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Thread: UL508A FLA Rating Question

  1. #1
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    UL508A FLA Rating Question

    Section 49.2 reads that at a minimum, the FLA marking include the sum of the ampere ratings of all loads that are able to be operated simultaneously plus the primary ampere rating of all control transformers connected to the input voltage. So, do power supplies (and their loads) counts as loads or as control transformers?

    To be more specific, the enclosure in question is part of a large material handling system. Due to some of the distances involved, some of the Ethernet connections must be run from the main IT room as fiber and then converted to copper at the location of the PCs. The customer wants all enclosures to be UL 508A listed. Inside these small 120VAC single phase enclosure is just a DC power supply that provides power to the Ethernet fiber/copper converter. Both the power supply and converter are UL listed components. The converter requires 95mA @ 24VDC. The power supply however is rated at 1.4A at 120VAC.

    What is the correct 508A FLA rating for this enclosure, 0.02A (the FLA power needed by the converter) or 1.4A (the rating of the DC power supply)?

    The real issue then extends to the NEC. If the answer is 1.4A, we are being told that the power to these boxes must be sized to supply the LISTED FLA not what the load really needs. If we have 20 of these boxes, then multiple dedicated 15A circuits would be required to supply the 0.4A these boxes are going to actually use. This is then further exacerbated by the power for the PCs, which have 150W power supplies, but as configured only use 50W (these are custom PCs set-up to be used only to control/monitor the equipment).

  2. #2
    Join Date
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    I am pretty sure UL does not mean "power supply" where it actually says "transformer".

    I would be inclined toward the idea that the actual loads are what matters.

    I would also be inclined to replace the way over sized power supplies with something a lot closer to the load actually in use just to avoid any issues. The question came up, so who knows where it could go from there.

    Does UL even allow one to mark a panel with FLA < 1 amp? I always round it up to the next full amp.
    Last edited by petersonra; 02-17-12 at 12:41 PM.
    Bob

  3. #3
    Join Date
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    If the power supply is being powered from the incoming 120V, that is what they are looking for, the connected load is immaterial. The purpose of the rating on the nameplate is to ensure, at a glance, that the circuit feeding it is sufficient for the maximum possible amps that it will theoretically be able to consume from the upstream circuit, as defined by FLA ratings of all devices inside. What the devices inside or down stream actually consume does not figure into it.

    To further the example, let's say you had a 480V feed and a motor starter, then a 480-120V CPT, then a 120-24VDC power supply. The nameplate data would be derived from the motor FLA and the primary current of the CPT. The power supply input current is ALREADY going to be part of the CPT circuit current.
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