OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at https://www.osha.gov.

March 31, 2004



Director, Directorate of Science, Technology, and Medicine
SUBJECT: Fixed Wiring in Research Facilities and High-Voltage Cable Tray Systems at Department of Energy (DOE) Research Laboratories


Thank you for your March 4, 2004 memorandum regarding the compliance audits at the DOE research laboratories and our Electrical, Subpart S, standards contained in 29 CFR Part 1910. Your paraphrased questions and our replies follow:

Question 1: Six of the research laboratories have accelerators, which have substantial electrical requirements and are used for physics research. At times, when the accelerator is shut down, some of the wiring must be moved or changed; but wiring often may be in place for years. Also, with respect to other large research equipment, such as particle detectors, there are a large number of devices that are not hard-wired and instead use flexible cords equipped with attachment plugs. Typically, the equipment is upgraded to seek continually higher levels of performance, or, in some cases, items are inserted, tested, modified, removed, re-modified, reinstalled, etc. This occurs during downtime periods, where the accelerator is brought down and new equipment is installed. This may be over a period of months or years.

Your Directorate's October 12, 1995 interpretation letter, regarding flexible cord use for research projects, to the Department of Navy was reviewed, and it does not appear to address our situation.

The impact of OSHA requiring fixed (permanent) wiring will have substantial financial implications for the DOE laboratories. What is the length of time or the criteria to determine when fixed (permanent) wiring is required in lieu of flexible wiring?

Reply: Both permanent wiring and temporary wiring may be either fixed (that is, fastened in place) or moveable (that is, connected by flexible cords or cables). Because your scenario involves the use of flexible cords, we explain when flexible cords or cables are permitted as a temporary wiring method and when they are permitted with permanent wiring.

29 CFR §1910.305(a)(2)(iii)(B) requires temporary branch circuits to be run as multiconductor cord or cable assemblies or as open conductors. Thus, flexible cords and cables are permitted to be used as branch circuit conductors whenever temporary wiring is permitted and is actually being used. The Temporary wiring requirements, which are contained in §1910.305(a)(2), lists permitted uses for temporary wiring, permits temporary wiring of 600 volts or less to be used for experimental work, and permits temporary wiring of more than 600 volts to be used during periods of tests and experiments. No specific time limit, in terms of days, weeks, or months, has been established for temporary wiring used in experimental work. Rather, for this type of work, temporary wiring is only permitted during the duration of an experiment.

In your scenario, it appears that the accelerator and other referenced research equipment are permanently-installed, and the research laboratories have chosen not to temporarily install the accelerators, other large equipment, and their supply wiring. Rather, the research laboratories have selected a temporary wiring method and installed it permanently. The research equipment wiring remains in place for years; thus, the wiring is permanent wiring. It should be noted, however, that any wiring that is added to or removed from permanently-installed research equipment to accommodate the running of specific experiments can be run as temporary wiring. In contrast to the permanently-installed equipment, this wiring would be installed temporarily and would need to be removed when the experiments calling for its use are concluded.

Even though the supply wiring for accelerators and other permanently-installed equipment must meet the requirements for permanent wiring methods, it may still be permissible to use flexible cords or cables. Paragraph (g)(1) of §1910.305 lists nine specific situations in which flexible cords may be used.3 (Even in these situations, flexible cords and cables must be approved and suitable for the conditions of use and location.4) The following provides a partial listing of the circumstances in which flexible cords and cables may be permitted for use at a research laboratory. While §1910.305(g)(1)(i) lists several additional circumstances, those listed below are the most likely to be applicable at a research laboratory:





  1. pendants (typically, a lampholder or cord-connector body suspended by a length of cord properly secured and terminated directly above the suspended device);
  2. wiring of fixtures;
  3. connection of portable lamps or appliances;
  4. connection of stationary equipment to facilitate their frequent interchange; and
  5. prevention of the transmission of noise or vibration (in some cases vibration may fatigue fixed wiring and result in a situation more hazardous than flexible cord).

Unless flexible cords and cables are specifically permitted for use in situations detailed in §1910.305(g)(1)(i), they must not be used. Furthermore, unless specifically permitted otherwise by any of the nine permitted conditions, §1910.305(g)(1)(iii) requires that flexible cords and cables may not be used:





  1. as a substitute for the fixed wiring of a structure;
  2. where run through holes in walls, ceiling, or floors;
  3. where run through doorways, windows, or similar openings;
  4. where attached to building surfaces; or
  5. where concealed behind building walls, ceilings, or floors.

Question 2: What are the OSHA requirements for the type and quantity of cable (weight loading) for cable trays, including the cable spacing requirements?

Reply: The Cable Trays requirements, contained in §1910.305(a)(3), address the general installation and use requirements, including the types of cable permitted to be installed in cable tray systems. In terms of the quantity (fill requirements) and weight (load requirements) of cable(s) permitted in a tray and the spacing requirements between cables, these installation specifications would be determined through the instruction included in the listing or labeling associated with the particular cable tray system. As mentioned above, electrical equipment must be approved and suitable for the conditions of use and location and used/installed in accordance with general requirements for all conductors and equipment contained in §§1910.303(a) and (b).

This installation and use information should be included in the manufacturer's instructions. Manufacturers typically incorporate recognized good engineering practice into their instructions, such as including references to the National Electrical Code (NEC) and National Electrical Manufacturers Association (NEMA) standards.

In summary, the OSHA electrical standards require electrical equipment to be approved and used/installed in accordance with any instructions included in the listing or labeling. Manufacturers generally will provide this information on their product specifications to ensure that their equipment and conductors are installed and used in accordance with the listing and labeling requirements.

We hope you find this information helpful. If you have any further questions, please feel free to contact the Office of General Industry Enforcement at (202) 693-1850.





1 The Department of Navy letter synonymizes the terms temporary wiring and flexible cords and cables within the context of the response. As noted in the response below, these terms are not synonymous, and there are separate and distinct requirements for temporary wiring and flexible cords and cables, which are contained in §§1910.305(a)(2) and 1910.305(g), respectively. [back to text]





2 This is further clarified in the Section 527.3(D) of the 2002 National Electrical Code, which requires temporary wiring to be removed upon completion of the purpose for which the wiring was installed. [back to text]





3 There is a definite need and place for flexible cords, but there also may be a temptation to misuse them because they are perceived to offer a quick and easy way to carry electricity. The basic problem with flexible cords is that they generally are more vulnerable than fixed wiring. In many cases in which flexible cord is used, the protection associated with a recognized permanent wiring method is diminished. Flexible cords may be used only when permitted by the standard.

Furthermore, the provisions contained in the Electrical Safety-Related Work Practices standards contain important safety requirements regarding the safe use of cord- and plug-connected equipment, including flexible cord sets and extension cords. Specifically, some of the requirements contained in the Use of equipment paragraph, 1910.334(a), address common safe work practices for portable electric equipment.
[back to text]





4 See §§1910.303(a) and (b) and §1910.305(g)(1) and also the definitions of approved, acceptable, certified and listed contained in 1910.399. [back to text]





5 For your information, the Metal Cable Tray Systems (NEMA VE 1) standard contains maximum allowable load specifications, and the Cable Tray Installation Guidelines (NEMA VE 2) require cable trays to be installed in accordance with the NEC. For example, section 392.6(E) of the NEC (2002) permits multiconductor cable rated at 600 volts or less (low-voltage) to be placed in the same cable tray. The high-voltage multiconductor cable requirements are contained in section 392.6(F), and high-voltage multiconductor cable is required to be 1) type MC (metal-clad cable) or 2) to be separated from low-voltage cable by a fixed barrier of a material compatible with the cable tray. [back to text]