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Grounding a construction transformer secondary circuit

Summary An OSHA regional office asked whether a described construction transformer secondary system had to be grounded. OSHA said the 120/240-volt configuration met the cited grounding condition and the ungrounded installation violated the rule. It explained the protection grounding would provide. OSHA now archives the memorandum as potentially outdated.
Document
Federal opinion letter
Agency
Occupational Safety and Health Administration (OSHA)
Document number
1988-02-24-0
Date issued
February 24, 1988
Status
Archived by the agency
OSHA archives this memorandum and says it may no longer represent OSHA policy.
Status last verified October 10, 2026
Topics
Workplace Safety OSHA 1926 OSHA 1910 Electrical Safety Grounding
Source
View the official document

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OSHA archives this memorandum and says it may no longer represent OSHA policy. It may no longer reflect the agency's current position. Treat this page as historical context, not current guidance, and verify current law before relying on anything here.
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NOTICE: This is an OSHA Archive Document, and may no longer represent OSHA Policy. It is presented here as historical content, for research and review purposes only.

February 24, 1988

MEMORANDUM FOR: LINDA R. ANKU
Regional Administrator

THRU: LEO CAREY, Director
Office of Field Programs

FROM: THOMAS J. SHEPICH, Director
Directorate of Compliance Programs

SUBJECT: Review of Electrical Utilities Procedures for Compliance with Subpart K of Part 1926

29 CFR 1926.404(f)(1)(iv) requires AC systems of 50 to 1000 volts to be grounded if the system can be so grounded that the maximum voltage to ground will not exceed 150 volts. (29 CFR 1910.304(f)(1)(iv) contains a similar requirement for general industry.) A system supplied by a transformer whose secondary voltage is 120 volts between conductors (or is 120 volts between each line conductor and a common neutral and 240 volts between the line conductors) must be grounded. Therefore the installation described in the attachment to Mr. Forbes' letter is in violation of this requirement. Grounding the system described by Mr. Forbes will protect the secondary circuit conductors from having the primary voltage impressed on them (in this case 600 volts) and will limit the voltage above ground on the secondary conductors.

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