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UL 508A: Industrial Control Panels

Standard for Safety, Industrial Control Panels

Last edit: 29/05/2023

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.

A new update of the standard was officially published on 28 July 2022. Being members of the UL 508A Rooster, hereafter we give you some insights of the main changes.

FULL-LOAD AMPERE 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).

TERMINATION TEMPERATURE

Another change in the 2022 release of UL 508A is related to how to choose between the 60°C and he 75°C columns in Table 28.1.
One of the important aspects of reducing the risk of fire is that the cable ampacity has to be determined also based upon the maximum temperature the terminals of, for example, a circuit breaker, can attain. That is clearly stated in the NEC at Article 110.14(C).

[NEC: 2020] 110.14 (C) Temperature Limitations.
The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. […]

The problem is that not all components clarify what is the maximum temperature of their terminations. Normally, they can be 60°C or 75°C. Therefore the clarification is needed for determining the ampacity of internal conductors since it is not clear how to use Table 28.1 relative to the differing ampacities based on the insulation temperature of the conductor (60°C column vs 75°C column). This revision aligns also with the requirements in Section 110. 14(C)(1) of the NEC. Although this section addresses field wiring terminations, it establishes the minimum requirements for temperature ratings of conductors inside an Industrial Control Panel used in circuits rated 100 amperes or less and circuits rated over 100 amperes.

[UL 508A: 2022] 29.6 Sizing
[…]
The wire temperature rating(s) marked on the component or in the instructions shall be used to select the appropriate column(s) from Table 28.1 as follows :
1)    […]
4)    If the component markings or instructions do not specify a wire temperature rating, the conductor size shall be determined using the ampacities in the 60°C column in Table 28.1 when the component is rated 100 amps or less, and the 75°C column in Table 28.1 when the component is rated greater than 100 amps.

 

AMBIENT TEMPERATURE

Finally it is possible to certify industrial control panels that are installed in ambient temperatures higher than 40°C. That was already allowed in Canada, and it is now possible in USA as well. You find the new language in section 1.1 of UL 508A 2022.

 

GFCI CLASS A NOW COMPULSORY ON RECEPTACLES

With the 2022 update of UL 508A, the standard is now 100% aligned with NFPA 79 (and with IEC 60204-1) regarding the prescription for a Class A GFCI on all receptacle for general use. You find the new language in section 66.4.4.

 

ENCLOSURE ACCESS

The 2022 update aligns the enclosure access requirements with NFPA 79: No door interlock required anymore, normally. No need any more to interlock all doors with the presence of energy inside the panel. No need any more to open the main panel disconnector in order to open the doors. We now have a similar international approach to the risk of direct contacts inside a Machinery Control Panel, since IEC 60204-1, NFPA 79, UL 508A, C22.2 N° 301 and C22.2 N°286 are somehow aligned in this respect. Please check this page on our website.

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

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