Load Center Outgoing Wiring

Discussion in 'Electrical Forum discussion & Blog' started by eweneek1, Nov 25, 2021.

  1. eweneek1

    eweneek1 Member

    Joined:
    Jun 23, 2009
    Installing new Loadcenter with a 24" 2 1/2" PVC pipe going to a 2 1/2" Conduit body. How many circuit wires are allowed inside the pipe? Have 3 15 amp circuits and 20 20 amp circuits outgoing.
     
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Each cable is required to be secured to the load center enclosure, NEC 312.5(C). There is an exception that would allow a number of cables to enter through a single section of conduit as you describe, but it has many conditions, so you have to check if you can use it. If you try to use a large 2-1/2" NM clamp in the enclosure to satisfy 312.5(C), the manufacturer of the NM clamp will list a maximum number of cables of a given size that the clamp can secure; I don't know of any that would allow 20 12/2 cables.

    If the conduit is entering the top of the enclosure, instead of a conduit body, you could stick a piece of horizontal wireway on top of the conduit. The wireway can have multilple KOs on top allowing each cable to be secured to the wireway with a typical 1/2" or 3/4" NM clamp, which may be listed for 2 or 3 cables per clamp. That would take care of the 312.5(C) issue, and the NM cable can proceed with its sheath intact through the 2-1/2" PVC conduit into the load center enclosure. You'd have to calculate the allowable conduit fill for the 2-1/2" PVC, but if it's under 24" long, you can use up to 60% of the internal cross sectional area. Each NM cable gets treated as if it had a circular cross section of diameter equal to its largest cross sectional dimension.

    There's also a serious derating issue if the PVC conduit is over 24" long. You can put at most 9 current carrying conductors in #12 or #14 NM cable inside a conduit longer than 24" while maintaining the usual ratings of a 20A circuit breaker for #12, 15A circuit breaker for #14. So that would mean at most 4 12/2 or 14/2 cables in one conduit. With the wireway approach, you could use multiple smaller conduits between the wireway and the panelboard to keep each conduit down to 4 cables.

    So the upshot is see if you meet the rules in 312.5(C) exception; otherwise, each cable needs to be clamped to the enclosure within a clamp that is listed for the number of cables in the clamp. And keep the conduit run to under 24" to avoid derating issues.

    Cheers, Wayne
     
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  4. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    I.e. NEC Chapter 9 Table 4 says 2-1/2" PVC Schedule 40 has an internal diameter of 2.445", and Southwire 12/2 NM has a width of 0.430" per https://www.southwire.com/wire-cable/building-wire/romex-brand-simpull-type-nm-b-cable/p/28828255

    So the maximum number allowed if the conduit is 24" or less would be floor (60% * 2.445^2 / 0.430^2) = 19 12/2 cables. Thus if you figure out the 312.5(C) issues, you can't put all 20 12/2 and 3 14/2 cables into a single short 2-1/2" PVC conduit. You'll either need a 3" conduit or to use at least two conduit entries.

    Cheers, Wayne
     
  5. eweneek1

    eweneek1 Member

    Joined:
    Jun 23, 2009
    Thanks Wayne,

    The Square D Qo Plug On Neutral load center is exterior and has a 2 1/2" KO at the top and bottom, and two 2 1/2" on the rear of the panel. Also, a 2" KO on the side and several 1/2" and 3/4" on the bottom. The meter could either use the top or side KO. The bottom 2 1/2" could feed wiring to the first floor and the top or rear 2 1/2" KO's could feed wiring to the second floor. The second floor only has 6 circuits. Our home was built in 1892 with balloon framing.
     
  6. WorthFlorida

    WorthFlorida The wife is still training me.

    Joined:
    Oct 28, 2009
    Occupation:
    Retired
    Location:
    Orlando, Florida
    Do you want to know the number a total individual wires (THHN) or NMC cables? It's not the number of circuits.
     
  7. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Then the only way you can bring NM cable into the panel is through holes in the back. NM cable is not allowed in conduit outside, which would occur if you came in through the sides, top or bottom. [Top entry for outdoor applications is limited to certain weatherproof techniques, as is side entry above the bottom of the bus bars]. So for side or bottom entry you'd need to first convert in an interior junction box to an outdoor wiring method.

    Cheers, Wayne
     
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