Last edit: 19/08/2025
UL 508A is the technical standard for Industrial Control Panels in the USA. The first edition was publised in 2001. Starting from the basics of electrical safety, stated by the NEC and looking at the prescriptions stated in NFPA 79, it gives indications for the design of industrial control panels.
While the NFPA 79 provides prescriptions for industrial machinery control panels, UL508A has a broader scope and it focuses on the criteria that guarantee the safety of a control panel. It also provides a well recognised method to calculate the Short-Circuit Current Rating of an Intustrial Control Panel.
In Europe, there is no equivalent standard, as you may know, to UL 508A. The equivalent of IEC 61439-2 (for switchboards) is UL 891 and of IEC 60204-1 (for industrial control panels for Machineries) is NFPA 79.
The third edition of this standard was published in April 2018. UL 508A Ed. 3 has been designated as an American National Standard (ANSI). This designation indicates that the standard has been developed under a fair and open process designed to protect the rights and interests of every participant.
JUNE 2025 UPDATE
A new update of the standard was officially published on 26 June 2025. Being members of the UL 508A Committee, hereafter we give you some insights of the main changes.
Limits to Control Circuit Voltages; § 66.12.1 and § 66.12.2
The reason for the change is to align the requirements in UL 508A to the ones in NFPA 79. The maximum voltage limit for control circuits is now set at 120 Vac or 250 Vdc
Similar prescriptions are also in IEC 60204-1, even if the limits are higher.
Need for Emergency Stop Function; § 66.11.2
In the past, the UL 508A standard required an emergency stop button to be installed on each industrial control panel for machinery equipped with operator controls, such as pushbuttons. NFPA 79 specifies that an emergency stop is required when indicated by a risk assessment, an approach also adopted by B11.19. ISO 13850, as referenced in NFPA 79, follows a similar philosophy.
The proposal aligns UL 508A with this approach.
[UL 508A: 2025] 66.11 Operator controls
66.11.2 An industrial control panel provided with operator controls, such as pushbuttons and selector switches, can be provided with an emergency stop button.
Disconnecting Means – Industrial Machinery; 66.6.1, 66.6.1A, Table 52.1
In the past, all industrial control panels (ICPs) for industrial machinery were required to have an external handle connected to a disconnecting means for each incoming supply source. In some cases, however, multiple panels might supply a single machine through a single external feeder line.
The 2021 edition of NFPA 79 – The Standard for Industrial Machinery now requires a disconnecting means with an external operating handle for each machine supply circuit. In addition to the main machine supply circuit disconnecting means, additional devices may be provided to disconnect electrical equipment so that work can be performed on individual portions of the machine. But that is not mandatory!
Under these requirements, panels that form part of a group controlling a single machine may be supplied from a panel containing the machine supply circuit disconnecting means. In such cases, these auxiliary panels are permitted—but not required—to have their own disconnecting means. If such a disconnecting means is provided, it does not need an external operating handle.
With this revision, UL 508A aligns with the same approach as NFPA 79.
Location of Ventilation Opening; 21.2.2 and Figure 21.1
Panels, that include drives, often need to include fans to provide positive airflow for cooling heatsinks. These panels are installed with other panels located on either side which limits the areas where the outflow air can be directed. In these cases, the front of the panel needs to be used for the ventilation openings. The requirement in 21.2.2 applies whenever any type of control button, switch HMI or handle is mounted on the front of the panel, regardless of the amount of operator interaction needed. There is no specific hazard that is addressed by this requirement, only to prohibit exhaust air from being directed to an assumed area an operator may occupy. For panels operating in normal conditions, the outflow air from semiconductor heatsinks does not pose a hazard to an operator.
The change addresses and relaxes this prescription, adding the following exception:
[UL 508A: 2025] 21.2 Location of ventilation opening
Exception No. 3: An enclosure containing external switch actuators or displays that only require a short time presence of an operator, such as handles for disconnecting means, emergency stop buttons, external overload reset buttons, simple control devices (such as start / stop pushbuttons, selector switches), indicator lights, non-interactive digital displays, electrical meter faces and similar devices, is not required to comply with 21.2.2.