neutral ground transformer Photo courtesy of Eaton
This ventilated dry-type transformer comes equipped with a factory-installed grounding terminal.

Neutral Grounding of Dry-Type Ventilated Transformers

What NEC Sec. 450.10 means to manufacturers and installers

In the 2014 Edition of the National Electrical Code (NEC), a revision was made that seems to have gone largely unnoticed by manufacturers of ventilated dry-type transformers. Section 450.10(A) of the 2017 NEC requires a “terminal bar” for all grounding and bonding conductor connections. Doing an Internet search reveals that a few good articles addressed this change around 2014; however, there seems to be very little movement by the major manufacturers of dry-type transformers to provide the terminal bar. So this places the responsibility of providing the terminal bar on the shoulders of the installer.

Photo courtesy of Eaton

This ventilated dry-type transformer comes equipped with a factory-installed grounding terminal.

For at least 100 years, electricians have been grounding dry-type transformers, and it always seems to be a struggle to do it with a method that meets Code requirements. This apparently was the driving force behind the proposal in the 2014 Code cycle (i.e, that a terminal bar be provided in ventilated dry-type transformers). Some of the more common methods that installers have been using in the past are:

1)    Using the vent holes in the bottom of the enclosure to terminate mechanical or compression type lugs. (Note: This is a Code violation as Sec. 450.10 doesn’t allow you to do this.)

2)    Using one large split-bolt connector with a threaded stud through either a drilled hole or a vent hole. (Note: This is typically ― at the very least ― a violation of the listing of the split bolt connector.)

3)    Stacking D-lugs under a single bolt that is either inserted through a vent hole or a drilled hole. (Note: This may be acceptable to the authority having jurisdiction.), and

4)    Installing a terminal bar in the bottom of the transformer as required by the Code.

Installing a terminal bar or a bus in the bottom of smaller dry-type transformers can be a challenge due to space limitations and avoiding the vent holes. For this reason, it would make more sense for the terminal bar ― or at least the means for a terminal bar ― to be provided by the transformer manufacturer in all cases. This, of course, would mean a change in product standards and the listing requirements for dry-type transformers.

Until the day arrives when this requirement is part of a product standard for all dry-type transformers, either purchase a transformer that includes the terminal bar, or check with the AHJ as to the grounding method that will be acceptable.

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