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  1. #1
    rimcanyon's Avatar
    rimcanyon is offline Titanium
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    Default OT: combining low and high voltage wiring in a box

    I've read HP Richter's Practical Electrical Wiring, and in some places it says low voltage and high voltage wiring should never be in the same elect. box or conduit. In others it seems to say that although low and high voltage wiring should not be in the same box, it is ok to do so.

    I'm adding controls for a multi-zone heating system, and need 110V, 24V and control wiring in the same box. This is for a pump relay (24V coil, 110V load), a 24V transformer, thermostat wires, and wires for boiler control and thermal detectors. I plan on labelling both the wires and the conduit locations. I do not see a way to put a metal barrier up between the low and high voltage wiring in the box, since the relay takes both. So the question is, what does the electrical code say about this kind of mix of voltages - what do I need to do to be in compliance?

    -Dave

  2. #2
    wippin' boy is offline Diamond
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    i do not know NEC's specific verbiage
    but it must be acceptable considering every heating system sold has just what you describe

  3. #3
    S_W_Bausch is offline Diamond
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    Molded plastic boxes have grooves for separator plates, not sure how separation is handled in metal boxes.

  4. #4
    mobile_bob is offline Stainless
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    you can do anything you like within the confines of a units electrical supply/control box, however from what i understand distribution boxes are a different matter where keeping thing separate is necessary.

    when it comes to a units control box, following a convention of trying to keep things separated to the best of your ability is probably good practice. such as keeping the higher voltage buss, on one side of the box and the 24vdc low voltage on the other side of the box as much as is practical.

    that and using appropriately sized conductors that are color coded, clearly marked and listed in a readily accessible schematic is also good form.

    bob g

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    doug8cat is offline Stainless
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    I dunno but my a/c unit has 240vac and a 24 vac relay in the same 6 x 4 x 3 inch enclosure

  6. #6
    HelicalCut is offline Stainless
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    Extra low voltage and low voltage are mixed in every machine I have seen which uses them.

    There are areas which Extra low voltage is probably mandatory such as pool lights.

    Low and high voltage are never mixed. 480V is considered low voltage by most supply authorities, 11Kv is considered high voltage.

  7. #7
    Jeff_M_PA is offline Cast Iron
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    How could you ever wire a transformer in a box if you didn't mix low and high voltage?

  8. #8
    David Utidjian's Avatar
    David Utidjian is online now Titanium
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    I think the general rule of thumb is to keep them segregated as much as possible. It has to do with why there even is low voltage. That is to say... if a system has low voltage controls(LVC), as many of the machine tools in my shop do, then the reason is for safety and they should be segregated as much as possible. Which they are... but the box with the motor contactors obviously has both high and low voltages present. The control boxes or heads only have low voltage and uses separate conduits and raceways from the high voltage.

    Most any other device that plugs into the wall has them mixed with little or no separation. A PC will have all the high voltage inside the power supply... yet all low voltage power and one or more low voltage control circuits eventually end up inside the power supply high voltage box.

    When I design my own circuitry I go to great lengths to insure it is very difficult to get them mixed up. All high voltage connections use a separate terminal strip from the low voltage terminal strips. I try to never use black or white colored wire for low voltage. I use terminal strips with clear plastic covers. I use ring terminals and binding head screws for all connections to the terminal strips. I print and stick a wiring diagram inside the cover of the enclosure. I group all high voltage components to one side or area in the box and route all high voltage wires separately within the box.

    -DU-

  9. #9
    MBensema is offline Hot Rolled
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    The problem is more with the line voltage and control cables in the conduit and not the junction box. With the control cables laying next to higher voltage line voltage wires over longer distances, the higher line voltage can induce a voltage in the lower voltage control wires and cause unpredictable reactions in the controls.

    Having said that, 120v is not all that high and I don't recall ever having a problem with 24v and 120v wires in the same conduit, but have had problems requiring the customer to run new separate conduit for control wires in 480V systems. You can reduce the chances of any problems by using twisted shielded pair wires for each control device, this reduces the chances of nearby higher voltage wires causing noise in the controls.

  10. #10
    JST's Avatar
    JST
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    The language covering this generally says that you can combine various low voltage wires (under 600V) so long as ALL the wires have a voltage rating (insulation) suitable for the *highest* voltage in the enclosure or raceway.

    There are a few things that cannot be combined.

    The problem you run into is that the insulation limit is fine for individual wires, but harder to satisfy when you want to run multiconductor wires, ethernet, etc in with others. Many such wires will not have suitable insulation, and will require separate housing on account of it.

    You also have to satisfy the conduit fill limits regardless.

    "Low" voltage is not above 600V.

    "Medium" voltage is from there up to limit I don't recall but which is likely somewhere in the 25 to 35kV area.

  11. #11
    rimcanyon's Avatar
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    Quote Originally Posted by wippin' boy View Post
    i do not know NEC's specific verbiage
    but it must be acceptable considering every heating system sold has just what you describe
    Exactly. But Richter gives examples, e.g. transformers that mount on box covers, so that the low voltage wiring is external to the box. He also says
    "Low-Voltage Wiring. This term includes wiring for doorbells, chimes and other signals, thermostats, and similar devices operating at low voltage. Usually this means 30 volts or less. The power for operating such low-voltage circuits is derived from small transformers. Under no circumstances may low-voltage wires be run in the same conduit or cable with other wires carrying full voltage. They must never enter an outlet or switch box containing full-voltage wires unless a metal barrier of the same thickness as the walls of the box separates the two types of wiring, or unless the power supply wires are introduced solely for energizing equipment to which the low-voltage wires are connected."
    He doesn't have much else to say on the topic, but he probably wrote the above 30 yrs. ago and it has been reprinted in succeeding editions of his book. The case I have, with low voltage used to actuate a relay that is switching high voltage, falls outside the paragraph above. The electrical box is 12x12x4 and is mounted in the wall. So clearly, if it is allowed, then there aren't really any restrictions about what can go into an electrical box (within the constraints of the orig. question), and it comes down to common sense to develop good practice, like the suggestions in David Utidjian's post.

    Thanks to everyone who replied.

    -Dave

  12. #12
    MwTech Inc is offline Stainless
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    I am in control boxes almost everyday built by manuf in the states and european markets.

    There is no real separation of any control wiring.

    24v thru 480 all tucked in the same raceway, however all wiring is 600 volt rated, assuming 480 supply.

    Most but not all will have the logic boards away from the supply and high amperage locations but I think it's mostly due to finding space or the board is a touch panel and needs to be at the operators position.

    You will not have any issues mixing 24v and 110v in the same box
    for your home heater, evey furnance in this world has them mixed together

  13. #13
    Tony Wells is offline Banned
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    I'm not sure this is current, so YMMV:

    E4204.1 Separation from other conductors.
    In cables, compartments,
    enclosures, outlet boxes, device boxes, and raceways,
    conductors of Class 2 circuits shall not be placed in any
    cable, compartment, enclosure, outlet box, device box, race-
    way, or similar fitting with conductors of electric light, power,
    Class 1 and nonpower-limited fire alarm circuits.
    Exceptions:
    1. Where the conductors of the electric light, power,
    Class 1 and nonpower-limited fire alarm circuits are
    separated by a barrier from the Class 2 circuits. In
    enclosures, Class 2 circuits shall be permitted to be
    installed in a raceway within the enclosure to separate
    them from Class 1, electric light, power and
    nonpower-limited fire alarm circuits.
    2. Class 2 conductors in compartments, enclosures,
    device boxes, outlet boxes and similar fittings where
    electric light, power, Class 1 or nonpower-limited fire
    alarm circuit conductors are introduced solely to connect
    to the equipment connected to the Class 2 circuits.
    The electric light, power, Class 1 and
    nonpower-limited fire alarm circuit conductors shall
    be routed to maintain a minimum of
    1/4 inch (6.4 mm)
    separation from the conductors and cables of the
    Class 2 circuits; or the electric light power, Class 1
    and nonpower-limited fire alarm circuit conductors
    operate at 150 volts or less to ground and the Class 2
    circuits are installed using Types CL3, CL3R, or
    CL3P or permitted substitute cables, and provided
    that these Class 3 cable conductors extending beyond
    their jacket are separated by a minimum of
    1/4 inch
    (6.4 mm) or by a nonconductive sleeve or
    nonconductive barrier from all other conductors.

    E4204.2 Other applications.
    Conductors of Class 2 circuits
    shall be separated by not less than 2 inches (51 mm) from conductors
    of any electric light, power, Class 1 or nonpower-limited
    fire alarm circuits except where one of the following
    conditions is met:
    1. All of the electric light, power, Class 1 and
    nonpower-limited fire alarm circuit conductors are in
    raceways or in metal-sheathed, metal-clad, nonmetallic-
    sheathed or Type UF cables.
    2. All of the Class 2 circuit conductors are in raceways or in
    metal-sheathed, metal-clad, nonmetallic-sheathed or
    Type UF cables.

  14. #14
    RDL
    RDL is offline Stainless
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    Quote Originally Posted by rimcanyon View Post
    I've read HP Richter's Practical Electrical Wiring, and in some places it says low voltage and high voltage wiring should never be in the same elect. box or conduit. In others it seems to say that although low and high voltage wiring should not be in the same box, it is ok to do so.
    ...

    -Dave
    I believe that the intent is a safety issue of having more than one feed to a panel rather than having high and low voltage within the same box. After all a lot of controls for high voltage equipment are powered by lower voltages. If there is only one breaker for the panel then switching off the power does so for the entire panel. If there is more than one feed a person could shut off a breaker and successfully do a few tests to confirm that there is "no power" yet get zapped on an active circuit.

    A panel should have a warning label on the outside if it has two or more power sources.

    Raymond

  15. #15
    jlrii's Avatar
    jlrii is online now Cast Iron
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    Our business has a UL listed panel shop, although that is not what I do. I believe the wire used, if not separated must be equal, to the highest insulation grade req'd. Any lower voltage wiring in the panel must be related to the panel. I could come up with more details on Monday if needed.

    JR

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