The nominal bare diameter of 40 AWG copper wire is 0.00314 inches (0.0799 mm). At this scale, you are no longer dealing with building wire or standard chassis wiring; 40 AWG is a micro-wire used almost exclusively in high-frequency RF transformers, medical micro-coils, aerospace sensor arrays, and precision thermocouples. Because it is thinner than a human hair (which averages 0.0039 inches), handling, terminating, and calculating thermal limits for 40 AWG requires entirely different standards than standard household electrical work.
Below is the complete reference data, thermal derating guidance, and practical handling advice you need when designing or repairing circuits with micro-wire.
The 40 AWG Specification Table (ASTM B258)
The physical dimensions and DC resistance of solid round copper wire are governed by ASTM B258. When reading the table below, note that the diameter columns represent bare copper. If you are using enameled magnet wire, you must consult the specific NEMA MW 1000 build grade (e.g., Grade 1 or Grade 2) to find the outside diameter (OD) with insulation, which will typically add 0.0002 to 0.0005 inches to the bare diameter.
| AWG Size | Bare Diameter (inches) | Bare Diameter (mm) | Cross-Section (cmil) | DC Resistance (Ω / 1000 ft @ 20°C) |
|---|---|---|---|---|
| 38 AWG | 0.00397 | 0.1008 | 15.7 | 653.5 |
| 39 AWG | 0.00353 | 0.0897 | 12.5 | 824.0 |
| 40 AWG | 0.00314 | 0.0799 | 9.89 | 1058.0 |
| 41 AWG | 0.00280 | 0.0711 | 7.84 | 1334.0 |
| 42 AWG | 0.00249 | 0.0633 | 6.20 | 1682.0 |
Source: ASTM B258 Standard Specification for Standard Nominal Diameters and Cross-Sectional Areas of AWG Sizes of Solid Round Wires Used as Electrical Conductors.
Current Capacity, Thermal Columns, and Derating Micro-Wire
When sizing standard building wire, electricians look at the NEC Article 310 ampacity tables. However, the NEC does not publish ampacity columns for 40 AWG. The National Electrical Code essentially stops general ampacity tables at 14 AWG. Using 40 AWG for any mains-adjacent, Class 1, or standard low-voltage building circuit (like a 24V thermostat) is a severe fire hazard; the wire will vaporize long before a standard breaker trips.
Which column applies to the reader's installation?
For 40 AWG, your governing thermal standards are NEMA MW 1000 (for enameled magnet wire in coils) and IPC-2221 (for internal electronics and PCB traces). Instead of an 'ampacity column,' you must look at the Thermal Class of the wire's insulation. Common classes for 40 AWG include:
- Class 155 (F): Polyurethane or polyesterimide. Rated for continuous operation up to 155°C.
- Class 180 (H): Polyesterimide with polyamide overcoat. Rated up to 180°C.
- Class 200 (N): Polyamide-imide. Rated up to 200°C, often used in aerospace and high-temp sensors.
A single 40 AWG wire in free air can typically carry 20mA to 30mA before exceeding a Class 155 thermal limit, but this drops drastically in bundled configurations.
How derating rows modify the base value
In building wire, derating is based on 'conduit fill' (how many current-carrying conductors share a pipe). In micro-wire applications, derating is dictated by the winding fill-factor inside a bobbin or toroid. When you wind 40 AWG wire into a multi-layer coil, the inner layers are thermally choked. They cannot shed heat to the ambient air.
If your coil has a high fill-factor (tightly packed layers with minimal air gaps), you must apply a 20% to 40% thermal derating to your free-air current limit. For example, if a single strand in free air handles 25mA safely, a densely wound 500-turn micro-transformer might require you to limit the RMS current to 15mA to prevent the inner layers from melting the enamel insulation and causing a shorted turn.
What the Table Cannot Tell You (Edge Cases & Handling)
The ASTM B258 chart gives you perfect-world geometry and DC resistance. It completely ignores the physics of high-frequency signals and the mechanical fragility of micro-wire.
The Skin Effect at High Frequencies
At DC or 60Hz, current flows evenly through the entire 0.0799 mm cross-section. But at 1 MHz, the 'skin depth' of copper—the depth at which current actually flows—is roughly 0.066 mm. Because 40 AWG has a radius of 0.0399 mm, it is actually thinner than the skin depth at 1 MHz, meaning it remains highly efficient. However, at lower RF frequencies (like 100 kHz), the center of the wire carries less current. If you need high current capacity at RF, engineers do not use a single thick wire; they use Litz wire (multiple strands of 40 AWG or 44 AWG woven together to maximize surface area).
Tensile Strength and Breakage
The breaking strength of annealed 40 AWG copper is roughly 0.12 to 0.15 lbs (about 55 to 65 grams of force). If you pull it tight by hand, it will snap. When winding bobbins, you must use a magnetic hysteresis tensioner or a felt-pad tensioner calibrated for micro-wire. Standard mechanical tensioners will stretch the copper, altering its diameter and increasing its resistance before it finally snaps.
Soldering and Termination
You cannot mechanically strip 40 AWG wire with standard wire strippers; the blades will cut the copper core. Instead, you rely on the insulation chemistry. If you are using Solderable Polyurethane (PU) enamel, the heat of a 380°C (715°F) soldering iron will melt the insulation and the flux will wet the copper simultaneously. If you are using Polyimide (PI) or high-temp amide-imide, it will not melt. You must strip it using a precision thermal stripper or a micro-abrasive eraser before tinning.
Frequently Asked Questions
What is the exact 40 AWG wire diameter in millimeters and inches?
The bare copper diameter of 40 AWG is exactly 0.00314 inches, which converts to 0.0799 millimeters. If you are purchasing enameled magnet wire, the outside diameter (OD) including the insulation layer will typically range from 0.0034 inches (0.086 mm) for a thin Grade 1 build, up to 0.0037 inches (0.094 mm) for a heavy Grade 2 build, per NEMA MW 1000 standards.
Can I use 40 AWG wire for household electrical wiring or low-voltage thermostat circuits?
No. 40 AWG is strictly for internal electronics, micro-coils, and sensor applications. It has a DC resistance of over 1,000 ohms per 1,000 feet, meaning it will suffer catastrophic voltage drop on any standard low-voltage run (like a 24V HVAC thermostat). Furthermore, it lacks the mechanical strength to survive terminal screw torque and will pose a severe fire hazard if subjected to standard control circuit currents.
How do you strip and solder 40 AWG enameled magnet wire without breaking it?
Do not use mechanical blade strippers. If your wire has a polyurethane (PU) coating, simply apply a hot soldering iron (set to roughly 380°C / 715°F) directly to the wire tip along with rosin-core solder; the heat will burn off the enamel and tin the wire in one step. For high-temp polyimide (PI) coatings that won't melt, use a specialized thermal wire stripper or gently ablate the enamel using a fiberglass scratch pen, then tin the exposed copper immediately to prevent oxidation.
What is the maximum current capacity for a 40 AWG copper wire?
Because the NEC does not cover 40 AWG, we rely on IPC-2221 and thermal class limits. A single 40 AWG wire in free air can safely carry about 20mA to 30mA continuous DC current before the temperature rises enough to degrade standard Class 155 enamel insulation. However, if the wire is tightly wound in a multi-layer coil where heat cannot escape, you must derate this value by up to 40%, limiting the safe continuous current to roughly 12mA to 15mA.
