1926.404(f)(1)(iv): Grounding AC Systems of 50 to 1000 Volts
AC systems of 50 volts to 1000 volts must be grounded if the system can be grounded so the maximum voltage to ground on ungrounded conductors does not exceed 150 volts, if it is a 480Y/277 or 240/120 volt 3-phase 4-wire system as described, or if a service conductor is uninsulated, unless exempted by (f)(1)(v).
Key takeaways
- 1926.404(f)(1)(iv) is part of OSHA's construction standards (29 CFR Part 1926, Subpart K).
- OSHA cited it 2 times in the last 3 years, in 2 inspections, with $15,527 in current penalties.
- 1 death is linked to inspections that cited this rule.
- Related part: 1926.404(f) Grounding (241 citations).
- Free 5-minute toolbox talk for your crew below.
In this article
What this rule covers
29 CFR 1926.404(f)(1)(iv) ("AC systems, 50 volts to 1000 volts") is one requirement inside 1926.404 Wiring design and protection, an OSHA construction standard (Part 1926, Subpart K). It includes 4 sub-paragraphs. The official text below is copied word for word from the Electronic Code of Federal Regulations (eCFR). The numbers, injury cases and training on this page show how OSHA enforces it on real job sites.
Official text
(iv) AC systems, 50 volts to 1000 volts. AC systems of 50 volts to 1000 volts shall be grounded under any of the following conditions, unless exempted by paragraph (f)(1)(v) of this section: 2 citations
(A) If the system can be so grounded that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts;
(B) If the system is nominally rated 480Y/277 volt, 3-phase, 4-wire in which the neutral is used as a circuit conductor;
(C) If the system is nominally rated 240/120 volt, 3-phase, 4-wire in which the midpoint of one phase is used as a circuit conductor; or
(D) If a service conductor is uninsulated.
Source: eCFR, text current as of 2026-10-15. Read all of 1926.404.
Injuries linked to this rule
1 death is linked to inspections where OSHA cited 1926.404(f)(1)(iv).
At 1:40 p.m. on June 6, 2024, Employee #1, employed by a company installing a temporary cooling chiller system, was working on electrical connections. The employee was running 50-foot cables (4-AWG) from the rooftop to connect to the electrical panel and the chiller's disconnect on…
Where and how often it was cited
Free toolbox talk
A 5-minute crew talk on 1926.404(f)(1)(iv), plus alerts when this rule changes. Free.
OSHA interpretation letters
None exist. OSHA has not published a letter of interpretation for 1926.404(f)(1)(iv).
Related parts of this rule
- 1926.404(f) Grounding 241 citations
- 1926.404(f)(6) Grounding path 234 citations
- 1926.404(f)(7) Supports, enclosures, and equipment to be grounded 4 citations
- 1926.404(f)(7)(iv) Equipment connected by cord and plug 3 citations
- 1926.404(f)(1) Systems to be grounded 2 citations
- 1926.404(f)(7)(i) Supports and enclosures for conductors
- 1926.404(f)(3)(i) Portable generators
- 1926.404(f)(3) Portable and vehicle-mounted generators
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Frequently asked questions
When must a 50 to 1000 volt AC system be grounded?
Under any of the listed conditions, such as when the maximum voltage to ground on ungrounded conductors would not exceed 150 volts.
Does a 480Y/277 volt system need grounding?
Yes, if it is nominally rated 480Y/277 volt, 3-phase, 4-wire with the neutral used as a circuit conductor.
Are there exceptions?
Yes. Systems exempted by paragraph (f)(1)(v) are not required to be grounded.
About this page
Sources
- 29 CFR 1926.404 (eCFR), text current as of 2026-10-15
- U.S. Department of Labor enforcement data: OSHA inspections, citations and accident records, copied nightly; data as of 2026-10-19
3-year window. Federal OSHA only; state-plan rules are not counted. Plain-language text is a summary, not legal advice; the official text controls. Black Belt Safety is not part of OSHA or the Department of Labor.