The National Fire Protection Association National Electrical Code is the basic safety standard for electrical design for buildings in the United States and has been adopted by reference in many building codes. In some cases, however, local codes may contain more restrictive requirements. The local ordinance should always be consulted.
The National Electrical Code, or the National Electrical Code Handbook, which explains provisions of the code, may be obtained from NFPA, 1 Battery March Park, Quincy, MA 02269-9101.
The American Insurance Association sponsors the Underwriters Laboratories, Inc., which passes on electrical material and equipment in accordance with standard test specifications. The UL also issues a semiannual List of Inspected Electrical Appliances, which can be obtained from the UL at 333 Pfingsten Road, Northbrook, IL 60062-2096.
Electrical codes and ordinances are written primarily to protect the public from fire and other hazards to life. They represent minimum safety standards. Strict application of these codes will not, however, guarantee satisfactory or even adequate performance. Correct design of an electrical system, over these minimum safety standards, to achieve a required level of performance, is the responsibility of the electrical designer.
Electrical Symbols
Table 15.1 illustrates the graphic symbols commonly used for electrical drawings for building installations. ANSI Y32.2, American National Standards Institute, contains an extensive compilation of such symbols.
Building Wiring Systems
The electrical load in a building is the sum of the loads, in kilowatts (kW), for lighting, motors, and appliances. It is highly unlikely, however, that all electrical loads in a building will be at full rated capacity at the same time. Hence, for economic selection of the electrical equipment in a building, demand and coincidence factors should be applied to the total connected load.
The demand factor is the ratio of the actual peak load of equipment or system to its maximum rating. An air-conditioning fan, for example, may require 8 hp at maximum load, but it will have a 10-hp motor (the standard available size). Therefore, its demand factor is 8/ 10. Lighting fixtures in a building, in contrast, can only operate at full load, or at a demand factor of 1.0.
The coincidence factor is the ratio of the maximum demand load of a system to the sum of the demand loads of its individual components and indicates the largest portion of all the electrical loads likely to be operating at one time. Diversity factor is the multiplicative inverse of the coincidence factor. Demand factors and coincidence factors or diversity factors can be obtained from a number of sources, such as the NFPA National Electrical Code.
Motor and appliance loads usually are taken at full value. Household and kitchen appliances, however, are exceptions. The National Electrical Code lists demand factors for household electric ranges, ovens, and clothes dryers. Some municipal codes allow the first 3000 W of apartment appliance load to be included with lighting load and therefore to be reduced by the factor applied to lighting.
For factories and commercial buildings, the electrical designer should obtain from the mechanical design the location and horsepower of all blowers, pumps, compressors, and other electrical equipment, as well as the load for elevators, boiler room, and other machinery. The load in amperes for running motors is given in Tables 15.8 and 15.9.
Building Wiring Systems
The electrical load in a building is the sum of the loads, in kilowatts (kW), for lighting, motors, and appliances. It is highly unlikely, however, that all electrical loads in a building will be at full rated capacity at the same time. Hence, for economic selection of the electrical equipment in a building, demand and coincidence factors should be applied to the total connected load.
The demand factor is the ratio of the actual peak load of equipment or system to its maximum rating. An air-conditioning fan, for example, may require 8 hp at maximum load, but it will have a 10-hp motor (the standard available size). Therefore, its demand factor is 8/ 10. Lighting fixtures in a building, in contrast, can only operate at full load, or at a demand factor of 1.0.
The coincidence factor is the ratio of the maximum demand load of a system to the sum of the demand loads of its individual components and indicates the largest portion of all the electrical loads likely to be operating at one time. Diversity factor is the multiplicative inverse of the coincidence factor. Demand factors and
Plans
Electrical plans should be drawn to scale, traced or reproduced from the architectural plans. Architectural dimensions may be omitted except for such rooms as meter closets or service space, where the contractor may have to detail his equipment to close dimensions. Floor heights should be indicated if full elevations are not given. Locations of windows and doors should be reproduced accurately, and door swings shown, to facilitate location of wall switches. For estimating purposes, feeder or branch runs may be scaled from the plans with sufficient accuracy.
Electrical plans may be drawn manually or by using a computer-aided drafting and design (CADD) system. Although a significant initial investment is required, CADD can make the preparation of drawings fast and efficient and can make the interchange of information between electrical and the other engineering disciplines much easier.
Indicate on the plans by symbol the location of all electrical equipment (Table 15.1). Show all ceiling outlets, wall receptacles, switches, junction boxes, panelboards, telephone and interior communication equipment, fire alarms, television master-antenna connections, etc.
A complete set of electrical plans should include a diagram of feeders, panel lists, service entrance location, and equipment. Before these can be shown on the plans, however, wire sizes should be computed in accordance with procedures outlined in the following paragraphs.
Where there is only one panelboard in an area, and it is clear that all circuits in that area connect to that box, it is not necessary to number the panel other than to designate it as, for example, apartment panel. In larger areas, where two or more panelboards may be needed, each should be labeled for identification and location;
for example, L.P. 1-1, L.P. 1-2 . . . for all panelboards on the first floor; L.P. 2-1, L.P. 2-2 . . . for panels on the second floor.
Branch Circuits
It is good practice to limit branch runs to a maximum of 50 ft for 120-V circuits and 100 ft for 277-V circuits by installing sufficient panelboards in efficient locations.
Connect each outlet with a branch circuit and show the home runs to the panelboard, as indicated in Table 15.1. General lighting branch circuits with a 15-A fuse or circuit breaker in the panelboard usually are limited to 6 to 8 outlets, although most codes permit 12. No more than two outlets should be connected in a 20-A appliance circuit.
It is good practice to use wire no smaller than No. 12 in branch circuits, though some codes permit No. 14. Special-purpose individual branch circuits for motors or appliances should be sized to suit the connected load.
Electric Services
For economy, alternating current is transmitted long distances at high voltages and then changed to low voltages by step-down transformers at the point of service.
Small installations, such as one-family houses, usually are supplied with threewire service. This consists of a neutral (transformer midpoint) and two power wires with voltage differing 180 in phase. From this service, the following types of interior branch circuits are available:
Single-phase two-wire 230-V by tapping across the phase wires Single-phase two-wire 115-V by tapping across one phase wire and the neutral Single-phase three-wire 115/230-V by using both phase wires and the neutral For larger installations, the service may be 480/277-V or 208/120-V, threephase four-wire system. This has a neutral and three power wires carrying voltage differing 120 in phase. From this service, the following types of interior branch circuits are available:
Single-phase two-wire 480-V or 208-V by tapping across two phase wires Single-phase two-wire 277-V or 120-V by tapping across one phase wire and the neutral
Two-phase three-wire 480/277-V or 208/120-V by using two phase wires and the neutral
Three-phase three-wire 480-V or 208-V by using three phase wires Three-phase four-wire 480/277-V or 280/120-V by using three phase wires and the neutral
