40 AWG wire is an ultra-fine solid copper conductor with a bare diameter of exactly 0.00314 inches (0.0799 mm). To put that in perspective, it is roughly the thickness of a human hair. You will never find 40 AWG wire in residential branch circuits, panel wiring, or standard low-voltage thermostat runs. Instead, it is the baseline standard for micro-coils, implantable medical devices, aerospace telemetry harnesses, high-frequency RF transformers, and precision thermocouples.
Because it sits far outside the scope of the National Electrical Code (NEC) branch circuit tables, sizing and handling 40 AWG wire requires a shift from standard electrician practices to micro-electronics engineering. Below is the complete reference data, derating rules, and handling protocols for working at this extreme end of the American Wire Gauge spectrum.
40 AWG Wire Specification Chart (ASTM B258)
| AWG Size | Diameter (in) | Diameter (mm) | Area (mm²) | Resistance (Ω/1000 ft) | Chassis Ampacity | Power Ampacity |
|---|---|---|---|---|---|---|
| 38 | 0.00397 | 0.1008 | 0.0080 | 678.1 | 0.052 A | 0.017 A |
| 39 | 0.00353 | 0.0897 | 0.0063 | 854.8 | 0.042 A | 0.014 A |
| 40 (Target) | 0.00314 | 0.0799 | 0.0050 | 1079.0 | 0.033 A | 0.011 A |
| 41 | 0.00280 | 0.0711 | 0.0040 | 1359.0 | 0.026 A | 0.009 A |
| 42 | 0.00249 | 0.0632 | 0.0031 | 1713.0 | 0.021 A | 0.007 A |
Note: The fusing current (the point at which the wire melts and acts as a fuse) for 40 AWG copper in open air is approximately 0.50 A. Exceeding the 33 mA chassis rating continuously will cause the insulation enamel to carbonize and fail long before the copper melts.
Applying Derating and Installation Rules to Micro-Wire
When designing a harness or winding a coil with 40 AWG wire, the raw numbers in the table above are only your starting point. Micro-wire is exceptionally sensitive to thermal and mechanical stress.
Which Column Applies to Your Installation?
If you are winding a high-frequency inductor on a ferrite core where the wire is exposed to ambient air or potted in a thermally conductive epoxy, use the Chassis Ampacity (33 mA). If you are routing a bundle of fifty 40 AWG telemetry wires through a sealed polyimide sleeve inside an aerospace enclosure, you must use the Power Transmission column (11 mA) as your baseline, because the inner wires cannot shed heat to the environment.
How Derating Modifies the Base Value
Bundle derating hits micro-wire harder than standard 12 AWG or 10 AWG home wiring. According to aerospace wire derating guidelines referenced by NASA NEPP, a bundle of 15 to 30 micro-wires requires a derating factor of 0.70. For a bundle exceeding 30 wires, the factor drops to 0.60.
Worked Example: You are routing 40 individual 40 AWG wires through a single braided shield. Your base power transmission ampacity is 11 mA. Applying the 0.60 derating factor for a 30+ wire bundle yields a safe continuous current limit of just 6.6 mA per conductor. If your sensor requires 10 mA, you must step up to 38 AWG or split the bundle.
What the Table Cannot Tell You
Standard AWG charts only define the bare copper. They omit three critical factors for 40 AWG applications:
- Insulation Build: 40 AWG magnet wire is typically coated in polyurethane or polyester-imide enamel. A "Single Build" (MW 35-C) adds roughly 0.0002 inches to the diameter, while a "Heavy Build" (MW 80-C) adds 0.0004 inches. This drastically affects your fill factor when winding micro-transformers.
- Skin Effect: At frequencies above 1 MHz, current travels only on the outer edge of the conductor. Because 40 AWG is already thinner than the skin depth at most RF frequencies, it is highly efficient for RF coils, but parallel strands (Litz wire) are often preferred to maximize surface area.
- Tensile Strength: 40 AWG annealed copper breaks at approximately 1.2 to 1.5 pounds of tension. Automated winding machines must be calibrated to micro-Newton tension limits, or the wire will snap mid-spool.
Frequently Asked Questions About 40 AWG Wire
Can I use 40 AWG wire for home electrical wiring or low-voltage thermostats?
No. The NEC does not recognize 40 AWG for any building wiring, and standard 18 AWG is generally the minimum for flexible low-voltage control circuits. Beyond code compliance, 40 AWG wire lacks the mechanical strength to survive terminal screw compression. If you attempt to terminate 40 AWG wire under a standard thermostat screw terminal, the copper will crush, deform, and create a high-resistance hot spot that will cause the connection to fail or arc.
How do I solder 40 AWG enameled copper wire without burning it?
You cannot strip 40 AWG magnet wire with mechanical blades; you will sever the conductor. Instead, use the "hot iron and flux" method. Apply a generous drop of rosin-based flux to the wire tip. Set your soldering iron to 350°C (660°F) and tin the tip with fresh 63/37 leaded or SAC305 lead-free solder. Press the fluxed wire into the molten solder bead on the iron tip for 2 to 3 seconds. The heat and flux will chemically dissolve the polyurethane enamel, tinning the wire in one step. Wipe the iron tip on a damp sponge immediately after to prevent carbonized enamel buildup.
What is the difference between 40 AWG and 0.08mm metric wire?
They are functionally identical in most commercial contexts, but mathematically distinct. 40 AWG has a bare diameter of 0.0799 mm. In the IEC metric standard, wire is sized by cross-sectional area rather than diameter, and 0.08 mm refers to the diameter of the nearest metric equivalent. When sourcing from European or Asian suppliers, "0.08mm" wire will seamlessly substitute for 40 AWG in micro-coil winding, with a negligible resistance difference of less than 0.2%.
