1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA DC 20-1992 (R2003, R2009, R2014)Residential Controls Class 2 TransformersNEMA Standards Publication DC 20-1992 (R2003, R2009, R2014) Residential ControlsClass 2 Transformers Published by: National Electrical Manufacturers As
2、sociation 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2009 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literary and Arti
3、stic Works, and the International and Pan American copyright conventions. 2009 National Electrical Manufacturers Association NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document
4、at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which the document contained her
5、ein is one, are developed through a voluntary consensus standards development process. This process brings together volunteers and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fair
6、ness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any perso
7、nal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory, directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, expressed or implied
8、, as to the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the information in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers pr
9、oducts or services by virtue of this standard or guide. In publishing and making this document available, NEMA is not undertaking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone els
10、e. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be
11、available from other sources, which the user may wish to consult for additional views or information not covered by this publication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs
12、, or installations for safety or health purposes. Any certification or other statement of compliance with any health- or safety-related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. NEMA DC 20-1992 (R2003,
13、R2009, R2014) Page i 2009 National Electrical Manufacturers Association Foreword This Standards Publication represents suggested standards for the construction and performance of Class 2 transformers designed primarily for use with residential controls. It has been developed in the public interest a
14、nd is designed to promote a better understanding between the manufacturer and the user and to assist the user in selecting and obtaining the proper transformer for a particular application. If a value for measurement as given in this Standards Publication is followed by an equivalent value in other
15、units, the first stated value is to be regarded as the requirement. The given equivalent value might be only approximate. This standard is periodically reviewed by the Residential Controls Section of NEMA for any revisions necessary to keep it up-to-date with evolving technology. Proposed or recomme
16、nded revisions should be submitted to: Vice President, Technical Services National Electrical Manufacturers Association 1300 North 17th Street Suite 900 Rosslyn, Virginia 22209 This Standards Publication was developed by the Residential Controls Section. Section approval of the standard does not nec
17、essarily imply that all section members voted for its approval or participated in its development. At the time this standard was approved, the group was composed of the following members: Apcom Inc.Franklin, TN Emerson Electric Company, White-Rodgers DivisionSt. Louis, MO GE Industrial SystemsMorris
18、on, IL Honeywell, Inc.Golden Valley, MN Therm-O-DiscMansfield, OH NEMA DC 20-1992 (R2003, R2009, R2014) Page ii 2009 National Electrical Manufacturers Association CONTENTS Page Foreword . i History of Standards iii Section 1 GENERAL 1.1 Scope . 1 1.2 References 1 1.3 Definitions 1 Section 2 RATINGS
19、AND CLASSIFICATIONS 2.1 Electrical Ratings . 3 2.1.1 Primary Voltage Ratings . 3 2.1.2 Secondary Voltage Ratings 3 2.2 Classification 3 Section 3 MOUNTING 3.1 General 4 3.2 Plane-Surface Mounting 4 3.3 Outlet-Box Mounting 4 Figure 3-1 Dimensions (in inches mm) for Mounting 4 Section 4 PERFORMANCE AN
20、D TEST 4.1 General 5 4.2 Regulation 5 4.2.1 Test Equipment 5 4.2.2 Test Procedures . 7 Figure 4-1 Test Circuit 7 4.2.3 Curves 7 4.2.4 Data Sheet 8 Table 4-1 Test Points 8 Table 4-2 Example Data Sheet . 8 Figure 4-2 Example Data Sheet . 9 Section 5 MARKING 5.1 General 10 5.2 Additional Marking . 10 N
21、EMA DC 20-1992 (R2003, R2009, R2014) Page iii 2009 National Electrical Manufacturers Association History of Standards In March 1958, the Residential Controls Section of NEMA adopted a standard regulation curve for Class 2 transformers. This standard was issued under the title “Voltage Ratings and Lo
22、ad Characteristics for Primary Control Equipment of 30 Volts and Less,“ and was a part of Publication No. DC 1, General Standards for All Automatic Temperature Controls. At that time the transformer in general use by the industry was one having a 40-volt-ampere rating. It was, therefore, around this
23、 rating that the standard was developed. Subsequently, transformers having other ratings were introduced. These transformers, although they might have had the same electrical ratings, in many cases varied quite radically in performance. This presented problems from an interchangeability standpoint.
24、Recognizing these problems, the Residential Controls Section in 1964 activated a program to write standards for all transformer sizes. Publication No. ST 2-1973 was the result of the development and revision of several drafts developed by the Residential Controls Section of NEMA and approved by the
25、Dry-Type and Specialty Transformer Section whose scope at that time included the products covered in this Standards Publication. It was reaffirmed and editorially revised in 1979. DC 20-1992 (R2003, R2009) revises and supersedes ST 2-1973 (R 1979). This current revision of the standard has deleted a
26、ll requirements which are contained in ANSI/UL 1585, and references that document as appropriate. Other superfluous information has been deleted. NEMA DC 20-1992 (R2003, R2009, R2014) Page iv 2009 National Electrical Manufacturers Association NEMA DC 20-1992 (R2003, R2009, R2014) Page 1 2009 Nationa
27、l Electrical Manufacturers Association Section 1 GENERAL 1.1 SCOPE This Standards Publication covers Class 2 transformers intended primarily for use in 30 volts rms maximum low-voltage residential control circuits. It includes definitions, electrical ratings, performance standards, mounting dimensio
28、ns, and marking requirements. 1.2 REFERENCES The following publications are adopted, in whole or in part as indicated, by reference in this Standards Publication and are available from the organizations below. National Fire Protection Association Batterymarch Park Quincy, Massachusetts 02269 America
29、n National Standards Institute 11 West 42nd Avenue New York, NY 10036 ANSI/NFPA 70 National Electrical Code Underwriters Laboratories, Inc. 333 Pfingsten Road Northbrook, IL 60062 American National Standards Institute 11 West 42nd Avenue New York, NY 10036 ANSI/UL 514 A Metallic Outlet Boxes ANSI/UL
30、 514 B Fittings for Cable and Conduit ANSI/UL 514 C Nonmetallic Outlet Boxes, Flush-Device Boxes and Covers ANSI/UL 1585 Safety Standard for Class 2 and 3 Transformers 1.3 DEFINITIONS class 2 transformer: A Class 2 transformer is a step-down transformer of the low-secondary-voltage type, suitable fo
31、r use in Class 2 remote-control low-energy circuits in accordance with ANSI/NFPA 70. It shall be of the energy-limiting type or of non-energy-limiting type equipped with over-current protection. energy-limiting transformer: An energy-limiting transformer is a transformer that is intended for use on
32、an approximately constant-voltage supply circuit and that has sufficient inherent impedance to limit the output current to a thermally safe maximum value. NEMA DC 20-1992 (R2003, R2009, R2014) Page 2 2009 National Electrical Manufacturers Association non-energy-limiting transformer: A non-energy-lim
33、iting transformer is a constant-potential transformer which does not have sufficient inherent impedance to limit the output current to a thermally safe maximum value. primary winding: The primary winding is on the input side. secondary winding: The secondary winding is on the output side. rating of
34、transformer: The rating of a transformer consists of a voltampere output, together with any other characteristics, such as voltage or current; other characteristics such as frequency and power factor may be included. voltampere rating of a transformer: The voltampere rating of a transformer is the o
35、utput which can be delivered continuously at rated secondary voltage and rated frequency without exceeding the specified temperature rise limitations. load: The load of a transformer is expressed in voltamperes and is the product of the secondary voltage and the secondary current, expressed as volta
36、mperes. NEMA DC 20-1992 (R2003, R2009, R2014) Page 3 2009 National Electrical Manufacturers Association Section 2 RATINGS AND CLASSIFICATION 2.1 ELECTRICAL RATINGS 2.1.1 Primary Voltage Ratings Primary voltage ratings shall be as follows: 120 volts at 60 hertz 208 volts at 60 hertz 240 volts at 60 h
37、ertz 277 volts at 60 hertz 480 volts at 60 hertz Transformers with multiple rated primaries shall be tested at each primary voltage ratings. 2.1.2 Secondary Voltage Ratings Secondary voltage ratings shall be as follows: No load: 26.5 0.5 volts 100 percent rated current: 24.0 1.0 volts 200 percent ra
38、ted current: 21.5 1.5 volts Secondary voltage ratings shall be applicable to all loads between 0.5 power factor and unity power factor with the primary connected to a source of rated voltage and frequency in accordance with 2.1.1. 2.2 CLASSIFICATION Class 2 transformers shall be one of the following
39、 types. NEMA Type Rated Secondary Current (in Amperes) at Nominal 24 volts A 0.4 B 0.8 C 1.2 D 1.6 E 2.0 . NEMA DC 20-1992 (R2003, R2009, R2014) Page 4 2009 National Electrical Manufacturers Association Section 3 MOUNTING 3.1 GENERAL Class 2 transformers shall conform to ANSI/UL 1585. The enclosure
40、of a Class 2 transformer shall be designed for mounting either on a plane surface or on an outlet box. 3.2 PLANE-SURFACE MOUNTING For mounting on a plane surface, the enclosure shall be so designed that it will make contact only at the points of support. An air space of not less that inch (6.4 mm) s
41、hall be maintained between the enclosure and the supporting surface. 3.3 OUTLET-BOX MOUNTING If a plate is provided for mounting on standard junction boxes, the dimensions and provisions for mounting shall be in accordance with Figure 3-1. Note I Holes A and B shall be optional. Note II 3/16 inch (4
42、.7) diameter holes shall be permitted to be any of the slotted shapes shown. Note III Openings into the box when the transformer is installed shall meet the requirements under “Closure of Openings” in ANSI/UL 514A. Note IV See 3.0 and ANSI/UL 1585, Section 8. Figure 3-1 DIMEMSIONS (IN INCHES (MM) FO
43、R MOUNTING NEMA DC 20-1992 (R2003, R2009, R2014) Page 5 2009 National Electrical Manufacturers Association Section 4 PERFORMANCE AND TEST 4.1 GENERAL All transformers shall meet the performance requirements for Class 2 transformers as described in ANSI/UL 1585. 4.2 REGULATION Transformers classified
44、 according to 2.2 shall meet the appropriate requirements of this standard when tested at no load, 100 percent rated load, and 200 percent rated load at load factors between 0.5 and unity. Load tests shall be performed at an ambient temperature between 68F (20C) and 80F (26.6C). 4.2.1 Test Equipment
45、 The test equipment listed in this subclause should be of good quality instrumentation; the accuracy of the meters being within 1 percent and periodically verified by the user. To ensure accuracy of test results, it is important to verify that measuring and recording instruments provide true rms rea
46、dings. The test equipment should be connected in accordance with Figure 4-1. All resistance measurements should be measured at the load switch with an accurate resistance bridge. a. Power Supply 1. Should be of the nonregulating type. 2. Should be capable of producing the rms test voltages of 120, 2
47、08, 240, 277, or 480 volts. 3. Should have a minimum capacity of 500 volt-amperes. b. Rms Voltmeter (for primary) 1. Should be of a proper range to provide accurate readings. c. Rms Voltmeter (for secondary) 1. Should be of a proper range to provide accurate readings. d. Rms Current Meter 1. Should
48、be of a proper range to provide accurate readings. 2. There should be a circuit impedance to meter resistance ratio of at least 50:1 e. Resistance Bridge f. Switches 1. Should have a maximum contact resistance of 0.02 ohms at 5 amperes rms. g. There should be four resistive loads, as follows: 1. 60
49、ohms + 1 percent, 25 watts (R1) 2. 30 ohms + 1 percent, 50 watts (R2) 3. 15 ohms + 1 percent, 75 watts (R3) 4. 7.5 ohms + 1 percent, 100 watts (R4) The resistors should be arranged so that they can be switched into the circuit, singly or in parallel, to produce the rated current or 200 percent rated current, as follows: NEMA DC 20-1992 (R2003, R2009, R2014) Page 6 2009 National Electrical Manufacturers Association 5. 60 ohms for 0.4 ampere (R1) 6. 30 ohms for 0.8 ampere (R2) 7. 60 plus 30 ohms for 1.2 amperes (R1 + R2) 8. 15 ohms for 1
