# Calculating Wire Length for Spiral Coil

• posted

I need to wind a spiral (pancake) coil from 0.4mm enamel wire. The ID is 20mm and the OD is 150mm.

What is the procedure for caluculating the length of wire needed?

I am not a math wizz, so an example would be much appreciated.

David King

• posted

Google for "spiral inductor calculator". For example:

```--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com ```
• posted

area of pancake is pi*75^2 - pi * 10^2 ~= 17357 mm^2

17357 / 0.4mm ~= 43392mm

-Lasse

• posted

Oops. 2nd try. I didn't notice the above didn't include the wire length. This should work: However, it assumes that you know the number of turns. So, I plugged in the following numbers: Inner Dia 20. mm Number turns --- Wire Dia 0.4 mm Wire spacing 0.0 mm I then tried different values for the Number of Turns until the Outer Diameter was equal to about 150 mm. I ended up with 162 turns and a length of 43.2 meters, which is fairly close to Lasse's calculation.

If you're actually going to build this coil, I don't think you'll be able to obtain 0.0 mm spacing between turns. The 0.4mm (#38 AWG???) might not include insulation thickness, lumps, kinks, and bends in the wire. I suggest you experiment with what spacing you can obtain, and use that value instead. (I'm also not a math wizz, but instead prefer to measure things).

```--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com ```
• posted

_This_ math whiz approaches problems like that by computing the wire length needed, cutting off about twice as much as the calculations say, then winding the coil. Often, I'll end up with extra. Very very rarely, it'll be half the wire used. Sometimes I'll come up short.

If you need to know for production, measure the starting length and the amount left over, and subtract.

```--
www.wescottdesign.com```
• posted

i will now starting winding and allow for a bit extra wire just in case.

David King

• posted

David King formulated on Sunday :

Why worry?? Just wind from the wire reel and cut off when you get there. Surly you are not going to buy the wire down th the last millimetre. :-Z

```--
John G Sydney.```
• posted

start with the area of a circle 150mm diameter , sutract the area of the 20mm hole, divide by the width of the wire.

```--
umop apisdn

--- news://freenews.netfront.net/ - complaints: news@netfront.net ---```
• posted

Yep, that's the way. It needs the EXTERNAL DIAMETER of the wire: if '0.4mm' is the wire gauge, you'll have to measure the insulation-plus-wire.

Another quasi-empirical approach is to wind one, and weigh it. That includes any stretching of the copper during the winding process.

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