Building Design and Construction Handbook - Merritt - Ventech!

15.12 SECTION FIFTEEN
15.4 ELECTRICAL LOADS
Electric services in a building may be provided for several different kinds of loads:
lighting, motors, communications equipment. These loads may vary in voltage and
times of service, as for example, continuous lighting or intermittent elevator motors.
Motors have high instantaneous starting currents, which can be four to six times
the running current, but which lasts only a brief time.
It is highly improbable that all of the intermittent loads will occur at once. To
determine the probable maximum load, demand factors and coincidence factors
(diversity factors) must be applied to the total connected load (see Art. 15.8).
Lighting Loads. The minimum, and often the maximum, watts per square foot of
floor area to be used in design are specified by building codes for various uses of
the floor area. Maximum wattages are set to conserve energy and should be followed
wherever possible. Electrical engineers, however, may exceed the minimum
wattages if the proposed use requires more. For example, lighting may be designed
to give a high intensity of illumination, which will require more watts per square
foot than the code minimum. (Recommended lighting levels are given in the Illuminating
Engineering Society ‘‘Lighting Handbook.’’)
Power Loads. In industrial buildings, the process equipment is normally the largest
electrical power load. In residential, commercial, and institutional buildings, the
power loads are mainly air-conditioning equipment and elevators. Some commercial
and institutional buildings, though, contain significant computer and communication
equipment loads, and special attention is required to properly serve these electronic
equipment loads.
Electronic Equipment Loads. The electric power from the utility company is
contaminated with electrical noise and spikes and is subject to sags, surges, and
other power-line disturbances. The sensitivity of electronic equipment requires that
the electrical system include equipment that will reduce the effect of these disturbances.
Selection of this protection equipment should be based on the functions to
be performed by the electronic equipment and a consideration of the consequences
disturbance might cause, such as disruption of service lost data equipment damage
and attendant costs. Most manufacturers specify the power quality needed for satisfactory
operation of their electronic equipment. In fact, manufacturers of many
computer systems furnish specific site-preparation instructions that address not only
electrical power, but also lighting, air conditioning, grounding, and room finishes.
For protection purposes, for a personal computer or workstation, it may be only
necessary to provide a good-quality plug-in strip with a transient-voltage surge
suppressor (TVSS). A medical imaging system, such as a CAT-scan machine, may
require a ‘‘power conditioner’’ that combines a voltage regulator, to eliminate sags
and surges, with a shielded isolation transformer, to block spikes and noise. In
critical installations, though, where equipment failure or an outage can have serious
effects, more extensive steps must be taken. For example, loss of service to a
satellite communication facility, a banking computer center, or an air-traffic control
tower can have severe adverse consequences. To prevent this, such facilities should
be provided with an uninterruptible power supply, backed up by either an alternate
utility service or a stand-by generator.
A commonly used uninterruptible power supply (UPS) has a rectifier that is
fed from the utility power line and delivers dc power to a large bank of batteries,