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Thread: ground wire in flexible steel conduit - and a de-rating question

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    Sprinkler Guy Wet_Boots's Avatar
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    Default ground wire in flexible steel conduit - and a de-rating question

    I have an application for running multiple circuits in flexible steel conduit. I know about fill limits (thank you, online calculators) and the idea of de-rating ampacity of bundled conductors, but I haven't yet found information that specifies any ground wire requirements for multiple circuits in a conduit. I know that the steel of the conduit has grounding capabilities, but I also know that modern flexible steel cable designated as AC has an aluminum bonding wire added for safety reasons, and I want to add a ground wire to gain those safety advantages. I also want to have a ground wire of sufficient size to satisfy requirements for non-conductive conduits. (In case that will work to my advantage in an inspection)

    I also have a de-rating question. I see some AC cable ratings that seem to match the 90-degree C ratings of the THHN wires inside. For instance, a 10/2 AC (steel jacket with aluminum bonding wire) shows a maximum ampacity of 40 amps. I assume it has a 30 amp rating for a lower temperature. So my question is, am I allowed to use a higher-temperature wire ampacity, and derate from there? The idea is that I can take advantage of the high-temperature ampacity to derate from, and get to a load rating equal to or better than what is customary for the wire gauge. This isn't about running on the edge of safe practices so much as trying to get circuit capacities to match up with the breakers.

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    DIY Senior Member Murphy625's Avatar
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    In 20 years of wiring machinery, I have never depended on a conduit to be a ground. Always run a separate wire for ground. Also, its always a good idea to bond it to both the machine and the end of the conduit connector.

    Remember that conductor ratings are for "Free air"... 10 gauge is 30 amp wire.. There is some special "high temp" oven wire that is rated for more than 30 amps but I doubt you have any of that.

    I suggest that 10ga is 30 amps.. derate from there.

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    Sprinkler Guy Wet_Boots's Avatar
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    I'm not actually planning on having conductors running anywhere near the 90 degree rating of THHN, even if there was a reading of the code that would allow for it, but I did want to take advantage of any 'surplus' ampacity ratings if it allows me to have two pairs of 12-gauge THHN in two 20-amp circuits, in a single 3/8 conduit.

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Yes you can use the 90 degree temp for de-rating purposes but no you can’t have 4 conductors in 3/8 flexible conduit. You would also count the equipment grounding conductor in the conduit fill.

    As long as the flexible conduit is no longer than six feet it can be the equipment grounding conductor

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    DIY Senior Member Murphy625's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    I'm not actually planning on having conductors running anywhere near the 90 degree rating of THHN, even if there was a reading of the code that would allow for it, but I did want to take advantage of any 'surplus' ampacity ratings if it allows me to have two pairs of 12-gauge THHN in two 20-amp circuits, in a single 3/8 conduit.
    Have you actually tried that yet? I'm not even sure if its possible to squeeze four 12ga wires thorough a 3/8 conduit.

    Also, one other thought, but first I want to point out I am not positive about this.. I do believe that 3/8 flex conduit is limited in its application to "Inter-device wiring" or lighting applications. I'm not up on the code as much as I used to be but I seem to recall that the 3/8 flex conduit could only be used on lighting circuits or on self contained machinery devices that have multiple interlocks, motors, etc etc. 3/8 conduit was not to be used as a feeder circuit for other than lighting.

    I could be mistaken but I seem to think I read that somewhere in the code...

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    Sprinkler Guy Wet_Boots's Avatar
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    Quote Originally Posted by jwelectric View Post
    Yes you can use the 90 degree temp for de-rating purposes but no you canít have 4 conductors in 3/8 flexible conduit. You would also count the equipment grounding conductor in the conduit fill.

    As long as the flexible conduit is no longer than six feet it can be the equipment grounding conductor
    Thanks for the de-rating detail.

    This all came up by way of planning the rewiring of a garage with a lath-and-mortar ceiling that I am considering as untouchable for the moment, so all the new wiring is to be in conduit. My fluorescent shop lamps plug into switched ceiling outlets. Since there are two outlet locations, and two switches for controlling the ceiling outlets, I have need for a four-conductor cable connecting the two switched ceiling outlet boxes.

    Maybe somewhere in this fair land of ours, is someone with 15-20 feet of 14/4 or 12/4 to sell me, but I'm not seeing it, either in AC or MC form, and I certainly won't buy a full roll of the stuff, so that leaves me with some steel conduit left over after I roll my own multi-conductor cable to connect the ceiling boxes. I'll know better after making the interconnect cable just how much I'd want to even consider making any longer ones.

    I do know about counting the grounding conductor as part of the conduit fill, for calculating fill percentage, and this is where it matters for me to know what size ground fits what size conductors, for when I get to larger conductors like 10 or 8 or 6 gauge (in 1/2-inch conduit)

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    Electrical Contractor/Instructor jwelectric's Avatar
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    The maximum number of #12 conductors allowed in 3/8 flex is 3 and it cannot be any longer than 6 feet.

    When dealing with electricity it is not a matter of what will work but what is safe and anything other than what the codes require is not safe

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    Sprinkler Guy Wet_Boots's Avatar
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    Yep. Even the fill calculator won't pass 4 #12 solid THHN wires in 3/8 flex, unless I conveniently leave off a grounding wire, and I'm not conveniently leaving off anything. (but I did throw in a #16 'bonding wire' like my AC cable has, just for grins, and still overshot the 40-percent fill mark by 0.18 percent)

    I'm fine with #14 for the garage lighting and power outlets. I did have the idea of running a pair of #12's to a new kitchen counter for toasters and what-not, but I can either live with #14's, or look to add the #12's to the fill of some 1/2-inch flex conduit that will feed an oven.

    What I had forgotten, or never noticed in the first place, is that the NEC ampacity (according to the Wiki page) for 14 and 12 gauge wire is at least 5 amps higher than the 15 and 20 amps I would expect, so that gives extra ampacity to work down from, for de-rating purposes, so it should all work with the usual breakers.

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    Sprinkler Guy Wet_Boots's Avatar
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    By the way, does NEC 250.122 (C) cover my ground wire question for multiple circuits in a conduit? It would benefit me greatly to only require a single ground wire, properly sized, of course, when it comes to some of the conduit fill calculations.

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    DIY Member Stuff's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    Maybe somewhere in this fair land of ours, is someone with 15-20 feet of 14/4 or 12/4 to sell me, but I'm not seeing it, either in AC or MC form, and I certainly won't buy a full roll of the stuff, so that leaves me with some steel conduit left over after I roll my own multi-conductor cable to connect the ceiling boxes.
    These guys sell 14/4 and 12/4 MC by the foot:
    http://www.wesbellwireandcable.com/c...wire-14-4.html

    I may have missed it in the text but why don't you use 1/2" EMT for the garage?

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    Sprinkler Guy Wet_Boots's Avatar
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    Besides matching the existing AC cable already in place, I want AC cable for this lighting circuit, because a steel beam runs the entire length of the garage, and the shortest path between the two lighting points crosses under it, with the lath-and-mortar ceiling being currently off limits to fish wires above.

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