Wire Gauge (AWG) Calculator

The AWG (American Wire Gauge) system is the North American standard for specifying the diameter of solid and stranded round wire. Despite its name suggesting a simple size number, AWG is a logarithmic scale — every 3-gauge decrease doubles the cross-sectional area, and every 6-gauge decrease doubles it again. This calculator exposes four practical tools in a single place: look up any gauge’s physical dimensions, reverse-engineer the nearest AWG from a known diameter, apply the correct NEC ampacity derating for your installation, and verify that a cable run stays within the recommended voltage-drop limits.

How it works

AWG to physical dimensions

The standard defines wire diameter with the closed-form expression:

d = 0.127 × 92^((36 − AWG) / 39) mm

From the diameter the calculator derives the cross-sectional area (A = π/4 × d²), the area in circular mils (d² × 10⁶ with d in inches), and the DC resistance per metre using material resistivity:

R/m = ρ / A

where ρ(copper) = 1.724 × 10⁻⁸ Ω·m and ρ(aluminium) = 2.65 × 10⁻⁸ Ω·m.

Ampacity and NEC derating

“Ampacity” is the maximum continuous current a conductor can carry before its insulation overheats. The NEC publishes base values in Table 310.15(B)(16) for three temperature ratings (60°C, 75°C, 90°C) and two conductor materials (copper, aluminium). The base value must then be derated for two real-world factors:

• Ambient temperature — the NEC 310.15(B)(2) correction factors assume a 30°C baseline; higher ambients reduce allowable current.
• Conduit fill — NEC 310.15(B)(3)(a) requires a derating when four or more current-carrying conductors share the same conduit, because the combined heat raises the effective operating temperature.

The derated ampacity = base × temperature factor × fill factor.

Voltage drop calculation

For a single-phase circuit, the full current flows out on one conductor and returns on the other, so the effective resistive length is twice the one-way run:

V_drop = 2 × L × I × ρ / A

For a balanced three-phase circuit the factor is √3 instead of 2:

V_drop = √3 × L × I × ρ / A

The percentage drop = V_drop / V_system × 100. The NEC recommends ≤ 3% for branch circuits and ≤ 5% for the combined feeder and branch.

Worked example

Scenario: You are running a 15 A, 120 V single-phase circuit in a residential garage. The panel is 25 m (82 ft) from the outlet. You plan to use AWG 14 copper wire in a conduit that already has 4 other circuits passing through it, and the garage can reach 40°C in summer.

Step 1 — Ampacity check (Ampacity tab):

• AWG 14 copper, 60°C base ampacity = 15 A
• Temperature correction (36–40°C) = 0.82 → 15 × 0.82 = 12.3 A
• Conduit fill with 4–6 conductors = 0.80 → 12.3 × 0.80 = 9.8 A

Result: the derated ampacity is only 9.8 A — insufficient for a 15 A circuit. Upsize to AWG 12 (base 20 A → derated 13.1 A, marginal) or AWG 10 (base 30 A → derated 19.7 A, comfortable).

Step 2 — Voltage drop check (Voltage Drop tab) with AWG 10:

• Current = 15 A, length = 25 m, system = 120 V, single-phase
• V_drop = 2 × 25 × 15 × 1.724×10⁻⁸ / (5.261×10⁻⁶) ≈ 2.46 V (2.05%)

Result: well within the 3% NEC recommendation. AWG 10 is the right choice for this installation.

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