1. Conductor size
In North America conductor size is given in American Wire Gauge. The scale runs backward: a larger number is a thinner wire. It is also geometric. Every three gauge steps roughly doubles or halves the cross-sectional area, and every six steps roughly doubles or halves the diameter. A 10 AWG copper wire has a resistance close to one ohm per thousand feet, which makes a convenient anchor: 13 AWG is about two ohms, 7 AWG about half an ohm.
Above 4/0 the gauge runs out and sizes are given by area in thousands of circular mils, written kcmil. A circular mil is the area of a circle one thousandth of an inch across. Metric specifications give area directly in square millimeters. The two systems do not line up exactly, so a metric size substituted for an AWG size is a change to the design and should be treated as one.
2. Stranding
The same gauge can be one solid wire or many fine ones. Stranding does not change the copper area much; it changes flexibility and fatigue life. A callout such as 19/32 means nineteen strands of 32 AWG.
| Construction | What it is | Where it is used |
|---|---|---|
| Solid | One wire | Building wire in small sizes, magnet wire |
| Concentric | Layers around a core: 7, 19, 37 strands | Most hookup, aerospace and power conductors |
| Bunched | Fine strands twisted together without a pattern | Cords and flexible leads |
| Rope lay | Groups of strands, themselves stranded | Welding cable, large flexible conductors |
Concentric strands give a round, predictable conductor that crimps consistently, which is why specifications for wire that will be terminated in crimp contacts insist on it.
3. Conductor material and plating
Copper is annealed soft for most wire. High-strength copper alloys are used in the smallest aerospace gauges, where pure copper would break. Aluminum appears in building wire, magnet wire and power cable, at a larger size for the same current.
Plating is chosen by temperature. Bare copper oxidizes as it gets hot. Tin protects it in ordinary service, silver at higher temperatures, and nickel higher still. The insulation’s rating and the plating’s limit have to agree: a high-temperature insulation over tin-plated strands is rated by the tin.
4. Insulation
The insulation material sets most of what the wire can do: its temperature limits, its resistance to fluids and abrasion, its weight and its behavior in a fire. PVC is economical and limited in temperature. Cross-linked polyethylene tolerates more heat and moisture. The fluoropolymers, ETFE, FEP and PTFE, run progressively hotter and resist nearly every fluid. Silicone stays flexible in the cold. The specification also fixes wall thickness, and with it the finished diameter, which matters when the wire has to fit a connector seal or a conduit.
5. Ratings
Two numbers. The temperature rating is the conductor temperature the wire can run at continuously, which includes the heat the current itself produces. The voltage rating is the working voltage the insulation is designed for. It is not the test voltage. Wire is proof-tested in production at a much higher voltage than it is rated for, and reading a test figure as a rating is a common mistake.
Some families carry further ratings in the type itself: wet-location suitability in building wire, oil and sunlight resistance in cord, smoke and flame performance in shipboard and transit cable.
6. Marking and color
The specification says how the wire identifies itself: a printed legend, a color, colored stripes, or a combination. In military part numbers color is a code at the end of the number. In building wire, color is set by convention and code for grounded and grounding conductors. The legend is worth requiring explicitly. It is what lets someone, years later, establish what a wire is and who made it.
Putting it together
Take M22759/16-20-9. The basic number names the specification family for fluoropolymer-insulated
aircraft wire. The slash sheet, 16, is a detail specification that fixes the insulation, plating, temperature
and voltage in one stroke. The next field is the size, 20 AWG, and the last is the color code, here white. Three
short fields, and nothing left open.
A commercial callout does the same job in words: 14 AWG, 19-strand tinned copper, type THHN, 600 V, 90 °C, black, listed. If a callout leaves one of the six out, the mill will fill the gap with whatever it normally makes, and the next mill will fill it differently.
One thing sits outside the six: the revision of the document cited. Standards change, and a bare number points at different requirements depending on when it is read. State the revision, or state that the revision in effect on the order date applies. Which family a callout belongs to, and who makes it, is covered in Wire types, and who makes which.