UFC 3 -520-01
June 10, 2002
4-1.6.2 If available, two full capacity taps should be provided above nominal and two
full capacity taps below nominal to allow increasing or decreasing the secondary
voltage. Although designs vary among manufacturers, transformers smaller than 15
kVA usually only have two 5 percent taps below normal to provide a 10 percent voltage
adjustment range. Larger transformers often have four 2.5 percent taps below normal
and two 2.5 percent taps above normal to provide a 15 percent voltage adjustment
4-1.6.3 Select the tap setting to optimize the range between the no -load voltage and
full-load voltage as well as possible. Taps are commonly rated at 2.5 percent of
nameplate rating and designated as FCAN (full capacity above normal) or FCBN (full
capacity below normal), meaning that the kVA rating of the transformer is not affected
when taps are adjusted. If taps are not rated as full capacity, then derating of the
transformer should be performed per the manufacturer's requirements.
4-1.7.1 All transformers transmit sound due to vibration g enerated within the
magnetic steel core. Depending on other nearby ambient noise, the transformer sounds
might not be noticeable. In low ambient noise areas, the transformer sound can be
noticed. Determine if noise rating is a required design consideration for the intended
4-1.7.2 A transformer located in low ambient noise level areas should have a low
decibel hum rating. The average sound level in decibels should not exceed the level
specified in NEMA ST 20, Dry Type Transform ers for General Applications, for the
applicable kVA rating range. Manufacturers readily provide sound ratings lower than
the limits listed in NEMA ST 20.
4-1.7.3 In addition to the transformer noise rating, consider the following actions to
improve the generated sound level:
4-126.96.36.199 Mount the transformer so that vibrations are not transmitted to the
surrounding structure. Small transformers can usually be solidly mounted on a
reinforced concrete floor or wall. Flexible mounting will be necessary if the transformer
is mounted to the structure in a normally low-ambient noise area.
4-188.8.131.52 Use flexible couplings and conduit to minimize vibration transmission through
the connection points.
4-184.108.40.206 Locate the transformer in spaces where the sound le vel is not increased by
sound reflection. For example, in terms of sound emission, the least desirable
transformer location is in a corner near the ceiling because the walls and ceiling function
as a megaphone.