Wire Gauge Calculator - AWG Size Reference & Specs

AWG wire gauge overview

The AWG (American Wire Gauge) system uses an inverse scale - lower numbers mean thicker wire. 4/0 (0000) is the largest common gauge; AWG 40 is fine enough for watch coils.

Choosing the right gauge

• Match wire gauge to the maximum continuous current, not peak current.
• Derate by 20–30% when bundling multiple conductors.
• For long runs, consider voltage drop in addition to current capacity.

AWG to mm² conversion table

Voltage drop calculation

Excessive voltage drop causes motors to run hot and lights to dim. The NEC recommends voltage drop ≤3% on branch circuits and ≤5% total (feeder + branch). The formula is:

V_drop = I × R
where R = (resistivity × 2 × length) / cross-sectional area

Example: 20 A circuit, 50 feet of 12 AWG copper: R ≈ 0.16 Ω -> V_drop ≈ 3.2 V (2.7% on a 120 V circuit - within the 3% limit).

Temperature rating and insulation types

NEC ampacity tables assume specific conductor temperature ratings. Common insulation types:

• THHN: 90°C dry / 75°C wet; most common building wire in conduit.
• THWN-2: 90°C wet; suitable for underground and outdoor conduit.
• NM-B (Romex): 90°C conductors, but derated to 60°C in NM cable ampacity tables; used in residential walls.

When conduit is fully loaded (more than 3 current-carrying conductors), ampacity must be derated per NEC Table 310.15(B)(3)(a). High ambient temperatures (attics, boiler rooms) also require derating.