mm² → AWG 12 (3.31 mm²) is slightly under → use AWG 10 (5.26 mm²)AWG properties — copper conductors:
| AWG | Diameter (mm) | Area (mm²) | R (Ω/100m) | Ampacity 75°C | Max 15A run (3% of 120V) |
|---|---|---|---|---|---|
| AWG 14 | 1.628 | 2.08 | 0.809 | 15 A | 14.8 m |
| AWG 12 | 2.053 | 3.31 | 0.508 | 20 A | 23.6 m |
| AWG 10 | 2.588 | 5.26 | 0.320 | 30 A | 37.5 m |
| AWG 8 | 3.264 | 8.37 | 0.201 | 50 A | 59.7 m |
| AWG 6 | 4.115 | 13.3 | 0.126 | 65 A | 94.9 m |
| AWG 4 | 5.189 | 21.2 | 0.0795 | 85 A | 150 m |
| AWG 2 | 6.544 | 33.6 | 0.0500 | 115 A | 238 m |
| Target Ampacity | Copper AWG | Aluminum AWG | Why Aluminum Needs Larger Size |
|---|---|---|---|
| 20 A | AWG 12 (3.31 mm²) | AWG 10 (5.26 mm²) | Al resistivity 1.58× higher |
| 30 A | AWG 10 (5.26 mm²) | AWG 8 (8.37 mm²) | Also Al expansion requires anti-oxidant |
| 100 A | AWG 1 (42.4 mm²) | AWG 1/0 (53.5 mm²) | Al cheaper per meter above AWG 4 |
| 200 A | AWG 3/0 (85 mm²) | AWG 350 kcmil (177 mm²) | Service entrance common in Al |
Frequently Asked Questions
Two criteria must both be met. First, ampacity: choose a wire whose rated ampacity (from NEC Table 310.15 or equivalent) exceeds your circuit current. For a 20 A circuit: AWG 12 copper (20 A rated at 60°C, 25 A at 75°C) is the minimum. Second, voltage drop: calculate V_drop = 2 × I × ρ × L / A and verify it is under 3% of supply voltage. If voltage drop requires a larger wire, use the larger size. The greater of the two requirements governs the selection.
NEC ampacity for copper conductors (75°C, THWN, in conduit): AWG 14: 15 A; AWG 12: 20 A; AWG 10: 30 A; AWG 8: 50 A; AWG 6: 65 A; AWG 4: 85 A; AWG 2: 115 A; AWG 1/0: 150 A; AWG 3/0: 200 A; AWG 4/0: 230 A. These ratings assume no more than three current-carrying conductors in conduit, ambient temperature 30°C (86°F), and 75°C insulation temperature rating. Higher ambient temperature or more conductors in conduit require derating.
AWG diameter: d (in) = 0.005 × 92^((36−AWG)/39). For metric: d (mm) = 0.127 × 92^((36−AWG)/39). Key relationships: every 6 AWG numbers doubles the cross-sectional area. Every 3 AWG numbers changes diameter by factor √2 ≈ 1.41. Every 10 AWG numbers changes diameter by factor 3.162 (≈√10). Examples: AWG 10 diameter = 2.588 mm; AWG 12 = 2.053 mm; AWG 14 = 1.628 mm. The ratio of consecutive diameters is 92^(1/39) = 1.1229, meaning each AWG step changes diameter by 12.3%.
Aluminum wiring is used for large conductors (AWG 4/0 and larger service entrances, utility feeders) where the cost and weight savings justify the extra precautions. Aluminum has 1.58× the resistivity of copper, so a larger aluminum conductor is required for the same ampacity: typically 1-2 AWG sizes larger. Never use aluminum for circuits smaller than AWG 8 — expansion/contraction and corrosion at connections cause loose connections that arc. When using aluminum for larger conductors: use CO/ALR rated devices, apply anti-oxidant compound at all connections, and use copper compression lugs or properly rated Al-Cu connectors.
Wire gauge (AWG number) describes the size of an individual conductor. Cable (e.g., 12/2 Romex, 14/3 NM-B) describes a group of conductors bundled together with insulation and a jacket. The first number is the AWG of the current-carrying conductors; the second is the count of conductors (not counting the bare ground wire). 12/2 cable contains two AWG 12 conductors (black hot and white neutral) plus a bare AWG 12 ground. 14/3 contains three AWG 14 conductors (black, white, red) plus ground — used for switch loops, 240 V circuits, and 3-way switch circuits.
12 V DC systems (automotive, solar, marine) require careful voltage drop analysis because any drop represents a high percentage of the 12 V supply. Maximum recommended drop: 0.5 V for most accessories (4.2%), 0.3 V for sensitive electronics. Formula: required area A = 2 × I × ρ × L / V_drop_max. Example: 30 A winch, 5 m run, limit 0.5 V → A = 2 × 30 × 1.68×10⁻⁸ × 5 / 0.5 = 1.008×10⁻⁵ m² = 10.08 mm² → AWG 8 (8.37 mm²) is slightly undersized → use AWG 6 (13.3 mm²). In automotive, always use the next gauge larger than calculated to account for connection resistance.