Electric devices utilizing secondary or low voltage in the United
States have been standardized by almost all manufacturers at 120/240 V.
While many utilities are following these standards for their systems, there
are a significant number operating at 115/230 V and some at 125/250 V
(and a small and diminishing number at 110/220 V). For polyphase or
three-phase loads, the established ratings are 208 and 416V for wye-connected
systems and 240 and 480 V for delta-connected systems. In a few
cities, and some downtown and heavy load centers, network systems
supply a single-phase voltage of 277 Volts and a three-phase voltage of
480 V. (A few two-phase 115/230 V systems still exist.)
With distribution systems designed for practical voltage tolerances
expressed in volts plus or minus in relation to their normal, “standard,”
base single-phase voltage of 115, 120, and 125 V, voltages of 110 to 130
V can exist at the terminals of the loads (lamps, appliances, etc.). This is
a spread of ± 10 V, or ± 8.3 percent on a 120-V base, to which a flicker
voltage drop of 3 V, or 2.5 percent, should be added to allow for motor
starts. This would then give a total spread of 23 V, or 19.1 percent, from
+ 8.3 to – 10.8 percent—approximately within most manufacturers’ rated
maximum tolerances of ± 10 percent for motors and heating devices.
Closer coordination between manufacturers and utilities could do much
to improve this situation. Electronic devices are more sensitive to voltage
variations, and difficulty may be experienced with their operation at
variations of this magnitude.
Some utilities provide for an estimated voltage drop of up to 2 V in
the consumer’s wiring by specifying normal voltage at the service point
2 V higher than mentioned earlier, i.e., 122 instead of 120 V (117 and 127
V on other bases). Their designs, however, provide for the same high
and low voltage limits, but the variations above and below the usual
base are unequal, e.g., 128 V high, 122 normal, 114 low. The 23-V spread
and—10.8 percent variation mentioned above will be the same.
Not only does satisfactory operation of lights, appliances, and
other devices make for good consumer relations, but the effect of high
and low voltages (principally because of unity power factor lighting
loads) on both revenue and fuel conservation measures should also be
considered.
Higher voltages—660 V, 2400 V, and others—are also employed
for larger motors, rectifiers, and some other purposes. Consumers using
such large voltages are usually served at primary voltages and meet
their own utilization voltage standards.
