A transformer installed in a hazardous location has to satisfy more than its normal electrical duty. The surrounding atmosphere may contain flammable gas or vapour, which means surface temperature, arcing components, wiring interfaces, and the complete certified construction become part of the specification.
For Class I, Division 2 installations, this does not automatically mean placing a conventional transformer inside an explosion-proof enclosure. Division 2 is based on an atmosphere that is not expected to contain ignitable concentrations during normal operation. The equipment still has to be suitable for the classified location, gas group, temperature requirement, and installation conditions, but several protection methods can satisfy those requirements.
That distinction is important when specifying a Class I Division 2 transformer. The area classification should be established first. Transformer construction and certification then have to match that classification rather than relying on a general description such as “hazardous-duty,” “sealed,” or “explosion-proof.”
What “Hazardous Location” Means in Electrical Design
A hazardous location is an area where the nature and expected presence of flammable gas, vapour, combustible dust, or easily ignitable fibres create an additional ignition risk for electrical equipment.
Hazardous-area classification is established for the installation location based on the hazardous material that may be present and the likelihood of an ignitable concentration. OSHA 29 CFR 1910.307 requires each room, section, or area to be considered individually when determining its classification. Electrical equipment and wiring installed in that area must then be suitable for the applicable hazardous location.
The same principle applies during transformer specification. The transformer manufacturer should be given the established area classification rather than being asked to determine from a process description whether the area is hazardous.
A gasoline transfer area, for example, may have a different classification at the pump, farther from the potential release point, and inside an adjacent electrical room. Similarly, a chemical facility may contain ordinary locations, Division 2 areas, and more restrictive areas within the same process unit. Select a hazardous-location transformer based on the classification at the actual installation point.
Understanding Class, Division, and Group Ratings
The traditional North American Division system describes three separate things: the type of hazardous material, how likely an ignitable atmosphere is to occur, and the ignition characteristics of that material. Those parts of the classification cannot be separated when equipment is selected.
Class I vs. Class II vs. Class III
| Class I | Class II | Class III |
|---|---|---|
| Locations where flammable gases or vapours may form an ignitable mixture with air. | Locations where combustible dust may create an ignition hazard. | Locations where easily ignitable fibres or flyings create a fire hazard. |
| Typical examples include petroleum products, natural gas, hydrogen, and solvents. | Typical examples include grain handling, coal processing, and certain metal-processing operations. | Typical examples include facilities handling materials that produce combustible fibres or flyings. |
A certification for one class does not automatically establish suitability for another. A Class I transformer, for example, is not necessarily suitable for a Class II dust location, and a dust-resistant enclosure does not by itself make equipment suitable for flammable gas service.
Division 1 vs. Division 2
The division number describes how likely a hazardous atmosphere is to occur.
| Class I, Division 1 | Class I, Division 2 |
|---|---|
| Ignitable gas or vapour concentrations may be present during normal operation. | Ignitable concentrations are not normally present during operation. |
| Hazardous concentrations may also occur frequently during maintenance, leakage, or equipment failure. | Flammable liquids or gases are generally contained within closed systems or controlled by ventilation. |
| Electrical equipment must be suitable for locations where the hazardous atmosphere may be present under normal conditions. | Hazardous concentrations are expected mainly after an accidental release, abnormal operation, or ventilation failure. Areas adjacent to Division 1 locations may also be classified as Division 2, where vapour migration is possible. |
The lower probability of an ignitable atmosphere in Division 2 affects which electrical protection methods are permitted, but it does not mean ordinary equipment can be installed without evaluation.
In Canada, Class I hazardous locations are generally addressed using the Zone system under the Canadian Electrical Code, although the Division system remains relevant to certain existing installations addressed under the Code’s Annex J provisions. Cross-border specifications should therefore identify whether the project uses Class/Division, Class/Zone, or both.
Gas and Vapour Groups: A, B, C, and D
Class I materials are further separated into groups according to their ignition and explosion characteristics. Acetylene represents Group A, hydrogen Group B, ethylene Group C, and propane Group D. These names are useful reference points, but the group is not simply a list of four gases — other gases and vapours are assigned according to comparable combustion characteristics.
The equipment approval must cover the gas group present at the installation. A specification stating only “Class I, Division 2” is therefore incomplete when the applicable group has not been identified. This becomes particularly important with gases such as hydrogen, where equipment suitable only for Group D service cannot simply be assumed suitable for the application.
Class I Division 2: What It Actually Requires
One of the most common specification mistakes is to treat Class I, Division 2 as synonymous with explosion-proof construction.
It is not.
For U.S. installations, NEC Article 501 contains specific requirements for Class I locations. NEC 501.100 addresses transformers and, for Division 2, refers dry-type transformer installation back to the applicable requirements of Article 450. At the same time, the broader hazardous-location rules require equipment to be suitable for the specific location and require control of ignition sources and excessive surface temperature.
This allows a Class I, Division 2 transformer to use a construction appropriate to Division 2 without requiring the entire enclosure to perform as a Class I, Division 1 explosion-proof enclosure.
Ignition-Capable Equipment Rules
Electrical equipment can become an ignition source in several ways. Switching contacts can create an arc. A loose termination can overheat. A component may develop a surface temperature above the ignition temperature of the surrounding gas. Accessories added to an otherwise suitable transformer can introduce their own contacts, relays, or heating elements.
Division 2 equipment can therefore use methods such as nonincendive construction, hermetically sealed components, or other accepted protection techniques where applicable. UL 121201 establishes requirements for nonincendive electrical equipment intended for Class I and II, Division 2, and certain Class III locations. Its scope addresses equipment that, during normal operation under prescribed conditions, is not capable of igniting the surrounding atmosphere.
This is different from an explosion-proof enclosure. UL 1203 addresses explosion-proof and dust-ignition-proof electrical equipment for use in hazardous (classified) locations. An explosion-proof enclosure is designed around the possibility that an ignition may occur inside the enclosure and that the resulting flame or pressure must not ignite the surrounding atmosphere.
For that reason, using “explosion-proof transformer” as a generic term for every Division 2 transformer can create the wrong specification. The required protection method should follow the classified area and the equipment certification.
Temperature (T-Code) Rating Requirements
Arcs and sparks are only part of the ignition problem. A transformer is inherently a heat-producing device, so maximum surface temperature also has to be considered.
Hazardous-location equipment uses temperature classes, or T-codes, to indicate the maximum permitted surface temperature associated with its certification. For example, T3 corresponds to a maximum surface temperature of 200°C, T3C to 160°C, and T4 to 135°C.
The selected equipment temperature rating has to remain below the ignition temperature applicable to the hazardous atmosphere. OSHA also requires hazardous-location equipment to be marked for its approved class, group, and operating temperature where the marking provisions apply. Equipment intended for ambient temperatures above the standard reference conditions requires the appropriate ambient and operating-temperature identification.
Transformer winding temperature rise should not be confused with the hazardous-location T-code. A transformer may have an 80°C or 115°C winding-temperature-rise design, but that number alone does not establish the certified maximum exposed surface temperature. The complete construction, loading condition, ambient temperature, and certification determine the applicable T-code. This is one reason a lower-temperature-rise design may become relevant in hazardous locations even where a higher-rise transformer would otherwise be acceptable electrically.
How Hazardous Location Transformers Are Built Differently
The construction of a Division 2 transformer depends on its rating and certification route. There is no single enclosure detail that converts an ordinary transformer into classified-location equipment. The design has to consider the core-and-coil assembly, exposed temperatures, terminations, wiring compartments, accessories, and enclosure as one certified system.
Enclosure and Sealing Requirements
Enclosure ratings and hazardous-location approvals address different risks. A Type 3R, 4, or 4X enclosure provides environmental protection but does not by itself establish Class I, Division 2 suitability.
For outdoor installations, weather protection, corrosion resistance, and hazardous-location approval must therefore be considered separately. Rex’s hazardous location transformers are available in Class I, Division 2 configurations with enclosure options suited to the application. The transformer construction, enclosure rating, and hazardous-location certification should be specified together.
Internal Component and Wiring Requirements
The classified-location requirement extends beyond the sheet-metal enclosure. Lead connections, terminal blocks, protective devices, space heaters, temperature-monitoring devices, surge protection, and other accessories can change the ignition assessment. A component that opens or closes a circuit may arc during normal service. A heater changes internal and external thermal conditions. Field-added devices can introduce equipment that was never evaluated as part of the original transformer.
The wiring method is equally important. The applicable electrical code and installation classification govern hazardous-location cable entries, conduit systems, fittings, seals, and grounding or bonding arrangements. A certified transformer cannot make an incorrectly installed raceway or unsuitable cable gland compliant.
For this reason, accessories should be identified before certification and production wherever possible. Adding them in the field is not necessarily equivalent to supplying them as part of the evaluated transformer construction.
Common Class I Division 2 Applications
A Division 2 rated transformer is typically used where power conversion is required inside or close to a process area in which flammable gas or vapour could be released under abnormal conditions.
Oil and Gas Facilities
Oil and gas installations contain many potential Division 2 applications because hydrocarbons are frequently handled in closed piping, vessels, compressors, and process equipment. A transformer may be required near a gas-compression package, pumping installation, storage or transfer facility, process skid, or other equipment where the surrounding area has been classified due to the possibility of an abnormal release.
The exact classification still comes from the facility’s area-classification study. “Oil and gas” alone is not a sufficient transformer specification because the same site may contain unclassified, Division 2, and Division 1 locations.
Mining Operations
Mining facilities create several different electrical hazards, and not all of them are Class I. Combustible dust can make some locations Class II, while areas involving fuels, solvents, natural gas, or process gases can create Class I conditions. Surface processing facilities can also have corrosive, wet, and high-contamination environments that require separate enclosure consideration.
For projects in this sector, the hazardous-location classification therefore has to be coordinated with the actual process and governing jurisdiction. Rex provides transformers for the broader mining industry, but a Class I, Division 2 design should only be specified where that classification applies to the installation point.
Chemical Processing Plants
Chemical facilities often contain closed piping systems, pumps, storage vessels, and process equipment handling flammable liquids or vapours. Areas around those systems may be classified as Division 2, where an ignitable atmosphere is expected only following leakage, equipment failure, or another abnormal condition.
The gas or vapour involved is especially important in these applications. Two process areas may both be Class I, Division 2 but require different gas-group and temperature ratings. A specification copied from another section of the plant can therefore be inadequate even when the voltage and kVA are identical.
Specifying a Hazardous Location Transformer: What to Provide Your Supplier
The best transformer specification starts with the area-classification documentation and then adds the normal electrical and mechanical requirements. At minimum, the supplier should receive:
- Installation jurisdiction and classification: NEC or CEC requirements, Class I Division 2 or applicable zone designation, gas group or specific gas, required T-code, and any project-specific certification requirements.
- Electrical duty: kVA, phase, frequency, primary and secondary voltage, taps, impedance requirements, winding material where specified, grounding arrangement, and any required shielding or harmonic capability.
- Environment: indoor or outdoor service, minimum and maximum ambient temperature, altitude, moisture, washdown, corrosion exposure, and the required NEMA or IP enclosure performance.
- Installation interface: conduit or cable entry requirements, certified hubs or glands where applicable, termination arrangement, mounting restrictions, and available clearances.
- Accessories: space heaters, temperature sensors, alarm or trip contacts, surge protection, and any other device that must form part of the supplied assembly.
- Certification and marking: required CSA, UL, or other recognized certification, together with the exact class, division, or zone, group, temperature, and ambient markings required by the project.
This information should be resolved before release for manufacture. Changing the gas group, T-code, or area classification later is not comparable to changing a paint colour or adding a nameplate. It can alter the certified construction and may require a different transformer design.
Standards and Certifications
Hazardous-location transformers must meet both transformer and classified-location requirements. In the United States, NFPA 70 and OSHA 29 CFR 1910.307 govern hazardous electrical installations, while UL 121201 applies to nonincendive Division 2 equipment and UL 1561 covers dry-type transformer construction.
In Canada, CSA C22.1 provides the installation framework, while CSA C22.2 No. 213 addresses nonincendive Division 2 equipment. Transformer standards such as CSA C9 and CSA C22.2 No. 47 apply within their respective scopes, but do not by themselves establish hazardous-location suitability.
Rex’s hazardous location transformers are available in certified Class I, Division 2 configurations — see the Rex hazardous location transformer catalogue for configurations and ratings. The required approval should always be confirmed for the specific rating and application, and additional qualification information is available in Rex Power Magnetics’ certifications.
Rex Power Magnetics Perspective
There is also value in avoiding unnecessary requirements. Specifying Division 1 explosion-proof construction where the project requires an approved Division 2 transformer can add complexity without improving compliance. At the same time, specifying only a Type 4 enclosure where Class I, Division 2 approval is required leaves the actual ignition hazard unresolved. The objective is to match the transformer to the classified location that actually exists.
Conclusion
Class I, Division 2 transformer specification starts with the hazardous-area classification, not with the enclosure.
Class I establishes that the hazard is a flammable gas or vapour. Division 2 establishes that an ignitable concentration is not expected during normal operation but may occur under abnormal conditions. The gas group defines the atmosphere the equipment must be suitable for, while the temperature rating limits exposed surface temperature relative to the ignition characteristics of that atmosphere. Those requirements sit alongside the transformer’s normal kVA, voltage, impedance, temperature rise, enclosure, and environmental requirements.
A correctly specified hazardous-location transformer brings those elements together in a certified construction. Identifying the area classification, gas group, T-code, ambient conditions, and certification requirements at the beginning of the project gives the manufacturer the information needed to select that construction correctly and avoids relying on terms such as “sealed” or “explosion-proof” that, by themselves, do not define the application.