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Thread: Can three #8 and one #10 wires be in ½” metal conduit?

  1. #1
    DIY Member electrotuko's Avatar
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    Default Can three #8 and one #10 wires be in ½” metal conduit?

    Would Code allow running three (L1, L2, Neutral) THHN 90C rated #8 wires and one #10 as non-current carrying for GND in ½” metal flex conduit? Circuit breaker will be 50A; cable will carry up to 50Amps.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Does not appear to be so...it's full with the three #8 wires, no room for the ground.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member electrotuko's Avatar
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    I would not agree, even four #8 wires fit, slide in easily and still ome room left.
    It is not about fits or not, it is more if code would alolow to do this.

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    DIY Member electrotuko's Avatar
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    Can a none Current‐Carrying Conductors be discounted form the estimations done by using Conduit Fill tables?
    For example, for 1/2" metal flex conduit max three 8 gauge wires are allowed. In a four wire system the GND wire is not carrying current, can be not counted, hence it is allowed to use three 8 gauge and one 10 gauge GND wire?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I only looked at a conduit fill table...code usually matches the manufacturer's recommendations. I do not have a copy of the actual codes, but if I were to guess (you need to know, not guess), I'd guess the answer is no. One of the pros with the code available should show up and advise. To keep it easier, could you just go to 3/4" conduit and there'd be no question...
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Sprinkler Guy Wet_Boots's Avatar
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    Quote Originally Posted by electrotuko View Post
    Can a none Current‐Carrying Conductors be discounted form the estimations done by using Conduit Fill tables?
    For example, for 1/2" metal flex conduit max three 8 gauge wires are allowed. In a four wire system the GND wire is not carrying current, can be not counted, hence it is allowed to use three 8 gauge and one 10 gauge GND wire?
    A conduit fill calculator wants to know about everything going into the conduit, whether it carries current or not, as jamming is a consideration, as is heat dissipation. I use one supplied by Southwire, and the only way you get three #8 THHN wires, plus a #10 ground into 1/2-inch flexible metal conduit, without exceeding a 40 percent fill limit, is if the #10 wire is bare copper. Three #8 THHN stranded {218 mils diameter} plus one bare #10 solid copper ground {101 mils diameter} gives a 37.89% fill in 1/2-inch flexible metal conduit.

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    DIY Member electrotuko's Avatar
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    Thanks, I will follow your advice go to ¾’ size conduit.
    By reading more I am getting in doubt that I am doing this project right.
    I want to add more outlets and light fixtures in to additionally built rooms in the house.
    It is no extra space in the main 100A panel.
    I was going to add in the crawl space a branch box: Siemens 8-Circuit 4-Space 125-Amp Main Lug Load Center, installing four dual single-pole 15A circuit breakers in it (two for each phase).
    This would give me eight 15A rated outputs to run eight # 16 cables to lights, outlets.
    In the main panel I was going to install dual-pole 50A breaker (main panel has room only for this).
    The plan was to use metal flex conduit to run four #8 wires to feed that branch box.

    Is my thinking is right.
    I used online calculator table which results confirmed that 50A circuit breaker is OK, 10 gauge GND wire in the cable is OK (going to a branch box with eight 15A rated outputs), but the three conductors are specified as 6 gauge, not the 8 gauge that I was thinking. This happened due to correction factor per note “Circuit is Branch Circuit that supplies multiple receptacles”, needs to follow 240.4(B) Devices Rated 800 Amperes or Less. The conductors being protected are not part of a multi outlet branch circuit supplying receptacles for cord and- plug-connected portable loads….and etc.
    If do not use that special condition, the calculator says that 8 gage wires are OK.

    Is my exercise is correct, should I consider an auxiliary box (8-Circuit 4-Space 125-Amp Main Lug Load) as
    “Circuit is Branch Circuit that supplies multiple receptacles” and use 6 gauge wires or I am mistaken and 8 gauge wires are OK?

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    Sprinkler Guy Wet_Boots's Avatar
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    There might be some limitation on the total number of circuits, no matter how you would group the breakers.

    By the way, you can download the complete NEC electric codes from Archive.org

    (by the way, just for grins, I did an additional fill calculation, and replaced the bare #10 solid ground wire with a bare #8 solid ground wire, and the fill calculated to 39.4 percent)
    Last edited by Wet_Boots; 08-01-2013 at 04:02 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    All wiring on a 15A circuit must be 14g or larger (as in 12, 10, etc.)...IOW, you CANNOT use 16g anywhere for supply lines to a fixture or receptacle or switch.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Moderator & Master Plumber hj's Avatar
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    My book says TWO #8 in a 1/2" conduit, four in 3/4".
    Licensed residential and commercial plumber

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    Electrical Contractor/Instructor jwelectric's Avatar
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    I/2 inch will only accept 3 #8 conductors without the ground. In order to install the ground it must be 3/4
    The panel must have 6 1/2 feet of head room so maybe the crawl space is not a good idea
    #14 is the smallest conductor allowed to be installed
    a copper #8 THW conductor is good for 50 amps

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    DIY Senior Member bluebinky's Avatar
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    Quote Originally Posted by electrotuko View Post
    ... I was going to add in the crawl space a branch box: Siemens 8-Circuit 4-Space 125-Amp Main Lug Load Center ...
    Not sure exactly what you mean by "crawl space", but my understanding is that breakers need to be readily accessible -- you should be able to walk up, reach out and touch them.

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    DIY Member electrotuko's Avatar
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    Thnaks for all yours help and advises. I also downloaded the NEC Code 2011, reading indepth, will try to understand sub-panel wiring requiremtns. It will be double 50A breaker in the main panel, three 8 gauge and one 10 gauge for GND wires inside 1/2" metal conduit (will try to clarify reading Code if fourth GND wire can be discounted anf 1/2" conduit acceptable), four double sided 16A brakers fill feed receptacles, lights through 14 gauge wires (#16 was a typo ofcourse).

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    Electrical Contractor/Instructor jwelectric's Avatar
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    No you can’t use ½ inch flex and yes you count every conductor installed when doing conduit fill.

    A bus has two mothers with new born babies wrecks and kills everyone board. How many died? Well I will see if the babies count as being aboard the bus.

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    Sprinkler Guy Wet_Boots's Avatar
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    By the way, did the tables change when large appliances went from 3-blade plugs to 4-blade plugs? Way back when, there wouldn't have been anything in a range outlet, for instance, for a fourth wire to connect to, so it wasn't going to be in the cable with the two hots, right?
    Last edited by Wet_Boots; 08-02-2013 at 08:31 PM.

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