SIZING OF SERVICE-ENTRANCE CONDUCTORS

The process of calculating the size of the service-entrance conductors supplying an industrial building is similar to the process we performed for a single-dwelling unit. The industrial load is divided into three groups:

1. General lighting loads

2. General-purpose receptacle loads

3. Special or equipment loads

Apply the appropriate demand factors and perform the standard calculations to determine the total load. Use table 3-5 to determine the correct service-entrance conductor size.

GROUND FAULT INTERRUPTERS

Ground fault interrupters (GFIs) are designed to provide reliable protection from line-toground faults, while other types of overcurrent protection may see the ground fault only as a load current. GFIs detect a ground fault by using a current transformer with the line and neutral conductors passing through the center of the transformer (fig. 3-1). A ground fault on any one of the conductors causes an unbalance in the circuit. The current transformer senses this unbalance and trips the circuit breaker quickly. All GFIs manufactured after 1 January 1976 are required to trip open automatically when the fault current reaches 5 milliamps.

The NEC^R requires all receptacles installed in bathrooms, garages, and outside areas to be protected by GFI circuit breakers or GFI receptacles. An exception to this rule applies to receptacles installed in garages to supply power to dedicated appliances, such as refrigerators, freezers, or gas dryers. Also, receptacles that are not readily accessible are not required to have GFI protection; for example, the receptacle for an overhead garage door motor.

GFI-protected receptacles are required for all 120-volt, single-phase, 15-amp and 20-amp circuits used by personnel on construction sites. This requirement can be fulfilled by installing GFI breakers on all receptacle circuits or by installing GFI receptacles at the jobsite.

Figure 3-1.\GFI circuit breaker.

ELECTRICAL INSTALLATIONS IN HAZARDOUS LOCATIONS

The material in this section is presented to develop your awareness of the special conditions to consider when you are installing and maintaining electrical equipment in hazardous locations. The requirements for each particular hazardous location project is beyond the scope of this chapter. Always consult the NECR, engineering department, or local authority when you are planning or constructing a project in a hazardous location.

CLASSIFICATION OF HAZARDOUS LOCATIONS

A "hazardous location" is defined as any location where fire or explosion hazards may exist because of flammable gases or vapors, flammable liquids, combustible dust, or ignitible fibers. Articles 501, 502, and 503 of the NECR classify these hazardous locations and specify the electrical components that are required to be installed in such locations to ensure safe operation.

The NEC^R divides hazardous locations into three classes: Classes I, II, and III and two divisions: Divisions 1 and 2.

Class I locations are those in which flammable gases or vapors are or may be present in quantities sufficient to cause explosions or fires. Class I locations include such facilities as fuel farms, flammable storage lockers, and paint spray booths. Class II locations are those where there is a potential for explosion or fire because combustible dust is present. Class III locations are ignitible-fiber locations.

Each hazardous class has two divisions. In Division 1 under each class, the particular hazard exists at any or all times of normal operation. Division 2 applies to locations where a hazard would exist if an accident, equipment failure, or unusual condition occurred. For example, Class I, Division 1 locations are areas where flammable gases or vapors are present during normal operating conditions, such as a paint spray booth. A flammable storage locker is considered a Class I, Division 2 location, and the hazard would exist only if the storage containers were accidentally ruptured or a spill occurred.