API RP 500 (R2021)

The purpose of this recommended practice is to provide guidelines for classifying locations Class I, Division 1 and Class I, Division 2 at petroleum facilities for the selection and installation of electrical equipment. Basic definitions given in the 2011 Edition of NFPA 70, National Electrical Code (NEC), have been followed in developing this recommended practice. This publication is only a guide and requires the application of sound engineering judgment.

Recommendations for determining the degree and extent of locations classified Class I, Zone 0, Zone 1, and Zone 2 are addressed in API 505, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2.

Electrical installations in areas where flammable liquids, flammable gases or vapors, or combustible liquids are produced, processed, stored or otherwise handled can be suitably designed if the locations of potential sources of release and accumulation are clearly defined. Once a location has been classified, requirements for electrical equipment and associated wiring should be determined from applicable publications. Applicable publications may include NFPA 70 and API 14F. Reference Section 2 for publications for other possible applications.

This document applies to the classification of locations for both temporarily and permanently installed electrical equipment. It is intended to be applied where there may be a risk of ignition due to the presence of flammable gases, flammable liquid-produced vapors, or combustible liquid-produced vapors, mixed with air, under normal atmospheric conditions (identified throughout this document as gases and vapors). Normal atmospheric conditions are defined as conditions that vary above and below reference levels of 101.3 kPa (14.7 psia) and 200 °C (68°F) provided that the variations have a negligible effect on the explosion properties of the flammable materials.