UFC 3 -520-01
June 10, 2002
power has no direct electri cal connection, including solidly connected grounds and
neutrals to another wiring system. A separately derived system is usually made when it
is desired to provide an isolated ground system for the wiring system.
4-4.1.2 When configured in a delta -wye configuration, the transformer provides a
power ground reference close to the point of use. This reduces common-mode noise
induced into the circuit from multiple ground loops upstream of the established
4-4.1.3 Isolation transformers provide a filtering function by separating the harmonic
frequencies between the source and the load. The delta -wye winding configuration
effectively cancels the third, ninth, fifteenth (and so on) harmonic currents in the delta
primary winding, thereby isola ting triplen harmonics from being fed back into the source.
(Refer to Chapter 12 for additional criteria related to power quality and harmonic
4-4.1.4 Isolation transformers can be used for retrofit applications to address existing
facility problems, but should not be arbitrarily used in new facilities because of the
higher per-kVA cost.
4-4.2.1. The buck-boost transformer has four separate windings, 2 windings in the
primary and 2 windings in the secondary. It is intended to be field connected as an
autotransformer to buck (lower) or boost (raise) the line voltage. Apply buck -boost
transformers only when required to achieve voltages beyond the capability of the
existing utilization equipment.
4-4.2.2. A buck-boost transformer can not be used to develop a 3 -phase, 4-wire, wye
circuit from a 3-phase, 3 -wire delta circuit. A delta to wye buck-boost configuration does
not provide adequate carrying capability to allow for unbalanced currents flowing in the
neutral of the 4 -wire circuit. The neutral current is not stable and will not provide the
desired line to neutral voltages under load. This connectio n also violates NEC Article
210.9 (2002 Edition) .
4-4.2.3. Do not use buck-boost transformers to correct for voltage drop on a long circuit
run in which the load fluctuates. Voltage drop varies with the load, but buck-boost
transformers are connected for a specific voltage drop. If a buck -boost transformer is
used to correct voltage drop under full load co nditions, high voltages can occur under
light load conditions.
4-4.2.4. Do not use buck-boost transformers to create a 120/240 volt single phase
service from a 208Y/120 volt 3 -phase supply. If done, two neutrals would exist on the
same circuit. Also, unbalanced line to neutral voltages would be created; one line would
be at 120 volts with the other line greater than 130 volts.