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Thread: Derating for low voltage

  1. #1
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    Derating for low voltage

    Does derating apply to low voltage (24VDC)? I have a 1" EMT pipe that will have 36 - 16awg TFFN wires in it if I use the plan given to me. All those wires will only be either +24VDC or -24VDC. They are used to carry I/O signals to and from a PLC. Is it against code to put that many wires in 1 raceway? I know on higher voltages and currents it is. Just was curious about the lower end. thanks in advance!

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    Jody Boehs:

    I don't know.

    You have not specified current.

    An insulated #16 is just over 0.1", but I will assume 0.1 With 36 wires of 0.1 dia in one configuration they occupy about 0.6 * 0.6 square inches, 0.36 sq-in. A 1" round pipe has an approx ID of 1, or about 3.2 * 0.5 * 0.5 = about 3/4 sq-in. Thus, wires will fit.

    #16 copper is about 4 ohms/1000 ft, or 0.04 ohms/10 ft. Suppose all wires each had 1 A of current on continuously. That is 0.04 W per wire or less than 0.04 * 40 = 1.6 W total per 10ft. You won't feel the temperature rise.

    Change the assumption to 10 A per wire and you get 160 W for 10 ft. 16 W / ft you should feel.

    The above numbers are for easy calculation.

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  3. #3
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    Quote Originally Posted by gar View Post
    180111-1613 EST
    Jody Boehs:

    I don't know.

    You have not specified current.

    An insulated #16 is just over 0.1", but I will assume 0.1 With 36 wires of 0.1 dia in one configuration they occupy about 0.6 * 0.6 square inches, 0.36 sq-in. A 1" round pipe has an approx ID of 1, or about 3.2 * 0.5 * 0.5 = about 3/4 sq-in. Thus, wires will fit.

    .

    Yes I know the wires will fit no problem...I think it was only like 30% fill or so. I'm only allowed 40%, although, I would never fill a conduit that full.

    As for the current, I said all these wires are used as PLC inputs and outputs. There's really no current to speak of on the inputs...the outputs, however, will be used for air solenoids, contactors, and relays. The current on these devices isn't a lot to speak of either.

  4. #4
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    Yes it was clear what you said, but the really critical information was not and still is not provided.

    If all your currents are under 1 A, and they might be, then you do not have a problem.

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  5. #5
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    Wire fill is about annular space in the raceway, a ratio of the inside area of the raceway and the aggregate outside diameter of the conductors, INCLUDING the insulation. It has no direct relationship to voltage. But if all circuits are 24V, the insulation of the wire can be thinner than say, 600V rated wire, so the diameter of each wire could be less, hence more actual wires could fit in a raceway up to the fill ratio. But if you are using say #14 THHN wire, the fill ratio is the same regardless of the voltage on the wire.
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  6. #6
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    Quote Originally Posted by Jody Boehs View Post
    Does derating apply to low voltage (24VDC)? I have a 1" EMT pipe that will have 36 - 16awg TFFN wires in it if I use the plan given to me. All those wires will only be either +24VDC or -24VDC. They are used to carry I/O signals to and from a PLC. Is it against code to put that many wires in 1 raceway? I know on higher voltages and currents it is. Just was curious about the lower end. thanks in advance!
    Since these are not power conductors but rather control circuits, this will depend on the class of the circuit. If they are class 1 see 725.51. If they are class 2 or 3 then no derating would apply.

  7. #7
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    Quote Originally Posted by texie View Post
    Since these are not power conductors but rather control circuits, this will depend on the class of the circuit. If they are class 1 see 725.51. If they are class 2 or 3 then no derating would apply.
    I agree, but even if they are class 1 and you need to derate the wire to 40% that still gives you 7 amps per wire. I'm sure a PLC I/O is much less than that. Probably a fraction of an amp.

    If you think about the PLC power supply rating, how many 24V, 7 amp outputs could it drive before it blew the fuse? Not very many.

    IMO, you don't need to worry about it.

  8. #8
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    Jody Boehs:

    Why did you start this thread? How or to what is the use of the word "derating" supposed apply?

    .

  9. #9
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    Quote Originally Posted by steve66 View Post
    I agree, but even if they are class 1 and you need to derate the wire to 40% that still gives you 7 amps per wire. I'm sure a PLC I/O is much less than that. Probably a fraction of an amp.

    If you think about the PLC power supply rating, how many 24V, 7 amp outputs could it drive before it blew the fuse? Not very many.

    IMO, you don't need to worry about it.
    An earlier reply/post said this:
    "Since these are not power conductors but rather control circuits, this will depend on the class of the circuit. If they are class 1 see 725.51. If they are class 2 or 3 then no derating would apply."

    Ok so i may be good on the derating and raceway fill aspects of this discussion, but help me understand the difference in Class 1, 2, and 3 circuits. Class 1 circuits 725.41 and 725.41(A) look to me like that's what I have, but then 725.121(A) talks about information technology and thermocouples (which I am using on this project). So which is it? Here is a pic of the panel I am terminating all those wires...I don't know if this helps or not...

    Name:  Liquid Rack Control Panel.jpg
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  10. #10
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    Figure out what the word "derate" means, what it applies to, and why you have no problem.

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