Article 90


  1. Purpose.
  2. Scope.
  3. Code Arrangement. This Code is divided into the intro‐ duction and nine chapters, as shown in Figure 90.3. Chapters 1, 2, 3, and 4 apply generally. Chapters 5, 6, and 7 apply to special occupancies, special equipment, or other special conditions and may supplement or modify the requirements in Chapters 1 through 7 .Chapter 8 covers communications systems and is not subject to the requirements of Chapters 1 through 7 except where the requirements are specifically referenced in Chapter 8.Chapter 9 consists of tables that are applicable as referenced. Informative annexes are not part of the requirements of thisCode but are included for informational purposes only.
  4. Enforcement. This Code is intended to be suitable for mandatory application by governmental bodies that exercise legal jurisdiction over electrical installations, including signal‐ ing and communications systems, and for use by insurance inspectors. The authority having jurisdiction for enforcement of the Code has the responsibility for making interpretations of the rules, for deciding on the approval of equipment and mate‐ rials, and for granting the special permission contemplated in a number of the rules.By special permission, the authority having jurisdiction may waive specific requirements in this Code or permit alternative methods where it is assured that equivalent objectives can be achieved by establishing and maintaining effective safety.This Code may require new products, constructions, or mate‐ rials that may not yet be available at the time the Code is adop‐ ted. In such event, the authority having jurisdiction may permit the use of the products, constructions, or materials that comply with the most recent previous edition of this Code adopted by the jurisdiction.
  5. Mandatory Rules, Permissive Rules, and Explanatory Material.
  6. Formal Interpretations. To promote uniformity of inter‐ pretation and application of the provisions of this Code, formal interpretation procedures have been established and are found in the NFPA Regulations Governing Committee Projects.
  7. Examination of Equipment for Safety. For specific items of equipment and materials referred to in this Code, examina‐ tions for safety made under standard conditions provide a basis for approval where the record is made generally available through promulgation by organizations properly equipped and qualified for experimental testing, inspections of the run of goods at factories, and service-value determination through field inspections. This avoids the necessity for repetition of examinations by different examiners, frequently with inade‐ quate facilities for such work, and the confusion that would result from conflicting reports on the suitability of devices and materials examined for a given purpose.It is the intent of this Code that factory-installed internal wiring or the construction of equipment need not be inspected at the time of installation of the equipment, except to detect alterations or damage, if the equipment has been listed by a qualified electrical testing laboratory that is recognized as having the facilities described in the preceding paragraph and that requires suitability for installation in accordance with this Code. Suitability shall be determined by application of require‐ ments that are compatible with this Code.Informational Note No. 1: See requirements in 110.3. Informational Note No. 2: Listed is defined in Article 100.Informational Note No. 3: Informative Annex A contains a list of product safety standards that are compatible with this Code .
  8. Wiring Planning.
  1. Measurement System of Preference. For the purpose of this Code, metric units of measurement are in accordance with the modernized metric system known as the International System of Units (SI).
  2. Dual System of Units. SI units shall appear first, and inch- pound units shall immediately follow in parentheses. Conver‐ sion from inch-pound units to SI units shall be based on hard conversion except as provided in 90.9(C).
  3. Permitted Uses of Soft Conversion. The cases given in 90.9(C)(1) through (C)(4) shall not be required to use hard conversion and shall be permitted to use soft conversion.
    1. Trade Sizes. Where the actual measured size of a product is not the same as the nominal size, trade size designators shall be used rather than dimensions. Trade practices shall be followed in all cases.
    2. Extracted Material. Where material is extracted from another standard, the context of the original material shall not be compromised or violated. Any editing of the extracted text shall be confined to making the style consistent with that of the NEC.
    3. Industry Practice. Where industry practice is to express units in inch-pound units, the inclusion of SI units shall not be required.
    4. Safety. Where a negative impact on safety would result, soft conversion shall be used.
  4. Compliance. Conversion from inch-pound units to SI units shall be permitted to be an approximate conversion. Compliance with the numbers shown in either the SI system or the inch-pound system shall constitute compliance with this Code.

Informational Note No. 1: Hard conversion is considered a change in dimensions or properties of an item into new sizes that might or might not be interchangeable with the sizes used in the original measurement. Soft conversion is considered a direct mathematical conversion and involves a change in the description of an existing measurement but not in the actual dimension.

Informational Note No. 2: SI conversions are based on IEEE/ ASTM SI 10-1997, Standard for the Use of the International System of Units (SI): The Modern Metric System.