Journeyman Electrician: Electrical Wires

Wire Insulation
The purpose of insulation on conductors is to prevent short circuiting between the wires.  Any number of insulating materials may be chosen depending on the voltage and current of the application and the environmental conditions where the conductors are used. 
The most common materials are:
- Plastic:  Used extensively for home and industrial power distribution as it is relatively cheap and durable.  Plastic jacketed 2- and 3- conductor cables (trade name Romex) are the most common form of wiring with this type of insulation.
- Rubber:  Used extensively in appliance power cords, it is moisture and heat resistant but is damaged by oil and solvents.  For this reason, Neoprene is sometimes used instead.
- PVC and Teflon:  Have superior durability but are usually limited to electronic applications because their cost is relatively high.
- Paper:  Once used extensively but its dielectric strength is lower than newer synthetic materials.
Overloading a conductor, operating at elevated ambient temperatures, or packing wires too densely in a conduit will generate excessive heat and may cause the insulation on the conductors to deteriorate.
If a conductor is encased within a material that is not recognized as electrical insulation by the code, it is referred to as a covered conductor.

Materials and Construction Used
The vast majority of electrical wire consists of a copper conductor and an insulator made from some form of synthetic material.  Silver is actually a better conductor than copper, but is not used extensively because of its cost.  Silver is sometimes used in small doses on electrical contacts to improve conductivity.
Aluminum was used in house wiring in the 1970s but was discontinued due to problems including corrosion.  Aluminum wire has the advantages of lower weight and cost.
Wire is sized by gauge and this refers to the conductor total cross sectional area which relates to the current carrying capacity, also called ampacity.
The wire conductor may be a single wire or consist of many smaller strands twisted together.  Solid wire is often preferred to stranded wire, because it can be shaped better and it is easier to connect without omitting stray strands.  The total diameter of stranded wire is larger than solid wire of the same gauge.

Conductivity of Wire Materials & Temperature
The resistance of a 10 gauge copper wire ten feet long is .01 Ohm.  For an aluminum wire about the same size and length, the resistance is less than .02 Ohm.  For shorter wires, any of the common conductive materials perform about the same.
In most cases, the resistance of a conductor increases with temperature.  The amount of increase is generally about 0.2% per degree
Fahrenheit.  

This means that if a copper conductor has a resistance of 1 Ohm at 70 degrees, then at 170 degrees its resistance is 100 x 0.2% = 20% higher, or 1.2 Ohms.  

Carbon is an exception as it resistance decreases with higher temperatures.  The resistance of carbon decreases about 0.03% per degree
Fahrenheit, so if a carbon motor brush has a resistance of 1 Ohm at 70 degrees, then at 170 degrees its resistance is 100 x -0.03% = 3% lower, or 0.97 Ohms.

Splicing and Insulating Wires
Splices are made by twisting two or more wires together, then soldering them, and finally insulating them with tape.  There are several different types of splices but all are made by twisting the wires tightly together.
A staggered splice is recommended for two conductor cable. 
It requires positioning the two splices at slightly different spots to avoid making a single large joint once the insulating tape is applied.
After soldering, insulation is added with the use of tape. 
The traditional method is to apply latex rubber tape first, followed by friction tape.  For each layer of rubber tape that is applied, it is stretched tight.  Pull the tape until it is ½ of its original width.
When completed the layers of tape blend into a solid rubber insulating cover.  Friction tape is then added for a protective covering.  It is made of cotton cloth that is impregnated with rubber.
Plastic electrical tape is often used instead of rubber and friction tape.  It is much thinner and withstands higher voltages.
Wire nuts are often used in place of a traditional splice.

Wire Color Code
The electrical code dictates only two colors for standard power system wiring, green and white.  There are other code requirements for special uses such as in medical facilities, and there are other common wire identification colors which are not specifically dictated by the code. Overall the convention is as follows:
- White or grey, or black with white stripes along the length of the wire: neutral, grounded conductor
- Black: hot, or ungrounded conductor
- Red (in 3-wire cable): hot, or ungrounded conductor
- Green, or bare uninsulated wire: Grounding wire connected to the equipment frame

For isolated power in health care facilities, two phase power uses orange and brown.  For three phase systems, yellow is also used.  If the conductors are wired into a single phase outlet, the orange wire is used for the grounded conductor.
For the special case of a delta wired system with the midpoint of one phase grounded, orange is used for the high leg.