The column this month will cover questions about transformer installations that were the subject of concern to subscribers to NECA’s on-line “Code Question of the Day.”
All answers are based on the current 1999 edition of the National Electrical Code (NEC); however, beginning with this month’s column, we are switching to the new numbering system that will be used starting with the next edition of the Code. That means using dots instead of dashes and capitalizing all sub-paragraph references. For example, 450-13(b) is shown as 450.13(B) as it will be in the 2002 edition of the NEC; might as well get used to seeing it that way.
QUESTION: I have a three-phase, three-wire delta, 480-volt ungrounded secondary. In one of your answers, you said that to ground a transformer like this, it is necessary to corner ground the secondary of the transformer. Isn’t there some sort of a transformer you can hook up to the secondary to get a grounded (neutral) conductor?
ANSWER: Yes, there is a grounding transformer called a zigzag transformer that can be used to convert a three-phase, three-wire delta ungrounded secondary to a three-phase, four-wire wye system and derive an artificial neutral that can be grounded. Generally, a zigzag transformer is used to gain the advantages of a grounded system and needs only be sized to carry ground-fault current for a short duration.
A zigzag transformer is classified as an auto-transformer and has no secondary winding. Two windings for each phase are wound on each core leg, but each pair of windings on the same core leg is wound in the opposite direction.
This winding technique creates a high resistance to three-phase currents and reduces the impedance that can interfere with the flow of ground-fault current.
The derived-grounded conductor can be used as the system grounded (neutral) conductor you are looking for. However, if you intend to use the artificial (derived) neutral for this application, the grounding transformer must be sized for the maximum unbalanced loads. In a new installation, this method of obtaining a grounded (neutral) conductor would probably be more costly and it would be to your advantage to use a standard three-phase, four-wire transformer.
The NEC does not define either a zigzag transformer or grounding transformer, but it does cover the requirements for their use in Section 450.5. This type of transformer is also referenced in Section 250.36(A)-Grounding Impedance Location.
Where the transformer is wound in this zigzag manner, it does two things. It limits the three-phase current to a level where it only has a magnetizing effect and it limits the impedance in the windings so that a high level of fault-current will flow. This high level of ground-fault current is desirable in most applications where it will serve to open the overcurrent protective device rapidly.
QUESTION: Is a disconnecting means required on the primary side of a dry-type transformer, rated 600 volts of less, and must it be in sight from the transformer location?
ANSWER: Using the NEC as a basis for my answer, the answer is “yes,” a disconnecting means is required on the primary side of this transformer, but the answer is “no” to a requirement that it be within sight from the transformer.
I cannot tell you what the basis is for not requiring a disconnecting means within sight of the transformer, as is the case with most other types of electrical gear. It is my opinion that there should be a means of disconnect visible from the transformer and I would recommend that it should be included in any design of electrical distribution systems using transformers. Remember that the NEC’s purpose is not to limit design, but to establish the minimum requirements for a safe installation.
The installation of a disconnecting means within sight from a transformer is, in my opinion, justified by the safety it would provide persons servicing or maintaining the transformer. It would also provide a means to quickly de-energize the transformer if a problem occurred within the transformer or at a point somewhere on the secondary system between the transformer secondary terminals and the first point of disconnect on the secondary.
In most instances, the location of the primary disconnect is not known by everyone involved and is not identified by signage. As I see it, Article 450 does not give guidance for the installation of disconnecting means on either the primary or secondary side of transformers. The requirements for overcurrent protection are given in Article 450 and Section 240.40 shows the requirements for the disconnecting means for fuses. Circuit breaker overcurrent protection provides disconnecting means as well.
QUESTION: Can the disconnecting means provided on the secondary of a transformer, rated 600 volts or less, consist of six switches or circuit breakers?
ANSWER: Yes, if the transformer is a utility transformer supplying service to a premises wiring system. Section 230-71(A) permits six switches or sets of circuit breakers to serve as the disconnecting means. If, however this is a separately derived system, Table 450.3(B) in Note 2 permits the overcurrent protection to consist of not more than six circuit breakers or six sets of fuses grouped in one location. The total of the overcurrent device ratings cannot exceed the allowed value of a single overcurrent device.
QUESTION: Can I locate a dry-type transformer, less than 600 volts, 6 feet above the distribution panelboard that it supplies without violating the working space requirements for that panelboard?
ANSWER: The dedicated space requirement in Section 110.26(F) for equipment within the scope of Article 384 is the space equal to the width and depth of the equipment and extending from the floor to a height of 1.8 m (6 feet) above the panelboard. No equipment foreign to the electrical installation can be located in that zone. At first look it appears that, since you are not in this space and since the transformer is not foreign to the installation, your installation is in conformance. However, by looking at Section 110.16(E) we find a requirement for minimum headroom about panelboards of 2.0 m (61/2 feet). This requirement is not complied with in the installation you propose.
QUESTION: Am I required to bond the grounded conductor of a separately derived system to the transformer enclosure at the source and to the enclosure of the first disconnecting means?
ANSWER: No, you are not required to bond at both points, but Section 250.30(A)(1) Exception No. 1 permits you to do so if you don’t establish a parallel path for the grounded circuit conductor. This is different from the requirements that allow a parallel path for the grounded conductor on the supply side of a service panel.
For separately derived systems, if there is a metallic connection, such as a steel conduit, run from the transformer to the main disconnecting means of the separately derived system, then you are only permitted to bond at one point, either at the transformer or at the main disconnecting means.
If a nonmetallic connection, such as a PVC raceway, runs from the transformer to the main disconnecting means of the separately derived system, then you must bond at both points. An equipment-grounding conductor can be used to bond the transformer to the main panel and then bonding to the grounded system conductor need only be done at one point.
The desired result of the bonding is that you establish a ground-fault path, which includes the transformer enclosure, that will open the system or circuit overcut device. It should be one that does not establish a parallel path for grounded conductor current between the metallic raceway and the grounded conductor during normal operation.
QUESTION: I wall-mounted a transformer in the storage area of a retail store. Now they have decided to make an employee lunchroom there and they put up a lay-in, suspended-type ceiling and my transformer is above the ceiling. I was told to move it because it isn’t readily accessible. What should I do?
ANSWER: You didn’t give me any details about the transformer characteristics. If this is a dry-type transformer of 600 volts or less and it doesn’t exceed 50 kVA, then Section 450.13(B) permits the installation in this location, provided it has adequate ventilation.
This installation does not require the transformer to be readily accessible. If it’s anything else, move it or permanently open the ceiling to the satisfaction of the Authority Having Jurisdiction.
TROUT is a technical consultant for Maron Electric Company of Chicago, and represents NECA as chairman of the National Electrical Code-making panel 12. He is also the principal author of ECMAG.com’s online feature, “Code Question of the Day.”