1. Definition of 'Fixture'

The NEC does not provide a specific definition for "fixture." However, industry practice and other codes, such as the National Electrical Code Handbook, define a fixture as a complete lighting unit consisting of a light source, a housing, and a means of mounting. This would typically include light fixtures, ceiling fans, and other devices that provide illumination.2. Wiring in Metal Cabinet Enclosure

The NEC does not specifically address the wiring of a flexible heater in a metal cabinet enclosure. However, the following articles may be relevant: Article 400 General Requirements for Electrical Installations

This article provides general requirements for the installation of electrical equipment, including the protection of conductors.

Article 430 Motors, Motor Circuits, and Controllers

While this article is primarily concerned with motors, it also contains provisions for the installation of flexible cords and cables.

Article 501 Hazardous (Classified) Locations

This article contains requirements for the installation of electrical equipment in hazardous locations. However, your enclosure is not classified as a hazardous location, so this article is unlikely to apply.Based on the available information, it appears that the following options may be considered:

Conduit

Running the wires through a conduit would provide protection against physical damage and potential short circuits.

Cable Tray

A cable tray could be used to support and protect the wires, provided it meets the requirements of Article 392.

Open Wiring: In certain cases, open wiring may be permitted, provided the wires are adequately secured and protected from damage.Ultimately, the best course of action would be to consult with a qualified electrician to determine the most appropriate method for installing the wires in your specific application. Flag for review