Twin-conductor magnet wire table

[shbazjinkens]

I'm forwarding this from a tube amp board because no one there can figure out the answer, I've had no luck at the library either so I thought maybe you guys could lend a hand:

Hi all,

Does anybody know where I can get a wire table for twin wire?

I'm going to wind a PT and I have a bunch of twin 29AWG. It's perfect for my current needs and will allow me to use less space, but I need to have a wire table for find out the resistance per 1000ft and turns per layer numbers.

Looking in the Radiotron Designers' Handbook 4thEd. there is mention of using twin 20 AWG in the example in the section on power transformer as opposed to 17 or 18 AWG "to save winding height".
The author used twin 20AWG and used 12 for the resistance per 1000ft. How did he arrive at that number? 20ga is 10.15 ohms per 1000, 21 is 12.8.
This is why I need a table.

The RDH4 has no other reference to twin wire, neither in the index nor anywhere else in the book.
Also, a Google search yielded nothing.

So where can I find this information?

Thanks in advance,
Henry

 

[alehman]

Never heard of twin magnet wire. If the two wires are joined at the ends, the DC resistance of two wires in parallel should be half of the resistance of a single wire.

 

[shbazjinkens]

Someone already thought of that..

I wish it were that simple but there is a problem with doing that.

In RDH4 the author uses 12 ohms for the twin 20AWG ohms/1000ft.
But 20AWG wire resistance is 10.15ohms/1000 ft.,
21AWG wire resistance is 12.80ohm/1000ft.

12 ohms is way over the number for 20AWG and almost to the amount for 21AWG. So this means I can't just divide the wire table number by two.

I suspect maybe it has something to do with the skin effect?

 

[RalphChristie]

Hi

Maybe contact some manufacturers?



Read through this file:

http://www.wiretron.com/magnet.html

In selecting & specifiying a magnet wire, there are several areas of concern:
1. Size or Diameter
2. Insulation Thermal Class
3. Insulation Build or Thickness
4. Additional Characteristics such as color or addition of bondable coating
--------------------------------------------------------------------------------
Conductors:
Although copper is the primary conductor material due to its superior conductivity & relatively low cost, other metals & alloys may be utilized.
--------------------------------------------------------------------------------
Thermal Classes:
There are a variety of film insulations available for magnet wire offering different characteristics. The primary classification factor for magnet wire is "Thermal Class." Thermal class is the recommended maximum continuous operating temperature for the insulation. Standard thermal classes range from 105 degrees Celsius up to 220 degrees Celsius.



Thus, there can be some variables, which can change the wire resistance.

 

[alehman]

The numbers you quote from RDH4 don't make sense to me.

Are you talking about DC resistance or AC resistance. Skin effect depends on frequency. For wire this size it would be negligable at 50 or 60Hz.

A quick google turned up the following formula.

Rac = ( pi*R^2 / (pi*R^2 - pi(R-e)^2 )) * DC resistance
where e = skin depth = (3160 / 2*pi ) * sqrt(res / (f * km))
res = resistivity of conductor
f = frequency
km = relative permeability