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Thread: Use of Conductors smaller than 18 awg for Class 1 systems

  1. #1
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    Use of Conductors smaller than 18 awg for Class 1 systems

    I work for an electronics company that specializes in PLC based control systems of remotely controlled field devices such as Door Locks, Utilities, etc.
    The door locks are usually powered by 120Vac or 24Vdc, Lights are either 277Vac or 120Vac and of course receptacles are 120Vac.
    We factory assemble our own industrial control panels which consist of PLC’s, Class-1 24Vdc power supplies, 24Vdc relays, single terminals and 24 gauge multiconductor cables (we don’t use Class 2 power for anything).
    Our multiconductor control cable is a Belden 9543, 24 gauge 25 conductor jacketed CMG cable rated at 600V.
    Our control cable connects PLC Outputs to 24Vdc relays for door control and Input modules connected to Door Position Switches located at the doors.
    We have been assembling these control panels on hundreds of facilities over a time span of more than three decades. Recently we have encountered a very strict security consultant that won’t allow us to combine the 24 gauge control cable in the same Panduit wireway as conductors of other voltages and conductor sizes. The consultants reason is; the 24 gauge multi-conductor cable is considered Class 2 even though it’s source of power is from a Class 1 power supply.

    Through my research of the NEC, I have found in code article 725.49 (A) permits 18awg and 16awg to be used as Class 1 circuits, but haven’t found any minimum conductor size for Class 2 or Class 3 circuits, nor have I found any exception that permits 24awg conductors powered from a Class 1 power supply to be permitted in factory assemblies or industrial control panels.

    Are you aware of any NEC code article that would permit 24awg control wiring to be installed in the same (Panduit) raceway within a factory assembled industrial control panel?

  2. #2
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    Wiring within control panels is outside the scope of the NEC. This is something that UL would determine. Are you a UL shop?

    -Hal

  3. #3
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    Quote Originally Posted by hbiss View Post
    Wiring within control panels is outside the scope of the NEC. This is something that UL would determine. Are you a UL shop?

    -Hal
    We are a certified UL-508 shop, however these control panels are for use in a detention facility. Once upon a time UL recognized and accepted control panels under the UL-508 listing, but approximately 8 years ago they made a decision not to allow UL listings in a detention facility (I don't know why).

    Can you lead me to a document that indicates wiring within control panels is outside the scope of the NEC? Or are you aware of any exception that would allow wiring smaller than 18awg to be permitted with Class 1 systems? Keep in mind that these 24awg conductors are part of a 25 conductor jacketed CMG cable rated for 600V.

  4. #4
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    Quote Originally Posted by Douglas Kaup View Post
    We are a certified UL-508 shop, however these control panels are for use in a detention facility. Once upon a time UL recognized and accepted control panels under the UL-508 listing, but approximately 8 years ago they made a decision not to allow UL listings in a detention facility (I don't know why).

    Can you lead me to a document that indicates wiring within control panels is outside the scope of the NEC? Or are you aware of any exception that would allow wiring smaller than 18awg to be permitted with Class 1 systems? Keep in mind that these 24awg conductors are part of a 25 conductor jacketed CMG cable rated for 600V.


    The reason your systems are not covered by UL508a anymore is because they are now covered by UL294. It specifically says this in UL508a. Probably has nothing to do with it being in a prison. Just the type of control system involved.

    1.21 Control equipment intended for use in physical access control systems, which provide an attended
    or unattended means of monitoring or controlling traffic through portals of a protected area for security
    purposes; or in key management systems, which regulate or control access to the use of a device by
    electrical, electronic or mechanical means, are covered by the Standard for Access Control System Units,
    UL 294.
    If you are claiming the field wiring are class 1 circuits they have to abide by the rules for such circuits. I don't see any provision for using #24 AWG conductors. Just out of curiosity, what rating of overcurrent protection are you using to protect these #24 AWG conductors?

    Having said that, I agree with the poster who said the NEC kind of stops at the control panel. Look closely at what the NEC says it covers.

    90.2 Scope.
    (A) Covered. This Code covers the installation and removal of
    electrical conductors, equipment, and raceways; signaling and
    communications conductors, equipment, and raceways; and
    optical fiber cables and raceways for the following:
    Note that it specifically excludes the internal design of a utilization device. Now over the years they have cheated on this philosophy a lot by putting design criteria for utilization devices in the code as specific requirements, but for the most part, it only covers the installation of utilization equipment and not the design of them.

    Personally, I would be designing to a recognized standard such as UL294 for this type of equipment.
    Last edited by petersonra; 07-18-18 at 11:44 AM.
    Bob

  5. #5
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    From an installation standpoint, 24 AWG is small (obviously), takes more care installation, I would not use anything smaller than 18 gage, easier to strip, and terminate
    Moderator-Washington State
    Ancora Imparo

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