# Current capacity of very thin (0.1mm ~ 0.5mm) copper wires?

I am looking at this very thin copper wire. It does not have much description other than the physical dimensions. What's the maximum current capacity of these wires? I guess they are not for power wiring (~ 200mA)?

• Not only are they pretty high resistance, so lots of voltage drop, but 0.1mm wire is very flimsy and will break easily. Suggest a minimum of about 0.25mm (AWG 30) unless you have a really good reason to use very fine wire. Commented Dec 29, 2014 at 19:22

## 3 Answers

That all depends on the length.

There's two factors to consider with wires:

1. The self-heating caused by the current
2. The voltage drop caused by the resistance

Number 1 is purely a factor of the wire diameter. Number 2 also has to take into account the length of the wire as well.

I find this table is a good reference: http://www.powerstream.com/Wire_Size.htm

0.1mm is roughly 38 AWG. So for that, for "chassis" wiring (i.e., short runs between boards, switches, lights, etc) the current limit is 130mA. For longer "power" runs, the limit is a mere 22.8mA.

The resistance of the wire is around 659.6Ω per 1000 feet, so about 0.659Ω per foot.

That kind if wire is usually used for winding your own inductors, transformers, or electromagnets. It's not often used for chassis wiring without an incredibly good reason.

• Thanks, I just need thin copper wires to debug my boards. I will get a more capable wire. Commented Dec 29, 2014 at 21:03
• I use 30 AWG wire-wrapping wire. Commented Dec 29, 2014 at 21:05

Your question is related with magnet enameled wires, and not house installation wires. Formal NEC does not dealling with fine magnet wires. There are many tables recommended this or that current based on a current capacity, but no one explain the choice of this J (A/cm^2 or inch^2). Voltage drop and consiquently length of conductor, confuse more the designer and finally cancelling when thermal equation solved. So length has a limited meaning on current capacity of a conductor.

A basic thermodynamic analysis it is the best way to calculate the ampacity of a wire. The remaining temperature on the wire is the generated heat minus the convected or emissed one. IEC recommends a maximum rise of 30 degr. Celc, above ambient allowing a good safety margin.

Bellow it is a graph made by me for the AWG 35, which is 0.143mm bare diameter (yours is 0.1mm i.e. AWG 38), assuming natural cooling convection (no fans etc), suspended vertically in air (not coiled). Ambient temperature is taken 25degr Celc. but recommend 35 for real designs.

The blue curve it is for bare wire capacity and purple is the current capacity with a typical magnet wire formavar insulation. You can ask the reason of this.

Once again this based in theory and valid for DC current ONLY, but it is agood start since you can not find souch data from manufacturers.

It depends on several factors. What is it being used for? How is it cooled (still air in a box, open air with a fan, in oil that may or may not be force-pumped, etc)? For winding an oil-cooled transformer I wouldn't go over 35mA with that, 25mA for better efficiency or air cooled. For short-run wiring it can do 150mA if not in a bundle and a fan on it. How long is it? More than a few feet and that will have a noticable voltage drop at 150mA (about 100mV per foot, which goes up as the wire heats up), esp when the source is only 5vdc or something (I prefer keeping drop under 2% where that is important). Magnet wire I've never used it for anything but, er, magnets (or transformers). For breadboard debugging usually 30awg wrapping wire is more than adequate for most things (ie can handle 3A or so with the usual Kynar insulation, if the length is short so v drop is low enough).