The NEC only requires the boxes to have threaded hubs. I believe that ordinary "Bell" boxes comply.

314.23(H)(1): A box shall be supported from a multiconductor cord or cable in an approved manner that protects the conductors against strain, such as a strain-relief connector threaded into a box with a hub.

That's just the point, there is no written code prohibiting this.

1910.303(b)(1)(i)
Suitability for installation and use in conformity with the provisions of this subpart. Suitability of equipment for an identified purpose may be evidenced by listing or labeling for that identified purpose.

1926.403(b)(1)(i)

Suitability for installation and use in conformity with the provisions of this subpart. Suitability of equipment for an identified purpose may be evidenced by listing, labeling, or certification for that identified purpose.

• Standard Number: 1910.305(a)(2)

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June 17, 1992

MEMORANDUM FOR: LINDA R. ANKU
REGIONAL ADMINISTRATOR

FROM: PATRICIA K. CLARK, DIRECTOR
DIRECTORATE OF COMPLIANCE PROGRAMS

SUBJECT: Acceptable Job-Made Extension Cords

This is in reference to your memorandum of June 12, 1991 to your area directors and district supervisors on the subject of extension cords acceptable for use (copy attached). We believe your interpretation is unduly restrictive and that it does not accurately reflect the requirements of the applicable standards. We have prepared the following analysis, which indicates that shop-made extension cords and other temporary wiring is acceptable in certain circumstances. It is not required in all circumstances that an extension cord be approved as an assembly.

Analysis

Normally, electrical equipment must be approved as an assembly by a nationally recognized testing laboratory to be acceptable under the General Industry or Construction Electrical Standards (Part 1910, Subpart S and Part 1926, Subpart K, respectively).

However, it is also true that cord sets, assembled in the field by qualified persons, are appropriately used in both general industry and in the construction industry, under limited circumstances. Such cord sets are considered to be temporary wiring extensions of the branch circuit.

Temporary electrical power and lighting wiring methods, as specified in 1910.305(a)(2) and 1926.405(a)(2), may be of a class less than that required for a permanent installation. Thus, temporary electrical power and lighting installations are permitted during the period of construction, remodeling, maintenance, repair or demolition of buildings, structures and equipment or similar activities. Such temporary wiring must be removed immediately upon completion of the work for which the wiring was installed.

When the temporary wiring consists of shop-made cord sets, etc., using approved parts, as permitted by 1910.305(a)(2) and 1926.405(a)(2) the requirements for listing by a nationally recognized testing laboratory do not apply.

The practice of assembling electrical extension cords is considered to be in compliance with OSHA standards provided the assembled cord sets are assembled in a manner equivalent to those that are factory-assembled and approved. Criteria for determining whether shop-made cord sets meet existing electrical standards include:

1. All components must be approved for the purpose by a nationally recognized testing laboratory (1910.303(a)) and (1926.403(a)). Individual components must be compatible for use with the other components of the completed assembly.

2. The cord set must meet all applicable requirements of 1910 Subpart S and 1926 Subpart K. For example, the assembly must be marked appropriately (1910.303(e)) and (1926.405(g)(2)(iv)); boxing intended for use in a permanent installation may not be used (1910.303(b)(1)(i) and 1926.403(b)(1)(i)); cords must be connected to devices and fittings so as to provide strain relief (1910.305(g)(2) (iii) and 1926.405(g)(2)(iv)); cords passing through holes in enclosures must be protected by bushings or fittings designed for the purpose (1926.405(g)(2)(v)- fittings designed to fasten cables to metal boxes are not acceptable); and no grounded conductor shall be attached to any terminal or lead so as to reverse designated polarity (1910.304(a)(2)) and (1926.404(a)(2)).

3. The cord set must be assembled by a qualified person.

4. The wiring of the completed assembly must be inspected by a qualified person before the cord set is used initially. For example, the following checks and tests, or equivalent, should be performed:

(a) Determine that all equipment grounding conductors are electrically continuous.

(b) Test all equipment grounding conductors for electrical continuity.

(c) Determine that each equipment grounding conductor is connected to its proper terminal.

(d) Test each receptacle and attachment plug to ensure correct attachment of the equipment grounding conductor.

If you have further questions on this matter, please contact the Office of General Industry Compliance Assistance [at (202) 693-1850].