Dry-Type Transformer, Grounding & Bonding Terminations

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Grounding and bonding termination points in dry-type transformers are seemingly an issue in that the NEC apparently is silent on where in a dry-type transformer grounding and bonding conductors should be landed (terminated). In addition, many times installers utilize poorly chosen termination means such as installing a lug over a vented portion of the bottom of the transformer. Are there such termination requirements in the NEC and if so, where are they located? PC


Up to this point in the NEC language, I would agree with your assessment that the NEC might be silent on this issue other than perhaps 110.3(B) and the transformer manufacturer’s specifications. However, a proposal has been accepted by CMP-9 for the 2014 NEC to address grounding and bonding connections in transformer enclosures (see ROP Proposal 9-144). This new proposed provision located at 450.10(A), incorporates specific requirements for installing an equipment grounding terminal bar in transformer enclosures in accordance with 250.12 (Clean Surfaces). Section 250.12 requires nonconductive coatings (such as paint, lacquer, and enamel) on equipment to be grounded to be removed from contact surfaces to ensure good electrical continuity. Under this proposed requirement, this terminal bar would be prohibited from installation on or over any vented portion of the transformer enclosure. This type of termination utilizing the venting openings results in a less than effective grounding and bonding connection. These venting openings have not been evaluated as grounding and bonding connections and should not be depended upon to serve as an effective ground-fault current-fault return path.

An exception to this rule has also been proposed to address transformers with pigtail leads used as the connection means incorporating any of the methods at 250.8 (Connection of Grounding and Bonding Equipment) in lieu of an equipment grounding terminal bar.

Keith Lofland – CMP-9
IAEI Director of Education

About the Author

L. Keith Lofland is the Director of Education, Codes and Standards for IAEI. He represents IAEI as a member of Code-Making Panel 9 for the National Electrical Code. He serves on NFPA’s Electrical Section Executive Committee and also is a member of UL’s Electrical Council. He is the principal contributor to several IAEI publications including Analysis of Changes, Soares Grounding and Bonding, and One- and Two-Family Dwelling Electrical Systems. Prior to his position with the International Office of IAEI, Keith spent 16 years with the City of Garland (Texas) serving as their chief electrical inspector during a 21-year career as an electrical inspector. Keith holds a master electrician license from the State of Texas. He also has obtained electrical inspector certifications from IAEI, NFPA/IAEI, and ICC. He served as chairman of the Texas Chapter of IAEI in 1989 and as the secretary/treasurer for the Texas Chapter-IAEI for 10 years from 1995–2004.