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What is the UL1558 Standard?

What is UL1558?

The full title for UL1558 is the “Standard for Metal-Enclosed Low-Voltage Power Circuit Breaker Switchgear.” It is a safety standard published by Underwriters Laboratories (UL), which creates many of the key safety standards for electrical equipment used in industrial, construction, and field applications. As its full name suggests, UL1558 outlines several elements to be applied in the construction and application of metal-enclosed switchgear to ensure the safety of operators and equipment.

The covered devices include:

  • Low-voltage power circuit breakers and other interrupting devices

  • Switches

  • Control equipment

  • Metering equipment

  • Instrumentation

  • Protective equipment

  • Regulating equipment

The UL1558 standard applies to switchgear assemblies with a maximum voltage of 600 volts AC. It covers aspects such as electrical safety, construction, performance, and markings to maintain safety and reliability standards for switchgear assemblies in their intended operating environments.

It is intended to be used in conjunction with ANSI C37.20.1 and ANSI C37.51, both of which concern metal-enclosed low-voltage circuit breaker switchgear. In addition, the standard is recognized by many authorities holding jurisdiction (AHJs) to demonstrate compliance with local electrical safety codes and standards.

Features of the UL1558 Standard: Construction

The UL1558 standard clearly defines how relevant switchgear should be built for all applications. As UL1558 covers low-voltage switchgear in standard settings, many of the construction requirements should be standard features for your electrical switchgear. Here are some important details to note:

Heat Threats to Wiring

Field or factory-installed wiring should be spaced at least 3 inches (76.2mm) from a control power transformer or a heating element. However, this may be exempted if the temperatures of the transformer’s operating surface don’t exceed the temperature rating of factory-installed wiring or 140°F for field-installed wiring.

Operator Safety

Load terminals should also be designed so that when de-energized, a technician will not need to reach across an uninsulated ground bus to make a load connection. They are also considered uninsulated live parts unless mounted directly on or in a permanent electrical connection with grounded dead metal.


There are several important spacing guidelines in UL1558, such as for power circuits and control circuit spacings. For power circuits, these range from ½” minimum spacing through air and ¾” over surfaces for live parts of opposite polarity to a max of 125 volts up to 1” through air and 2” over surfaces at 1000 volts max. For control circuits for live parts of opposite polarity or uninsulated live parts and exposed or uninsulated dead metal parts, there should be ¼” through air and 1/8” over surfaces at 125 volts up to ½” through air and 3/8” over surfaces at a max of 1000 volts.

Corrosion Protection

Copper and brass are not acceptable for steel wire binding screws, nuts, and stud terminals. Instead, these should be plated cadmium, zinc, tin, silver, or similar. Other corrosion protection measures to be included in UL1558 switchgear can be found in ANSI C37.20 1-2002, where section 7.2.2 outlines the requirements for compliance.

Viewing Panes

Viewing panes must provide mechanical protection for the parts within. For openings less than 4” in any dimension, the glass must be at least 0.055” thick, while panes up to 12” for any dimension must have at least 0.115” thick glass and any other transparent material used for the pane must have the same resistance to impact and pressure. For panes with any dimension over 12” the glass or other transparent material must not shatter, crack, or become dislodged when both sides are subjected to the tests outlined in UL1558 11.5 and 11.6.

Working Space

The metal enclosure of a UL1558 switchgear assembly must be designed and built in a way that allows all doors to be able to open to 90° from a closed position.

Features of the UL1558 Standard: Performance

There are several performance requirements outlined in the UL1558, including these elements:

  • The ampacity of insulated conductors of various metals

  • The provision of disconnects with mechanical, key or electrical interlocking

  • Requirements for ground-fault protection

  • Temperature ratings for thermoplastic tape if it is to be used

  • Width of insulating barriers

The specific performance requirements for UL1558 switchgear are covered in the American National Standard for Conformance Testing of Metal Enclosed Low-Voltage AC Power Circuit Breaker Switchgear Assemblies (ANSI C37.51) and the American National Standard for Metal-Enclosed Low Voltage Power Circuit Breaker Switchgear (ANSI C37.20.1).

Features of the UL1558 Standard: Markings

Like all electrical safety equipment, specific guidelines cover w

why and how it must be presented. Examples of these markings include:

  • “Suitable for Use as Service Equipment,” where a UL1558 switchgear section is intended for that purpose.

  • “WARNING – Do not connect grounding conductors to these or any other neutral terminals. To do so will defeat ground-fault protection” on the part of the neutral bus for load termination on UL1558 switchgear with ground-fault protection.

  • “Emergency Transfer Switch Section,” where there is an automatic transfer switch for emergency situations under certain conditions.

Those markings that are required to be visible with the cover closed are allowed to be located behind a viewing pane or inside a hinged door. Tables, diagrams, and shop drawings should be provided together and in a specific pocket for that purpose. Where the UL1558 switchgear has been manufactured at different factories, the finished section of each factory should include an identifier for them.

Design UL1558-Compliant Switchgear with Enercon

The UL1558 standard is a safety standard for metal-enclosed low-voltage switchgear circuit breaker assemblies. It outlines several minimum requirements for the design and construction phase of these switchgear and the operating performance requirements. UL1558 is recognized as a key standard for protecting human health by AHJs across North America. As a result, it is necessary to comply with local health and safety standards.

Enercon has a large purpose-built facility where we design, fabricate and test our products. This allows us to maintain quality standards throughout the production process for UL1558 switchgear and other equipment. To learn more about how Enercon can help you design, create, and install UL1558-compliant switchgear, read more here or talk to our team.

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