Size Matters

You'll probably have noticed wires come in many different sizes.  Take apart a relay and the coil has fine coper wire thinner than human hair.  On the other side are thick cables feeding power into homes and businesses.  

With increasing size, wire can handle more electron flow (current, measured in Amperes).  Just like a garden hose, a larger diameter hose carries more water in a given time over a given distance.

Wire sizes are most often measures in standardized American Wire Guage (AWG) sizes.  The electronics industry uses the diameter of the wire in predefined sizes for manufacturability and known electrical characteristics.  

^{The Adafruit Ruler has a handy wire gauge finder built in from 8 to 28 AWG}

Below is a table of American Wire Gauge standard sizes and the characteristics of each wire size.  For a particular gauge, it has a set diameter and cross-sectional area.  When winding wire around for an inductor, coil, or transformer it is handy to know how thick the wire will stack up, which increases by wire gauge.  An important measure is the electrical resistance of the wire which increases as the gauge goes higher and the diameter decreases.  This is measured on ohms per kilometer or milliohms per meter.  Finally the last column for Americans lists national electrical code current capacity for larger gauge wires - it is a good demonstration of current capacity changing as wire area changes.

It is common to see the even size gauges in sizes 10 AWG and above.  The smaller odd sizes are not seen as often.

For a majority of projects, getting the exact gauge of wire is not a critical factor - close enough is usually fine. Here are some things to consider for choosing a gauge:

1. Power - if you have high current requirements in a curcuit like large motors or many LED lights, select thicker wire starting at 18 AWG and lower.  The size is based on handling the current while remaining cool plus providing lower resistance.  This goes for wearables also - do not sew too many power hungry LEDs with the thinnest wire that cannot handle the current draw.
   
   
2. Breadboards - typically breadboard holes only handle a small range of wire gauges.  Too small a wire and it falls out or makes a poor connection.  Too large a wire will bend out a hole or just not fit.  It is best to buy some wire specifically marketed for breadboards, most often 22 gauge.  Alternatively, if you run accross some solid telephone wire from a recent installation, it's a tad smaller but works.
   
   
3. Resistance - Although much of the time you can design circuits thinking that wires have near zero electrical resistance to current flow, there are times where you must choose wire so that resistance is not a big factor. If temperature is a factor in resistance measurements, take the resistance per kilometer above (we'll label that R₀), and for copper wire, resistance related to temperature is R = R₀ * (1 + 0.004 * (T - 20)) where the temperature T is in Celcius.  You can take resistance R and multiply by the number of kilometers for ohms, meters for milliohms.