UL 508 A: Industrial Control Panels
Standard for Safety, Industrial Control Panels
Last update 18/06/2022
UL 508A is the technical standard for Industrial Control Panels in the USA. 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 lastest release of this standard was 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.
In 2022 a new update of the standard will be published. Being members of the UL 508A Rooster, hereafter we give you some insights of the main changes.
FULL LOAD AMPACITY vs FULL LOAD CURRENT
In the past we, as GT Engineering, have always struggled with the fact the terms FLA and FLC were not used in a precise way throughout the UL 508A standard. Therefore, in 2021, we submitted a Proposal Request to align the use of the two terms with what is in the NEC. Hereafter some considerations for the changes.
“UL 508A, in Clause 52.1 (that refers back to 49.1) requires the Panel Nameplate to provide the "Full-load Ampere rating" and be written in the nameplate. NFPA 79 calls it "full-load current" (Article 16.4). NEC (NFPA 70) requires in article 670.3(A)(1), the nameplate to include a list of information among which the "Full-load Current" of the panel.
The use of full-load ampere rating in UL 508A should be changed to full-load current, where the meaning is the same, as stated in NFPA 79 or NEC. In particular: 29.6.1 of UL 508A calls the currents in Table 50.1 full-load ampere rating. The table is equivalent to NEC 430.250 and the name used is full load current. The title of Table 50.1 is "Full-load motor-running currents in amperes corresponding to various a-c horsepower ratings", while the equivalent table in NEC is table 430.250 " Full-Load Current, Three-Phase Alternating-Current Motors" (NEC has several tables instead of just one in UL 508A). We propose to change the title of Table 50.1 to "Full-Load currents in amperes corresponding to various a-c horsepower ratings".
The proposal was well welcomed and it is now implemented in the new edition of the standard. Probably the most important aspect is that, on the name plate, referring to the whole control panel, we will not write anymore “Total FLA” but the more correct term “Full Load Current Rating (FLC).
UL508A covers the engineering and manufacturing of Industrial Control Panels, while for Switchboards (in Europe we would also call them Distribution Panels) UL891 is the reference standard.
Every component, in order to be used in an Industrial Control Panel, must be approved by UL ("listed" or "recognised"). However, this is not enough to guarantee that the Industrial Control Panel is compliant to the UL508A.
The sizing of motor startings and of the main protection have different rules from the ones listed in EN 60204-1. The main problems that can be encountered while approving Industrial Control Panels in USA are:
- Circuit breakers not compliant to the UL standards or installed in the wrong way. In addition to sizing, their use should be considered very carefully because, depending on their certification (UL1077, UL489 and so on), they shall have definite applications in a control panel.
- Non-compliances regarding the spacing between cables or devices.
- Absence of the control panel plate or plate with incomplete data.
- Transformers not certified to the correct UL standard