# I took some nichrome wire from a heater, how can know it's gauge? Answered

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Here ya go, TG:  a table of AWG gauge numbers both mm and inches.

The numbers in this table were were copied from the Wikipedia article on American Wire Gauge, I linked to previously, here:
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes

 AWG Diameter (inch) (mm) 0000 (4/0) 0.46 11.68 000 (3/0) 0.41 10.4 00 (2/0) 0.36 9.27 0 (1/0) 0.32 8.25 1 0.29 7.35 2 0.26 6.54 3 0.23 5.83 4 0.2 5.19 5 0.18 4.62 6 0.16 4.12 7 0.14 3.67 8 0.13 3.26 9 0.11 2.91 10 0.1 2.59 11 0.09 2.31 12 0.08 2.05 13 0.07 1.83 14 0.06 1.63 15 0.06 1.45 16 0.05 1.29 17 0.05 1.15 18 0.04 1.02 19 0.04 0.91 20 0.03 0.81 21 0.03 0.72 22 0.03 0.64 23 0.02 0.57 24 0.02 0.51 25 0.02 0.46 26 0.02 0.41 27 0.01 0.36 28 0.01 0.32 29 0.01 0.29 30 0.01 0.26 31 0.01 0.23 32 0.01 0.2 33 0.01 0.18 34 0.01 0.16 35 0.01 0.14 36 0.01 0.13 37 0 0.11 38 0 0.1 39 0 0.09 40 0 0.08
You'll have to measure it. Gauge = Diameter in inches.

 40 = 0.0031 38 = .0040 36 = .0050 34 = .0063 32 = .0080 31 = .0089 30 = .0100 29 = .0113 28 = .0126 26 = .0159 25 = .0179 24 = .0201 22 = .0253 21 = .0285 20 = .0320 19 = .0359 18 = .0403 17 = .0453 16 = .0508 15 = .0571 14 = .0641 11 = .091

Here's a link to an online conversion tool (from mm's to inches). Once you have the diameter of your nichrome (in mm's), use the conversion tool to figure out which gauge you have. (I had to type out the chart from info I have here at home, and its only in inches).

There are two ways to do this.

The so called "gauge" of wire is a specification related to its thickness.  So one way to measure this, is to measure the diameter of the wire using a ruler, or a  micrometer,
http://en.wikipedia.org/wiki/Micrometer
if you've got one of those.
Then look up that diameter in in a table, like this one:
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes

For sort of a sanity check, you can examine your unknown wire, side by side, with some other pieces of wire you know the gauge of.  Frequently this number printed on the insulation of the wire, or well known for certain kinds of well known wire.  E.g. CAT5 network cable has size 24 AWG solid copper conductors, and you might have some of that lying around.

The other way to measure gauge,  is to realize that nichrome wire has certain resistance per unit length, that depends on the gauge (i.e. how thick the cross section is).  So you can take a length of this wire.  Then measure the electrical resistance with an ohmmeter.  Then divide that number of ohms by the length of the wire.

For example, if you have 2.0 meters of wire, and the electrical resistance of this measures 6.6 ohms,  then the resistance per unit length is 3.3 ohms/m, or 3300 mΩ/m.  The resistivity of nichrome is about 64 times that of copper.  So divide 3300 by 64, to get 51.6 mΩ/m, for the same piece of wire made out of copper.  Then you can look that number up in the AWG table I linked to above, and again here:
http://en.wikipedia.org/wiki/American_wire_gauge#Table_of_AWG_wire_sizes
and find that  22AWG copper has a resistance per unit length of 52.96 mΩ/m.  Then you sort of guess that your wire is also 22 AWG, or something close to that, since 52.96 is the closest number you can find to your measurement of 51.6.

You can read more about the resistivity of nichrome wire here
http://en.wikipedia.org/wiki/Nichrome
and that article is where I am getting my numbers from.

This second method, based on measuring electrical resistance, might sound like more trouble than its worth, but if you are building a heating element, then the actual electrical resistance of the heating element is the number you are interested in anyway.  I mean that's assuming you're driving it with a voltage source, and the power it dissipates is just P=V2/R (from Ohm's law), and that's the number that determines in part whether you are making the wire warm, hot, or melting it.

The wire will get hottest in those places where it is best thermally insulated, e.g. the middle of the wire, surrounded by still air.  If you stick the hot wire into a block of foam, naturally it will cool down somewhat , as heat is transferred to the melting foam.  I mean that's sort of the game that's being played here.  P=V2/R puts heat into the wire, and then heat transfer through materials surrounding the wire take the heat out of it.

I guess what I am saying is that number P=V2/R, the total power dissipated by your hot wire, is an important number, and to measure R you don't actually need to know the AWG gauge number of the wire, you just need to know its resistance per unit length, and you can measure that with an ohmmeter and a tape measure.

I forgot to mention, for using an ohmmeter to measure resistances this small, i.e just a few ohms, you should take into account the resistance in the probe wires plus contact points.

First you take a measurement with both probes touching the nichrome wire, and with the probes touching each other.  That is to say a zero length piece of nichrome wire is contributing to the resistance measurement.  Suppose the meter says 1.2 Ω.

Then do another measurement through 1 meter of nichrome wire, and the meter says 4.5 Ω.   Then do a measurement through a 2 meter piece of nichrome wire, and the meter says 7.8 Ω.

That is to say, with each additional meter of wire, the resistance increases by 3.3 Ω , and that's the number you're interested in.  I.e you are expecting proportional increases in the length of the wire, to result in proportional increases in the resistance of the wire.