Fuses and circuit breakers are designed to interrupt the power to a circuit when the current flow exceeds safe levels. For example, if your toaster shorts out, a fuse or breaker should "trip", protecting the wiring in the walls from melting. As such, fuses and breakers are primarily intended to protect the wiring -- UL or CSA approval supposedly indicates that the equipment itself won't cause a fire.
Fuses contain a narrow strip of metal which is designed to melt (safely) when the current exceeds the rated value, thereby interrupting the power to the circuit. Fuses trip relatively fast which can sometimes be a problem with motors which have large startup current surges. For motor circuits, you can use a "time-delay" fuse (one brand is "fusetron") which will avoid tripping on momentary overloads. A fusetron looks like a spring-loaded fuse. A fuse can only trip once, then it must be replaced.
Breakers are fairly complicated mechanical devices. They usually consist of one spring loaded contact which is latched into position against another contact. When the current flow through the device exceeds the rated value, a bimetallic strip heats up and bends. By bending it "trips" the latch, and the spring pulls the contacts apart. Circuit breakers behave similarly to fusetrons - that is, they tend to take longer to trip at moderate overloads than ordinary fuses. With high overloads, they trip quickly. Breakers can be reset a finite number of times - each time they trip, or are thrown when the circuit is in use, some arcing takes place, which damages the contacts. Thus, breakers should not be used in place of switches unless they are specially listed for the purpose.
Neither fuses nor breakers "limit" the current per se. A dead short on a circuit can cause hundreds or sometimes even thousands of amperes to flow for a short period of time, which can often cause severe damage.
WIRING MATERIALS
Insulators and Conductors:
- Some materials such as wood and glass have high resistance and are called insulators.
- Other materials have low resistance and are called conductors such as copper and aluminum
To keep electricity from flowing where it is not supposed to go, conductors are covered with insulators (insulated wire)
Insulators:
Amount and type of insulation is determined by the voltage that will exist between them.
Type of insulation is decided under which the conductors must operate with regard to heat, moisture, or other conditions that might have a deleterious effect on insulation.
NEC has established a system of letters that indicate their characteristics.
It may be noted that the letters R, RU and T refer to material (rubber, latex rubber, or thermoplastic)
Letters H refer to high temperature or heat resistant and W is moisture resistant.
Conductors:
Any metallic substance conducts an electric current.
The relative ability of a material to conduct is determined by its resistivity, expressed in ohms-circular mil per foot.
Best conductor is silver but very expensive. Choice is narrowed to copper and aluminum.
Copper is the most common and is stronger than aluminum.
For larger conductors, aluminum is preferred because of its lower cost and weight.
Heat on Conductors:
Greatest hazard conductors must endure is heat. Continued exposure to excessive heat causes insulation to become soft, melt and in extreme cases to burn.
This heat comes from two sources: ambient air surrounding the conductors or from the current the conductors must carry.
NEC has a table for each specific insulation that determine the current-carrying capacity (or ampacity) of each size of conductor.
Wire Sizes:
Wires are usually round and the unit for measuring the cross-sectional area of wires is the circular mil (abbreviated cmil).
A circular mil is a circle 0.001 inch in diameter.
A circular mil-foot is a circular wire 1 foot in length and I mil in diameter. This is used to express resistance of a wire.
American Wire Gauge (AWG):
AWG was developed to assure the manufacturers of conductors in sizes that will be suitable for all applications.
It assigns a number to a particular size of wire.
It starts with #40 as the smallest with a diameter of 3.145 mils.
The gauge numbers then descend in order to #000, the largest with a dia of 460 mils.
NEC is used to select a wire size for a given insulation as it establishes the allowable current capacity for each insulated wires.
Stranded Wires:
Consists of a group of wires which are usually twisted to form a metallic string.
Stranding improves the flexibility of a wire.
Multiply the circular-mil area of each strand by the number of strands to find the total circular-mil cross section.
An insulated stranded wire is called a cord.
Wire Size and Ampere of Circuit:
What size wire should I use? Here's a quick table for normal situations.
Gauge - Amps
14 - 15
12 - 20
10 - 30
8 - 40
6 - 65
RACEWAYS
A channel for holding wires, cables, or bus bars
May be in the form of a pipe called conduit; a thinner wall conduit called electrical metallic tubing; or a square sheet metal duct of which one side has a removable cover
All raceways are mechanically installed as a complete system with all necessary outlet boxes and fittings. Afterwards the conductors are installed (pulled through) the raceway.
A cable is a complete assembly consisting of conductors and raceways as a unit.
The raceway is actually a covering that may be either metallic or non-metallic.
Wiring Methods and Type of Raceway:
a. Metal-Clad Cable
- Code permits two types of metal-clad cable: AC and ACL
- The metal covering for these cables is a steel spiral wrapping that forms a flexible raceway
- Manufactured as a complete assembly with the conductors installed.
- AC is commonly known as BX cable and cannot be buried in concrete or damp or wet locations.
- ACL is a cable with lead-covered conductors available for wet locations.
b. Non-Metallic Sheathed Cable
- Has a nonmetallic covering of fabric or plastic.
- Available in size nos. 14 to 1 AWG in copper and nos. 12 to 2 AWG in aluminum conductors.
- Used extensively for wiring of buildings
- Nonmetallic sheathed cable is called Romex.
c. Electrical Metallic Tubing (EMT)
- Has a thin wall that does not permit threading.
- Connectors and couplings are secured either by compression or set screws.
- An excellent raceway for conductors, can be buried in concrete but must not be subject to continuous moisture.
d. Rigid Conduit
- Has all the outward appearances of plumber’s pipe
- Have a smooth enameled interior to facilitate the pulling and installation of wires.
- Connections to boxes are made with locknuts and bushings after conduit is threaded.
- Used in most severe cases where the possibility of mechanical injury or the presence of moisture presents a problem.
e. Wireways
- Sheet-metal troughs with removable covers.
- Cannot be concealed, but are very useful in maintaining a complete raceway system when many devices must be interconnected in a limited area.
Selection of Raceways:
- A conduit or tubing system must be installed completely before conductors are inserted.
- In anticipation of larger loads, conduits larger that necessary may be installed.
- A maximum number of conductors are permitted in the standard sizes of conduit or tubing for new applications. (refer to tables)
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