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Thread: Equipment Grounding Conductor Required for 24 VDC System?

  1. #1
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    Equipment Grounding Conductor Required for 24 VDC System?

    I have a distributed control system where the system power source originates from 480 VAC powering a 20 amp 24 VDC power supply. The DC common is grounded at the power supply. 24 volt supply circuits are run to various field or machine mounted components. Some of these cables are part of an industrial machine but some of them are part of the building as they are run in cable trays which interconnect various components throughout the building. My question is: Does an equipment grounding conductor need to be a part of the cable for these circuits, or can a two conductor cable supply the power without a third, equipment ground? If this were a 120 volt AC circuit, i would already know the answer, but I am not sure if it being 24 VDC makes any difference.

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  3. #3
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    Did not hellp

    Thanks, but I could not find an answer for me in that.

    My 24 V supply will have the secondary grounded. UL 508A 16.1 requires it in my case. My question is, am i required to run an equipment grounding conductor in the cable with the DC + and DC common when it leaves the panel? The common practice in the automation assembly industry is to NOT provide that equipment grounding conductor in the cable.

    I am questioning that since we are building the UL 508A panel which will be providing the 24 VDC receptacles to supply those components in the field.

  4. #4
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    I guess I don't see an exception from NEC 250.110 (and all of Part VI) due to low voltage.

    Although maybe you would be compliant with 720 or 725 which wouldn't require compliance with 250.110
    Ron

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    Could the 24VDc start a fire if there was a short? Low Voltage, or Art 725 Class II wiring has the power and voltage limits set to prevent a fire or shock
    My answer is yes as its from a 20 amp power supply, so this system is not a Art 725 Class II system, unless you have fuses or CBs and limit the current to each device.
    If its not a Art 725 Class II system, then you need to wire it in a chapter 3 method.
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    Ancora Imparo

  6. #6
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    There are two circuits being supplied by the 20 amp power supply. Each is protected by a 10 amp UL489 branch type circuit breaker.

    I don't see how a fire would be likely under a short to any conductive metal objects, like the enclosures the cables are supplying power to or a part of the equipment to which the enclosures or cables are attached.

    The breaker would trip either by finding a path back to the grounded power supply or if there were no path back, a "floating" or poorly bonded enclosure would become energized at 24 VDC.

    Could this be considered a Class 2 system? I am really not clear on the conditions of this requirement.

  7. #7
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    Quote Originally Posted by richwaskowitz View Post
    There are two circuits being supplied by the 20 amp power supply. Each is protected by a 10 amp UL489 branch type circuit breaker.

    I don't see how a fire would be likely under a short to any conductive metal objects, like the enclosures the cables are supplying power to or a part of the equipment to which the enclosures or cables are attached.

    The breaker would trip either by finding a path back to the grounded power supply or if there were no path back, a "floating" or poorly bonded enclosure would become energized at 24 VDC.

    Could this be considered a Class 2 system? I am really not clear on the conditions of this requirement.
    7215.121 tells us what a class 2 power source is, for your application it sends you to Table 11 (A). At ten amps it appears you do not have a class 2 system. So it must be wired in a chapter 3 wiring method, and I don't see an exception for an EGC
    Moderator-Washington State
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  8. #8
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    I think I found the exception for my 24 volt question

    Ron, You stated earlier "I guess I don't see an exception from NEC 250.110 (and all of Part VI) due to low voltage." Perhaps I did not make it clear that my circuits were Class 1 Remote-Control circuits. Please consider 250.112....

    250. 112 (I) covers my application and I believe is where it is stated that an equipment grounding conductor is not required to be provided along with the supply conductors in a two-conductor supply cord between a 24 V power supply and field located devices (such as IP 67 I/O blocks, the 24 V interfaces provided within field installed VFDs (like the Allen Bradley ArmorStarts)).

    It states "Remote-Control, Signaling, and Fire Alarm Circuits. Equipment supplied by Class 1 circuits (which mine are since they are powered by 20 A non-Class 2 power supplies at 24 V protected with 10 A OCPDs) shall be grounded unless operating at less than 50 volts."

    Can you or anyone provide me another article which contradicts or negates this?

  9. #9
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    Start here:

    VIII. Direct-Current Systems
    250.160 General. Direct-current systems shall comply
    with Part VIII and other sections of Article 250 not specifically
    intended for ac systems.
    250.162 Direct-Current Circuits and Systems to Be
    Grounded. Direct-current circuits and systems shall be
    grounded as provided for in 250.162(A) and (B).

    AC and DC have different requirements.
    The more they overthink the plumbing, the easier it is to stop up the drain

  10. #10
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    Not talking about grounding the system here

    Dkidd,

    This is not about grounding the power source, it is about whether or not an equipment ground to connected devices is required. The system IS grounded and complies with 250.160 and 162, in that the negative terminal of the 24 V power supply is connected to the grounding electrode system. 250.112 is not about grounding the source, it is about connecting an equipment grounding conductor to the devices and whether or not that conductor is required.

    One can have an electrical system which is required to be a grounded system (for example a typical residence) but that does not mean that everything connected to it has to carry an equipment ground (a table lamp with a two-prong plug).

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