8/3 AC cable perpendicular to joists?

Discussion in 'Electrical Forum discussion & Blog' started by DIYorBust, Sep 1, 2020.

  1. DIYorBust

    DIYorBust Active Member

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    Mar 5, 2019
    Location:
    Long Island, New York
    NEC 334.15 (C) Says "Where cable is run at angles with joists in unfinished basements and crawl spaces, it shall be permissible to secure cables not smaller than two 6 AWG or three 8 AWG conductors directly to the lower edges of the joists."

    I would like to run 8/3 AC cable in an unfinished basement. Would you interpret this to mean that 8/3 AC cable can be run perpendicular to, and attached to the bottom of joists in an such a circumstance?

    I don't intended to run it a long distance this way, but in some areas it could save me a lot of work to cross the joists bay. What do you think?

    Thanks,

    DIY
     
  2. Stuff

    Stuff Well-Known Member

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    Need to sort out terms first. NEC section 334 is for non-metallic (NM) cable. AC is Amored Cable and has different rules from NEC section 320 and the restriction is not there. Do you mean air conditioning cable? If NM cable then 8/3 would pass. Note that 8/3 means 3 conductor plus ground - usually white, black, red, copper. NM 8/2 would not qualify as the copper portion is normally 10 gauge.
     
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  4. DIYorBust

    DIYorBust Active Member

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    Thansk stuff, I see your point. I wanted the cable to serve at heat pump that calls for a 50A line on a 60A breaker. My understanding is the NM cable has to be used at the 60 degree ampacities, so 8/3 NM-B cable would be rated for only 40A. I could run 6/3 NM-B, but I thought using the 8/3 AC might be easier, and perhaps a bit better protected. Are you saying the AC cable can run across the bottoms of joists?

    320.15 says for Type AC Cable:
    Exposed Work
    Exposed runs of cable, except as provided in 300.11(A), shall closely follow the surface of the building finsih or of running boards. Exposed runs shall also be permitted to be installed on the underside of joists where supported at each joist and located so as not to be subject to physical damage.
     
  5. wwhitney

    wwhitney Well-Known Member

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    Location:
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    Most heat pumps would be 240V only, does your really require a neutral? And are you actually talking about AC cable, where the sheath has a bonding wire and is EGC, as opposed to MC cable, which has an wire-type EGC inside?

    The restriction in 334.15(C) is only for NM cable, so for other wiring methods, unless the associated article references part of Article 334, it would not apply.

    I suggest considering Al SEU/SER cable, #6 Al is good for 50 amps with 75C insulation.

    Cheers, Wayne
     
  6. DIYorBust

    DIYorBust Active Member

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    Hi Wayne, thanks for your help. Yes I'm talking about 8/3 AC cable with a bonding strip and a neutral wire. I don't know yet if a neutral is needed, but I usually make a habit to run the neutral in case it's needed later. Grounding to the sheath saves room in the boxes and requires fewer splices, although making a grounding tail on #8 stranded is a bit annoying. Is the SEU cable ok for running across joist bays? All else equal, I do think that the cable armor provides some added protection in an unfinished space, but maybe not. I can buy 100 feet of 8/3 AC cable for a little over 100 bucks, so it's not a huge savings to use armorless wire. One issue with going to aluminum #6 is that I don't have tools or connectors for making splices in wire that large so it would be a bit more work. Not a huge deal but it's something.
     
  7. wwhitney

    wwhitney Well-Known Member

    Joined:
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    Location:
    Berkeley, CA
    Both SE and NM are subject to the same installation requirements. So as far as attaching to the bottom of the joists, it's OK if it's 8/3 or 6/2 or larger.

    AC cable doesn't have that size limitation, you can run it on the bottom of the joists as long as it's not subject to damage (a vague standard that's up to the AHJ and that applies to NM and SE as well). So if you're happy using 8/3 AC, go for it.

    [Edit: to be clear, AC cable, like NM, is only rated for indoor use. So this would just be for the indoor portion of the run, although it could come into the backside of a disconnect mounted on the wall of the building.]

    As for terminating larger wires, mostly you just have to know how to strip them properly. Mechanical lug connections just require a torque driver.

    Cheers, Wayne
     
    Last edited: Sep 2, 2020
  8. DIYorBust

    DIYorBust Active Member

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    Location:
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    Thanks Wayne! Regarding the outdoor portion, my plan was to use a liquid tight whip. Can I just cut back the armor on the ac cable and run it through the whip, or do I need to splice it to labeled thhn? In this scenario I would run a ground back to the junction box where the whip originates.
     
  9. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Don't you need a disconnect at the point where the liquid tight originates? [And the disconnect needs to have 30" wide by 36" deep of clear space in front of it, i.e. don't put the disconnect behind the outdoor unit.] In which case you're going to have connections there anyway.

    But to answer your question: THHN alone is not rated for outdoor, use, you need THWN (which most THHN is also dual labeled as). If the AC cable inner conductors have the THWN rating printed on them, then you can use them in your liquid tight whip. If not, you can't.

    Cheers, Wayne
     
  10. DIYorBust

    DIYorBust Active Member

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    Yes! I forgot about the disconnect box. I guess that doesn't matter then, and I won't have to make the splice. I have yet to encounter thhn not dual rated, but yes I mean thwn. Thank you.
     
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