Participants at one of my recent workshops had questions pertaining to reverse-fed transformers (back-fed). The input side was 480 volts (V), three-phase, and the output side was 120/208V. They wanted to know if the output side of 120/208V transformer could be used as an input side, and if so, how was the electrician to bond and ground the transformer?
In my opinion, when a step-up transformer is necessary, it should be designed and identified for such application. This article does not address such problems, but it addresses the general requirements that are outlined in the National Electrical Code (NEC).
The big question seemed to be whether the grounded conductor (usually a neutral) is required to be connected to the XO termination point. Of course, the answer is no. When the grounded conductor is used in this type of installation, it must not be connected to the XO terminal and must not be bonded to the enclosure or earth-grounded. For operational purposes, just connect the ungrounded phase conductors to the output terminals that are usually identified as L1, L2 and L3, respectively. Then an equipment grounding conductor must be routed with the supply circuit and used to bond and ground the noncurrent-carrying parts of the transformer enclosure.
Note that a grounded neutral conductor cannot be derived from a reverse-fed transformer because standard transformers are not usually manufactured with a four-wire primary side.
Therefore, if the 480V side is grounded, one of its phase conductors must be connected to an earth-grounded electrode so that the transformer has a grounded conductor that will provide an effective ground-fault path, should a ground-fault occur on one of the ungrounded phase conductors. Note that one of the ungrounded phase conductors is grounded. The NEC requires grounded conductors to be identified either white or gray in accordance with sections 310.110(A) and 200.6(A) or (B).
The transformer’s output side
The 480V input side is now used as the output side. There is no XO termination; therefore, one of the ungrounded phase conductors must be connected to the earth ground if the installer desires the electrical system to be grounded and equipped with a grounded conductor.
First, corner-grounded delta systems will stabilize the voltages of the other two ungrounded conductors. Second, this setup aids in reducing the generation of transient overvoltages and, finally, provides an adequate path for fault currents when a ground-fault condition occurs.
The system is not able to provide dual-voltage service for lighting and power loads. Also, the grounded conductor must be positively indentified throughout the electrical system. Finally, a higher line-to-ground voltage (by a factor of 1.732) exists on the other two ungrounded phase conductors than it does on a wye neutral-grounded system.
The service equipment
At the service equipment location, the grounded conductor must again be connected to the earth ground as outlined in sections 250.24, 250.26, 250.50, 250.52 and 250.53 of the NEC. Section 240.85 permits the use of circuit breakers with a straight marking of 480V as long as the nominal voltage between any two conductors does not exceed the circuit breaker ratings. Warning: If two-pole circuit breakers are installed in a corner-grounded delta system, they must be marked to indicate such suitability.
Slash marked circuit breakers such as 120/240V or 277/480V must not be used on corner-grounded delta systems. Section 230.90(B) generally prohibits an overcurrent protection device to be placed in the grounded conductor if it does not open all the circuit conductors simultaneously.
Section 240.22 states that no overcurrent protection device can be inserted in any conductor that is intentionally grounded unless the poles of the circuit breaker open all the circuit conductors simultaneously. However, for motor overload conditions, an exception is permitted. See Section 430.37 of the NEC.
I suggested applying NEMA PB 1 and PB 2, which cover requirements for installing delta corner-grounded transformers and methods pertaining to the corner-grounded B phase. UL 67, UL 891, and UL 489 specifically addresses delta corner-grounded systems. When installing, inspecting and approving this type of system, see sections 110.2, 110.3(B) and 90.7 of the NEC.
It is a given that the circuits of a corner-grounded delta system are at higher voltage stress than a more widely used standard wye system because the two-ungrounded phases are constantly operating at higher voltage relative to ground. Of course, any surge activity would be in addition to the constant system voltage stress.
STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.