OT? Line Voltage And Lower Voltage Circuits And The Code
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  1. #1
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    Default OT? Line Voltage And Lower Voltage Circuits And The Code

    This is for those who are well acquainted with the NEC. I want the official poop, not just some guesses.

    In building a circuit that uses lower Voltages, probably a DC supply of some sort and housing it in standard electrical boxes which would be attached to either the building or to equipment in it, what would be the code's requirements for isolation of the low Voltage wiring vs. the line Voltage. It is my understanding that they should not be in the same box or conduit. But I could be wrong.

    Now I know that electronic devices are routinely built with AC power being brought into an enclosed chassis that also has the lower Voltage electronics inside it. It is usually routed to a power switch, fuse holder, and transformer with wires that are as short as possible. And there is no objection to this in the NEC. But what happens if the low Voltage circuit (the electronics) is to be housed in a standard electrical box.

    I built one such circuit once. I used a transformer which mounted in a knock-out hole and the line connections were passed through that hole. So the power was in one box and the low Voltage wiring was in a second one that was attached along side of it. The line connections passed through the aligned holes in the two. Was this OK?

    And what does the NEC say about running 115 or 230 Volt power in the same conduit as perhaps a 24VDC or even a cable with logic level signal? Is this ever permitted?

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    Here in EU land, the negative side of the DC needs to be bonded to machine ground, assuming the DC is going outside the enclosure to anything and especially if its to control panels etc. Also DC wiring must be ran separate to AC. Having the transformer - power supply in a control enclosure is fine so long as you don't do it in such a way as a the low voltage could be brought upto high voltage in a failure.

    Normally that means separate conduit or trunking in the box too. May be as simple as 2 separate rows of P clips with a space between em each carrying there own voltage. Basically the HV and LV wires are never running touching side by side or coming into contact. Very much so separate conduit on anything leaving the box. Even if both are heading to the same box else were on the machine. I belive the codes - std here specify a minimum separation distance between the varying voltages based on potential.

    Control signal stuff is also ran separate once more think Cat 5E etc, the only thing your allowed to drop in any conduit you wish is optical fibre, but even then it must be non conductive cased.

    Aligned holes would not be acceptable over here, they would have to have some kinda gland such that they could not guillotine the cable if moved. Simple panels bolted together with holes would strictly speaking void the enclosures ratings here. You may well get away with something as simple as 2 nuts and a externally threaded conduit sleeve. I believe that impacts your enclosures UL rating there, but its been too long since i learnt that stuff to remember it all. A lot of the stuff here in the UK if for export is oftern done to meet the most stringent stds so it can go any were and pass.

    Now all that said, its very different for a OEM if the machine is monolithic - no controls leave the machine, they have a lot more freedom under the rules to manage the risks as they wish to some degree. Think the control cabinets inside a enclosed CNC etc. Because these lines then never get exposed outside the machine the risks are very different to how say a larger factory wide system must be done.

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    First if you want real answers you are asking in the wrong forum, a forum of machinists with a few electricians thrown in will give you random answers, a forum of electricians with a few machinists is a better bet. There is an inspectors forum I read that can be confusing, but has highly technical answers.

    30 seconds of searching tells me that anyone is going to need to know a lot more about precisely what you are doing to give you a useful answer.

    It appears common conduit is a bad idea

    When I wired an unusual setup requiring inspection I had the unit plug into outlets making the inspector less worried about what was going on after that.

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    As someone who is building a house right now, I can tell you the only correct answer is to ask the local inspector. The enforced code varies based on location, the type of building, and if the inspector has had his morning coffee or not.

    That said, running Ethernet or similar next to line voltage is considered bad form. It adds interference, or worse, induced currents from the line voltage, which can seriously mess with the devices on either end. You want to shield the com wires with distance ideally.

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    You are asking a difficult question in that the way you are describing the device borders on the edge of NEC Code vs UL Standards.

    If the device is a plug in to a wall outlet then UL applies. If it is wired in directly, then NEC applies.

    This type of application is exactly why wall warts are so prolific. Buy an off the shelf, properly sized low voltage power supply that plugs into your device that is UL rated and the low voltage side is pretty much taken care of.

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    I have been told low voltage wiring will get a lot of attention and code revision over the next several years. Low voltage wiring for LED lights is becoming more common as they move away from individual light power supplies and transition to area power supplies.

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    Check local inspector for your requirements.

    But it is easier to service and maintain if separate conduits.

    Safety grounds can be a concern as different low voltage applications vary.

    But we understand code as "low voltage" combines any voltage from zero to 600 volts.

    This implies that 24 volt control lines are treated same as 480 volt supply lines.

    Sent from my SAMSUNG-SM-G930A using Tapatalk

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    Why not also read the code?

    Free online access to the NEC(R) and other electrical standards - NFPA

    All you have to do is submit an email address and a password and you have free access to NFPA 70

    Tom

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    Ok to run low voltage with line if low voltage insulation is rated for 600 volts. Bad idea if comm wire from a noise standpoint. Even leaving comm panel door open near line ac will cause reliability issues.

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    What I am doing is not a specific project. It is writer's research. I am trying to get a general idea of what can and can not be done, both in terms of safety and legality. I am not limiting this to any single city, state, or even country. I want it to be as general as possible and I am sure that the various codes are somewhat similar. But I do intend to lean somewhat heavily on the US NEC and I may wind up actually reading it. I have read sections of it in the past and it can be very difficult to understand if you are not well versed in it's use and the meaning of the terms that they use. Reading it and UNDERSTANDING what it is saying could be very time consuming. That is why I am seeking some basic information first.

    This is not the only place where I am asking. I just have a great respect for the pool of knowledge here so I figured it was a good place to ask.

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    One of the rules in the NEC is that you are supposed to install and use components in accordance with the manufacturer's instructions. The first thing I would investigate is if there are wall boxes intended to be used to house circuitry.

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    Deltap has it, basically.

    You need to have all the wire in the enclosure or raceway be insulated for the highest voltage in the same space. So, if it is all 240 or lower voltage, it is only going to need 300V class wire.

    From 300.3 (C)

    (C) Conductors of Different Systems.
    (1) 1000 Volts, Nominal, or Less. Conductors of ac and dc
    circuits, rated 1000 volts, nominal, or shall be permitted
    to occupy the same equipment wiring enclosure, cable,
    or raceway. All conductors shall have an insulation rating
    equal to at least the maximum circuit voltage applied to any
    conductor within the enclosure, cable, or raceway.


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